From cea3e47938322b97c318dea38dc0d649e196dc1b Mon Sep 17 00:00:00 2001
From: Ben Culkin <scorpress@gmail.com>
Date: Tue, 16 Aug 2022 23:03:27 -0400
Subject: Refactor to add backtracking support

This probably doesn't help w/ error messages, but it enables some cool
ideas where syntax can be reused in cases where it would otherwise be
invalid
---
 JPratt/src/main/java/bjc/pratt/PrattParser.java    | 267 +++++++++++++--------
 .../java/bjc/pratt/blocks/ChainParseBlock.java     |  19 +-
 .../java/bjc/pratt/blocks/GrammarParseBlock.java   |  53 ++--
 .../src/main/java/bjc/pratt/blocks/ParseBlock.java |   4 +-
 .../java/bjc/pratt/blocks/RepeatingParseBlock.java |  10 +-
 .../java/bjc/pratt/blocks/SimpleParseBlock.java    |  13 +-
 .../java/bjc/pratt/blocks/TriggeredParseBlock.java |   8 +-
 .../bjc/pratt/commands/AbstractInitialCommand.java |   4 +-
 .../java/bjc/pratt/commands/BinaryCommand.java     |   9 +-
 .../java/bjc/pratt/commands/CommandResult.java     |  97 ++++++++
 .../java/bjc/pratt/commands/InitialCommand.java    |   4 +-
 .../java/bjc/pratt/commands/NonInitialCommand.java |   4 +-
 .../pratt/commands/impls/BlockInitialCommand.java  |   3 +-
 .../bjc/pratt/commands/impls/ChainCommand.java     |  17 +-
 .../bjc/pratt/commands/impls/ConstantCommand.java  |   5 +-
 .../commands/impls/DefaultInitialCommand.java      |   3 +-
 .../commands/impls/DefaultNonInitialCommand.java   |   3 +-
 .../bjc/pratt/commands/impls/DenestingCommand.java |   8 +-
 .../bjc/pratt/commands/impls/GroupingCommand.java  |   9 +-
 .../bjc/pratt/commands/impls/InitialCommands.java  |  13 +-
 .../java/bjc/pratt/commands/impls/LeafCommand.java |   5 +-
 .../bjc/pratt/commands/impls/PanfixCommand.java    |  39 +++
 .../pratt/commands/impls/PostCircumfixCommand.java |  11 +-
 .../bjc/pratt/commands/impls/PostfixCommand.java   |   5 +-
 .../pratt/commands/impls/PreTernaryCommand.java    |  21 +-
 .../bjc/pratt/commands/impls/TernaryCommand.java   |  47 ++--
 .../commands/impls/TransformingInitialCommand.java |   9 +-
 .../bjc/pratt/commands/impls/UnaryCommand.java     |  10 +-
 .../java/bjc/pratt/tokens/ExpectionNotMet.java     |  23 ++
 .../java/bjc/pratt/tokens/StringTokenStream.java   |  34 ++-
 .../main/java/bjc/pratt/tokens/TokenStream.java    |  56 ++---
 31 files changed, 563 insertions(+), 250 deletions(-)
 create mode 100644 JPratt/src/main/java/bjc/pratt/commands/CommandResult.java
 create mode 100644 JPratt/src/main/java/bjc/pratt/commands/impls/PanfixCommand.java
 create mode 100644 JPratt/src/main/java/bjc/pratt/tokens/ExpectionNotMet.java

(limited to 'JPratt/src/main/java/bjc/pratt')

diff --git a/JPratt/src/main/java/bjc/pratt/PrattParser.java b/JPratt/src/main/java/bjc/pratt/PrattParser.java
index 9887aa0..c7d40fc 100644
--- a/JPratt/src/main/java/bjc/pratt/PrattParser.java
+++ b/JPratt/src/main/java/bjc/pratt/PrattParser.java
@@ -1,16 +1,24 @@
 package bjc.pratt;
 
+import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
 
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.commands.MetaInitialCommand;
 import bjc.pratt.commands.MetaNonInitialCommand;
 import bjc.pratt.commands.NonInitialCommand;
 import bjc.pratt.commands.impls.DefaultInitialCommand;
 import bjc.pratt.commands.impls.DefaultNonInitialCommand;
+import bjc.pratt.tokens.ExpectionNotMet;
 import bjc.pratt.tokens.Token;
 import bjc.pratt.tokens.TokenStream;
+import bjc.data.TransformIterator;
 import bjc.data.Tree;
 import bjc.utils.funcutils.NumberUtils;
 import bjc.utils.parserutils.ParserException;
@@ -20,14 +28,11 @@ import bjc.utils.parserutils.ParserException;
  *
  * @author EVE
  *
- * @param <K>
- *        The key type for the tokens.
+ * @param <K> The key type for the tokens.
  *
- * @param <V>
- *        The value type for the tokens.
+ * @param <V> The value type for the tokens.
  *
- * @param <C>
- *        The state type of the parser.
+ * @param <C> The state type of the parser.
  *
  *
  */
@@ -36,30 +41,33 @@ public class PrattParser<K, V, C> {
 	 * Default commands that error when used.
 	 */
 	private final NonInitialCommand<K, V, C> DEFAULT_LEFT_COMMAND = new DefaultNonInitialCommand<>();
+	private final List<NonInitialCommand<K, V, C>> DEFAULT_LEFT_LIST = Arrays.asList(DEFAULT_LEFT_COMMAND);
+
 	private final InitialCommand<K, V, C> DEFAULT_NULL_COMMAND = new DefaultInitialCommand<>();
+	private final List<InitialCommand<K, V, C>> DEFAULT_NULL_LIST = Arrays.asList(DEFAULT_NULL_COMMAND);
 
 	/*
 	 * Left-commands that depend on what the null command was.
 	 */
-	private final Map<K, Map<K, NonInitialCommand<K, V, C>>> dependantLeftCommands;
-	private final Map<K, Map<K, MetaNonInitialCommand<K, V, C>>> dependantMetaLeftCommands;
+	private final Map<K, Map<K, List<NonInitialCommand<K, V, C>>>> dependantLeftCommands;
+	private final Map<K, Map<K, List<MetaNonInitialCommand<K, V, C>>>> dependantMetaLeftCommands;
 	/*
 	 * The left commands.
 	 */
-	private final Map<K, NonInitialCommand<K, V, C>> leftCommands;
-	private final Map<K, MetaNonInitialCommand<K, V, C>> metaLeftCommands;
+	private final Map<K, List<NonInitialCommand<K, V, C>>> leftCommands;
+	private final Map<K, List<MetaNonInitialCommand<K, V, C>>> metaLeftCommands;
 
 	/*
 	 * The initial commands.
 	 */
-	private final Map<K, InitialCommand<K, V, C>> nullCommands;
-	private final Map<K, MetaInitialCommand<K, V, C>> metaNullCommands;
+	private final Map<K, List<InitialCommand<K, V, C>>> nullCommands;
+	private final Map<K, List<MetaInitialCommand<K, V, C>>> metaNullCommands;
 
 	/*
 	 * Initial commands only checked for statements.
 	 */
-	private final Map<K, InitialCommand<K, V, C>> statementCommands;
-	private final Map<K, MetaInitialCommand<K, V, C>> metaStatementCommands;
+	private final Map<K, List<InitialCommand<K, V, C>>> statementCommands;
+	private final Map<K, List<MetaInitialCommand<K, V, C>>> metaStatementCommands;
 
 	/**
 	 * Create a new Pratt parser.
@@ -82,185 +90,244 @@ public class PrattParser<K, V, C> {
 	/**
 	 * Parse an expression.
 	 *
-	 * @param precedence
-	 *        The initial precedence for the expression.
+	 * @param precedence  The initial precedence for the expression.
 	 *
-	 * @param tokens
-	 *        The tokens for the expression.
+	 * @param tokens      The tokens for the expression.
 	 *
-	 * @param state
-	 *        The state of the parser.
+	 * @param state       The state of the parser.
 	 *
-	 * @param isStatement
-	 *        Whether or not to parse statements.
+	 * @param isStatement Whether or not to parse statements.
 	 *
 	 * @return The expression as an AST.
 	 *
-	 * @throws ParserException
-	 *         If something goes wrong during parsing.
+	 * @throws ParserException If something goes wrong during parsing.
 	 */
-	public Tree<Token<K, V>> parseExpression(final int precedence, final TokenStream<K, V> tokens, final C state,
+	public CommandResult<K, V> parseExpression(final int precedence, final TokenStream<K, V> tokens, final C state,
 			final boolean isStatement) throws ParserException {
-		if(precedence < 0) throw new IllegalArgumentException("Precedence must be greater than zero");
+		if (precedence < 0)
+			throw new IllegalArgumentException("Precedence must be greater than zero");
 
 		ParserContext<K, V, C> parserContext = new ParserContext<>(tokens, this, state);
 
+		Tree<Token<K, V>> ast = null;
+		CommandResult<K, V> result;
+
+		tokens.mark();
 		final Token<K, V> initToken = tokens.current();
 		tokens.next();
+		tokens.mark();
 
 		final K initKey = initToken.getKey();
+		Iterator<InitialCommand<K, V, C>> nullCommandIter = getInitialCommand(isStatement, initKey, parserContext);
+		do {
+			if (!nullCommandIter.hasNext()) {
+				// Restore to the state we were in before we tried to parse this token.
+				// Need the commit because rollback doesn't remove marks
+				tokens.rollback();
+				tokens.commit();
+				tokens.rollback();
+				return CommandResult.backtrack();
+			}
 
-		InitialCommand<K, V, C> nullCommand = getInitialCommand(isStatement, initKey, parserContext);
-		Tree<Token<K, V>> ast = nullCommand.denote(initToken, parserContext);
-
-		parserContext.initial = initKey;
+			InitialCommand<K, V, C> nullCommand = nullCommandIter.next();
+			try {
+				result = nullCommand.denote(initToken, parserContext);
+			} catch (ExpectionNotMet enm) {
+				// TODO: Should enm be used for something here?
+				result = CommandResult.backtrack();
+			}
+			switch (result.status) {
+			case SUCCESS:
+				ast = result.success();
+				break;
+			case FAIL:
+				return result;
+			case BACKTRACK:
+				tokens.rollback();
+				break;
+			default:
+				throw new IllegalStateException("Unhandled result status " + result.status);
+			}
+			parserContext.initial = initKey;
+		} while (result.status != Status.SUCCESS);
+		tokens.commit();
+		// Think this is right...
+		// Will get rid of all our active marks.
+		tokens.commit();
 
 		int rightPrec = Integer.MAX_VALUE;
 
-		while(true) {
+		outer: while (true) {
+			tokens.mark();
 			final Token<K, V> tok = tokens.current();
 
 			final K key = tok.getKey();
 
-			NonInitialCommand<K, V, C> leftCommand = getNonInitialCommand(key, parserContext);
-
-			final int leftBind = leftCommand.leftBinding();
-
-			if(NumberUtils.between(precedence, rightPrec, leftBind)) {
-				tokens.next();
-
-				ast = leftCommand.denote(ast, tok, parserContext);
-				rightPrec = leftCommand.nextBinding();
-			} else {
-				break;
-			}
+			Iterator<NonInitialCommand<K, V, C>> leftCommandIter = getNonInitialCommand(key, parserContext);
+			do {
+				if (!leftCommandIter.hasNext()) {
+					// Restore to the state we were in before we tried to parse this token.
+					// Need the commit because rollback doesn't remove marks
+					tokens.rollback();
+					tokens.commit();
+					tokens.rollback();
+					return CommandResult.backtrack();
+				}
+
+				NonInitialCommand<K, V, C> leftCommand = leftCommandIter.next();
+				final int leftBind = leftCommand.leftBinding();
+
+				if (NumberUtils.between(precedence, rightPrec, leftBind)) {
+					tokens.next();
+					tokens.mark();
+
+					try {
+						result = leftCommand.denote(ast, tok, parserContext);
+						rightPrec = leftCommand.nextBinding();
+					} catch (ExpectionNotMet enm) {
+						result = CommandResult.backtrack();
+					}
+
+					switch (result.status) {
+					case SUCCESS:
+						tokens.commit();
+						tokens.commit();
+						ast = result.success();
+						break;
+					case FAIL:
+						return result;
+					case BACKTRACK:
+						tokens.rollback();
+						break;
+					default:
+						throw new IllegalStateException("Unhandled result status " + result.status);
+					}
+				} else {
+					tokens.commit();
+					break outer;
+				}
+			} while (result.status != Status.SUCCESS);
 		}
 
-		return ast;
+		return CommandResult.success(ast);
 	}
 
 	/**
 	 * Add a non-initial command to this parser.
 	 *
-	 * @param marker
-	 *        The key that marks the command.
+	 * @param marker The key that marks the command.
 	 *
-	 * @param comm
-	 *        The command.
+	 * @param comm   The command.
 	 */
 	public void addNonInitialCommand(final K marker, final NonInitialCommand<K, V, C> comm) {
-		leftCommands.put(marker, comm);
+		leftCommands.computeIfAbsent(marker, mrk -> new ArrayList<>()).add(comm);
 	}
 
 	/**
 	 * Add a initial command to this parser.
 	 *
-	 * @param marker
-	 *        The key that marks the command.
+	 * @param marker The key that marks the command.
 	 *
-	 * @param comm
-	 *        The command.
+	 * @param comm   The command.
 	 */
 	public void addInitialCommand(final K marker, final InitialCommand<K, V, C> comm) {
-		nullCommands.put(marker, comm);
+		nullCommands.computeIfAbsent(marker, mrk -> new ArrayList<>()).add(comm);
 	}
 
 	/**
 	 * Add a statement command to this parser.
 	 *
-	 * The difference between statements and initial commands is that
-	 * statements can only appear at the start of the expression.
+	 * The difference between statements and initial commands is that statements can
+	 * only appear at the start of the expression.
 	 *
-	 * @param marker
-	 *        The key that marks the command.
+	 * @param marker The key that marks the command.
 	 *
-	 * @param comm
-	 *        The command.
+	 * @param comm   The command.
 	 */
 	public void addStatementCommand(final K marker, final InitialCommand<K, V, C> comm) {
-		statementCommands.put(marker, comm);
+		statementCommands.computeIfAbsent(marker, mrk -> new ArrayList<>()).add(comm);
 	}
 
 	/**
 	 * Add a dependent non-initial command to this parser.
 	 *
-	 * @param dependant
-	 *        The dependent that precedes the command.
+	 * @param dependant The dependent that precedes the command.
 	 *
-	 * @param marker
-	 *        The token key that marks the command.
+	 * @param marker    The token key that marks the command.
 	 *
-	 * @param comm
-	 *        The command.
+	 * @param comm      The command.
 	 */
 	public void addDependantCommand(final K dependant, final K marker, final NonInitialCommand<K, V, C> comm) {
-		Map<K, NonInitialCommand<K, V, C>> dependantMap = dependantLeftCommands.getOrDefault(dependant,
+		Map<K, List<NonInitialCommand<K, V, C>>> dependantMap = dependantLeftCommands.getOrDefault(dependant,
 				new HashMap<>());
 
-		dependantMap.put(marker, comm);
+		dependantMap.computeIfAbsent(marker, mrk -> new ArrayList<>()).add(comm);
 	}
 
 	/**
 	 * Lookup an initial command.
 	 * 
-	 * @param isStatement
-	 *        Whether to look for statement commands or not.
+	 * @param isStatement Whether to look for statement commands or not.
 	 * 
-	 * @param key
-	 *        The key of the command.
+	 * @param key         The key of the command.
 	 * 
-	 * @param ctx
-	 *        The context for meta-commands.
+	 * @param ctx         The context for meta-commands.
 	 * 
 	 * @return A command attached to that key, or a default implementation.
 	 */
-	public InitialCommand<K, V, C> getInitialCommand(boolean isStatement, K key, ParserContext<K, V, C> ctx) {
-		if(isStatement) {
-			if(metaStatementCommands.containsKey(key))
-				return metaStatementCommands.get(key).getCommand(ctx);
-			else if(statementCommands.containsKey(key)) return statementCommands.get(key);
+	public Iterator<InitialCommand<K, V, C>> getInitialCommand(boolean isStatement, K key, ParserContext<K, V, C> ctx) {
+		if (isStatement) {
+			if (metaStatementCommands.containsKey(key)) {
+				List<MetaInitialCommand<K, V, C>> lst = metaStatementCommands.get(key);
+
+				return new TransformIterator<>(lst.iterator(), (itm) -> itm.getCommand(ctx));
+			} else if (statementCommands.containsKey(key)) {
+				return statementCommands.get(key).iterator();
+			}
 		}
 
-		if(metaNullCommands.containsKey(key)) {
-			return metaNullCommands.get(key).getCommand(ctx);
+		if (metaNullCommands.containsKey(key)) {
+			List<MetaInitialCommand<K, V, C>> lst = metaNullCommands.get(key);
+
+			return new TransformIterator<>(lst.iterator(), (itm) -> itm.getCommand(ctx));
 		}
 
-		return nullCommands.getOrDefault(key, DEFAULT_NULL_COMMAND);
+		return nullCommands.getOrDefault(key, DEFAULT_NULL_LIST).iterator();
 	}
 
 	/**
 	 * Lookup a non-initial command.
 	 * 
-	 * @param key
-	 *        The key of the command.
+	 * @param key The key of the command.
 	 * 
-	 * @param ctx
-	 *        The context for meta-commands.
+	 * @param ctx The context for meta-commands.
 	 * 
 	 * @return A command attached to that key, or a default implementation.
 	 */
-	public NonInitialCommand<K, V, C> getNonInitialCommand(K key, ParserContext<K, V, C> ctx) {
-		if(dependantMetaLeftCommands.containsKey(ctx.initial)) {
-			Map<K, MetaNonInitialCommand<K, V, C>> dependantCommands = dependantMetaLeftCommands
-					.get(ctx.initial);
+	public Iterator<NonInitialCommand<K, V, C>> getNonInitialCommand(K key, ParserContext<K, V, C> ctx) {
+		if (dependantMetaLeftCommands.containsKey(ctx.initial)) {
+			Map<K, List<MetaNonInitialCommand<K, V, C>>> dependantCommands = dependantMetaLeftCommands.get(ctx.initial);
+
+			if (dependantCommands.containsKey(key)) {
+				List<MetaNonInitialCommand<K, V, C>> lst = dependantCommands.get(key);
 
-			if(dependantCommands.containsKey(key)) {
-				return dependantCommands.get(key).getCommand(ctx);
+				return new TransformIterator<>(lst.iterator(), (itm) -> itm.getCommand(ctx));
 			}
 		}
 
-		if(dependantLeftCommands.containsKey(ctx.initial)) {
-			Map<K, NonInitialCommand<K, V, C>> dependantCommands = dependantLeftCommands.get(ctx.initial);
+		if (dependantLeftCommands.containsKey(ctx.initial)) {
+			Map<K, List<NonInitialCommand<K, V, C>>> dependantCommands = dependantLeftCommands.get(ctx.initial);
 
-			if(dependantCommands.containsKey(key)) {
-				return dependantCommands.getOrDefault(key, DEFAULT_LEFT_COMMAND);
+			if (dependantCommands.containsKey(key)) {
+				return dependantCommands.getOrDefault(key, DEFAULT_LEFT_LIST).iterator();
 			}
 		}
 
-		if(metaLeftCommands.containsKey(key)) {
-			return metaLeftCommands.get(key).getCommand(ctx);
+		if (metaLeftCommands.containsKey(key)) {
+			List<MetaNonInitialCommand<K, V, C>> lst = metaLeftCommands.get(key);
+			return new TransformIterator<>(lst.iterator(), (itm) -> itm.getCommand(ctx));
 		}
 
-		return leftCommands.getOrDefault(key, DEFAULT_LEFT_COMMAND);
+		return leftCommands.getOrDefault(key, DEFAULT_LEFT_LIST).iterator();
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/ChainParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/ChainParseBlock.java
index 038b6ea..2717e42 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/ChainParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/ChainParseBlock.java
@@ -3,6 +3,8 @@ package bjc.pratt.blocks;
 import java.util.Set;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -51,9 +53,11 @@ public class ChainParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> parse(ParserContext<K, V, C> ctx) throws ParserException {
-		Tree<Token<K, V>> expression = iner.parse(ctx);
-
+	public CommandResult<K, V> parse(ParserContext<K, V, C> ctx) throws ParserException {
+		CommandResult<K,V> resOuter = iner.parse(ctx);
+		if (resOuter.status != Status.SUCCESS) return resOuter;
+		
+		Tree<Token<K, V>> expression = resOuter.success();
 		Token<K, V> currentToken = ctx.tokens.current();
 		if(indicators.contains(currentToken.getKey())) {
 			Tree<Token<K, V>> res = new SimpleTree<>(trm);
@@ -63,16 +67,19 @@ public class ChainParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 				res.addChild(new SimpleTree<>(currentToken));
 				ctx.tokens.next();
 
-				Tree<Token<K, V>> innerExpression = iner.parse(ctx);
+				CommandResult<K,V> resInner = iner.parse(ctx);
+				if (resInner.status != Status.SUCCESS) return resInner;
+				
+				Tree<Token<K, V>> innerExpression = resInner.success();
 				res.addChild(innerExpression);
 
 				currentToken = ctx.tokens.current();
 			}
 
-			return res;
+			return CommandResult.success(res);
 		}
 
-		return expression;
+		return resOuter;
 	}
 
 }
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/GrammarParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/GrammarParseBlock.java
index 2432d6e..3f79665 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/GrammarParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/GrammarParseBlock.java
@@ -4,6 +4,8 @@ import java.util.function.Function;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.PrattParser;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.pratt.tokens.TokenStream;
 import bjc.data.Tree;
@@ -15,23 +17,17 @@ import bjc.utils.parserutils.ParserException;
  *
  * @author bjculkin
  *
- * @param <K>
- *        The key type of the outer tokens.
+ * @param <K>  The key type of the outer tokens.
  *
- * @param <V>
- *        The value type of the outer tokens.
+ * @param <V>  The value type of the outer tokens.
  *
- * @param <C>
- *        The state type of the outer parser.
+ * @param <C>  The state type of the outer parser.
  *
- * @param <K2>
- *        The key type of the inner tokens.
+ * @param <K2> The key type of the inner tokens.
  *
- * @param <V2>
- *        The value type of the inner tokens.
+ * @param <V2> The value type of the inner tokens.
  *
- * @param <C2>
- *        The state type of the outer parser.
+ * @param <C2> The state type of the outer parser.
  */
 public class GrammarParseBlock<K, V, C, K2, V2, C2> implements ParseBlock<K, V, C> {
 	private final PrattParser<K2, V2, C2> innr;
@@ -46,12 +42,14 @@ public class GrammarParseBlock<K, V, C, K2, V2, C2> implements ParseBlock<K, V,
 	/**
 	 * Create a new grammar parser block.
 	 *
-	 * @param inner The inner grammar to parse.
-	 * @param precedence The precedence of the expression to parse.
-	 * @param isStatement Is the expression being parsed in statement context?
-	 * @param tokenTransform Function to transform to the new token type.
-	 * @param stateTransform Function to toggle between state types.
-	 * @param expressionTransform Function to transform  back to the normal token type.
+	 * @param inner               The inner grammar to parse.
+	 * @param precedence          The precedence of the expression to parse.
+	 * @param isStatement         Is the expression being parsed in statement
+	 *                            context?
+	 * @param tokenTransform      Function to transform to the new token type.
+	 * @param stateTransform      Function to toggle between state types.
+	 * @param expressionTransform Function to transform back to the normal token
+	 *                            type.
 	 */
 	public GrammarParseBlock(final PrattParser<K2, V2, C2> inner, final int precedence, final boolean isStatement,
 			final Function<TokenStream<K, V>, TokenStream<K2, V2>> tokenTransform,
@@ -66,16 +64,27 @@ public class GrammarParseBlock<K, V, C, K2, V2, C2> implements ParseBlock<K, V,
 	}
 
 	@Override
-	public Tree<Token<K, V>> parse(final ParserContext<K, V, C> ctx) throws ParserException {
+	public CommandResult<K, V> parse(final ParserContext<K, V, C> ctx) throws ParserException {
 		final C2 newState = stteTransform.to(ctx.state);
 
 		final TokenStream<K2, V2> newTokens = tkenTransform.apply(ctx.tokens);
 
-		final Tree<Token<K2, V2>> expression = innr.parseExpression(prcedence, newTokens, newState,
-				isStatemnt);
+		final CommandResult<K2, V2> res = innr.parseExpression(prcedence, newTokens, newState, isStatemnt);
+		switch (res.status) {
+		case SUCCESS:
+			break;
+		case FAIL:
+			return CommandResult.fail();
+		case BACKTRACK:
+			return CommandResult.backtrack();
+		default:
+			throw new IllegalStateException("Unhandled status " + res.status);
+		}
+
+		Tree<Token<K2, V2>> expression = res.success();
 
 		ctx.state = stteTransform.from(newState);
 
-		return xpressionTransform.apply(expression);
+		return CommandResult.success(xpressionTransform.apply(expression));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/ParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/ParseBlock.java
index 81ba508..fede096 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/ParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/ParseBlock.java
@@ -1,6 +1,7 @@
 package bjc.pratt.blocks;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.utils.parserutils.ParserException;
@@ -34,6 +35,5 @@ public interface ParseBlock<K, V, C> {
 	 *         If something goes wrong during parsing, or the block fails
 	 *         validation.
 	 */
-	Tree<Token<K, V>> parse(ParserContext<K, V, C> ctx) throws ParserException;
-
+	CommandResult<K, V> parse(ParserContext<K, V, C> ctx) throws ParserException;
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/RepeatingParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/RepeatingParseBlock.java
index 4c21358..722e395 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/RepeatingParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/RepeatingParseBlock.java
@@ -3,6 +3,8 @@ package bjc.pratt.blocks;
 import java.util.function.UnaryOperator;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -72,13 +74,15 @@ public class RepeatingParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> parse(final ParserContext<K, V, C> ctx) throws ParserException {
+	public CommandResult<K, V> parse(final ParserContext<K, V, C> ctx) throws ParserException {
 		final Tree<Token<K, V>> ret = new SimpleTree<>(mark);
 
 		Token<K, V> tok = ctx.tokens.current();
 
 		while(!tok.getKey().equals(term)) {
-			final Tree<Token<K, V>> kid = innerBlock.parse(ctx);
+			final CommandResult<K,V> resKid = innerBlock.parse(ctx);
+			if (resKid.status != Status.SUCCESS) return resKid;
+			Tree<Token<K, V>> kid = resKid.success();
 			ret.addChild(kid);
 
 			tok = ctx.tokens.current();
@@ -90,7 +94,7 @@ public class RepeatingParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 			}
 		}
 
-		return ret;
+		return CommandResult.success(ret);
 	}
 
 }
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/SimpleParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/SimpleParseBlock.java
index 83e1d91..b674815 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/SimpleParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/SimpleParseBlock.java
@@ -3,6 +3,8 @@ package bjc.pratt.blocks;
 import java.util.function.Predicate;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.utils.parserutils.ParserException;
@@ -49,15 +51,18 @@ public class SimpleParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> parse(final ParserContext<K, V, C> ctx) throws ParserException {
-		final Tree<Token<K, V>> res = ctx.parse.parseExpression(pow, ctx.tokens, ctx.state, false);
-
+	public CommandResult<K, V> parse(final ParserContext<K, V, C> ctx) throws ParserException {
+		final CommandResult<K,V> resBlock = ctx.parse.parseExpression(pow, ctx.tokens, ctx.state, false);
+		if (resBlock.status != Status.SUCCESS) return resBlock;
+		
+		Tree<Token<K, V>> res = resBlock.success();
 		if(term != null) {
 			ctx.tokens.expect(term);
 		}
 
-		if(validatr == null || validatr.test(res)) return res;
+		if(validatr == null || validatr.test(res)) return CommandResult.success(res);
 
+		// TODO: Figure out the right way to handle error context w/ CommandResult
 		throw new ParserException("Block failed validation");
 	}
 
diff --git a/JPratt/src/main/java/bjc/pratt/blocks/TriggeredParseBlock.java b/JPratt/src/main/java/bjc/pratt/blocks/TriggeredParseBlock.java
index bfe5ab3..df17595 100644
--- a/JPratt/src/main/java/bjc/pratt/blocks/TriggeredParseBlock.java
+++ b/JPratt/src/main/java/bjc/pratt/blocks/TriggeredParseBlock.java
@@ -3,6 +3,8 @@ package bjc.pratt.blocks;
 import java.util.function.UnaryOperator;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.utils.parserutils.ParserException;
@@ -45,13 +47,15 @@ public class TriggeredParseBlock<K, V, C> implements ParseBlock<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> parse(final ParserContext<K, V, C> ctx) throws ParserException {
+	public CommandResult<K, V> parse(final ParserContext<K, V, C> ctx) throws ParserException {
 		final C newState = onEntr.apply(ctx.state);
 
 		final ParserContext<K, V, C> newCtx = new ParserContext<>(ctx.tokens, ctx.parse, newState);
 
-		final Tree<Token<K, V>> res = sourc.parse(newCtx);
+		final CommandResult<K,V> res = sourc.parse(newCtx);
 
+		if (res.status != Status.SUCCESS) return res;
+		
 		ctx.state = onExt.apply(newState);
 
 		return res;
diff --git a/JPratt/src/main/java/bjc/pratt/commands/AbstractInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/AbstractInitialCommand.java
index 50e884b..48f9ba7 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/AbstractInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/AbstractInitialCommand.java
@@ -21,7 +21,7 @@ import bjc.utils.parserutils.ParserException;
  */
 public abstract class AbstractInitialCommand<K, V, C> implements InitialCommand<K, V, C> {
 	@Override
-	public Tree<Token<K, V>> denote(final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) throws ParserException {
 		return intNullDenotation(operator, ctx);
 	}
@@ -39,7 +39,7 @@ public abstract class AbstractInitialCommand<K, V, C> implements InitialCommand<
 	 * @throws ParserException
 	 *                         If something went wrong while parsing.
 	 */
-	protected abstract Tree<Token<K, V>> intNullDenotation(Token<K, V> operator,
+	protected abstract CommandResult<K, V> intNullDenotation(Token<K, V> operator,
 			ParserContext<K, V, C> ctx) throws ParserException;
 
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/BinaryCommand.java b/JPratt/src/main/java/bjc/pratt/commands/BinaryCommand.java
index 069de78..7a65052 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/BinaryCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/BinaryCommand.java
@@ -1,6 +1,7 @@
 package bjc.pratt.commands;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -39,12 +40,14 @@ public abstract class BinaryCommand<K, V, C> extends BinaryPostCommand<K, V, C>
 	protected abstract int rightBinding();
 
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand,
 			final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		final Tree<Token<K, V>> opr
+		final CommandResult<K,V> opr
 				= ctx.parse.parseExpression(rightBinding(), ctx.tokens, ctx.state, false);
+		
+		if (opr.status != Status.SUCCESS) return opr;
 
-		return new SimpleTree<>(operator, operand, opr);
+		return CommandResult.success(new SimpleTree<>(operator, operand, opr.success()));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/CommandResult.java b/JPratt/src/main/java/bjc/pratt/commands/CommandResult.java
new file mode 100644
index 0000000..d27400f
--- /dev/null
+++ b/JPratt/src/main/java/bjc/pratt/commands/CommandResult.java
@@ -0,0 +1,97 @@
+package bjc.pratt.commands;
+
+import bjc.data.Tree;
+import bjc.pratt.tokens.Token;
+
+/**
+ * Represents the result of executing a command.
+ * 
+ * @author bjcul
+ *
+ * @param <K> The key type of the tokens
+ * @param <V> The value type of the tokens
+ */
+public class CommandResult<K, V> {
+	/**
+	 * Represents the status of a command execution
+	 * 
+	 * @author bjcul
+	 *
+	 */
+	public static enum Status {
+		/**
+		 * The command successfully parsed.
+		 */
+		SUCCESS,
+		/**
+		 * The command failed, in a non-recoverable way
+		 */
+		FAIL,
+		/**
+		 * The command failed. Attempt recovery via backtracking
+		 */
+		BACKTRACK
+	}
+
+	/**
+	 * The status of this command.
+	 */
+	public final Status status;
+	
+	private Tree<Token<K, V>> success;
+	
+	private CommandResult(Status status) {
+		this.status = status;
+	}
+
+	/**
+	 * Get the success value of this command, or null if it failed.
+	 * 
+	 * @return The success value of the command
+	 */
+	public Tree<Token<K, V>> success() {
+		return success;
+	}
+
+	/**
+	 * Create a success result
+	 * 
+	 * @param <K2> The key type of the token
+	 * @param <V2> The value type of the token
+	 * 
+	 * @param succ The tree produced by the command
+	 * 
+	 * @return A command result representing a success
+	 */
+	public static <K2, V2> CommandResult<K2, V2> success(Tree<Token<K2, V2>> succ) {
+		CommandResult<K2, V2> result = new CommandResult<>(Status.SUCCESS);
+		result.success = succ;
+		return result;
+	}
+
+	/**
+	 * Create a non-backtracking failure result.
+	 * 
+	 * @param <K2> The key type of the token
+	 * @param <V2> The value type of the token
+	 * 
+	 * @return A command result representing a non-backtracking fail
+	 */
+	public static <K2, V2> CommandResult<K2, V2> fail() {
+		CommandResult<K2, V2> result = new CommandResult<>(Status.FAIL);
+		return result;
+	}
+
+	/**
+	 * Create a backtracking failure result.
+	 * 
+	 * @param <K2> The key type of the token
+	 * @param <V2> The value type of the token
+	 * 
+	 * @return A command result representing a backtracking fail
+	 */
+	public static <K2, V2> CommandResult<K2, V2> backtrack() {
+		CommandResult<K2, V2> result = new CommandResult<>(Status.BACKTRACK);
+		return result;
+	}
+}
diff --git a/JPratt/src/main/java/bjc/pratt/commands/InitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/InitialCommand.java
index 3a2a8ff..ba544b4 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/InitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/InitialCommand.java
@@ -31,10 +31,10 @@ public interface InitialCommand<K, V, C> {
 	 * @param ctx
 	 *        The context for the command.
 	 *
-	 * @return The tree for this command.
+	 * @return The result of executing the command.
 	 *
 	 * @throws ParserException
 	 *         If something goes wrong during parsing.
 	 */
-	Tree<Token<K, V>> denote(Token<K, V> operator, ParserContext<K, V, C> ctx) throws ParserException;
+	CommandResult<K, V> denote(Token<K, V> operator, ParserContext<K, V, C> ctx) throws ParserException;
 }
diff --git a/JPratt/src/main/java/bjc/pratt/commands/NonInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/NonInitialCommand.java
index 12eecb6..02826a9 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/NonInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/NonInitialCommand.java
@@ -32,12 +32,12 @@ public abstract class NonInitialCommand<K, V, C> {
 	 * @param ctx
 	 *        The state needed for commands.
 	 *
-	 * @return The tree this command forms.
+	 * @return The result of executing the command.
 	 *
 	 * @throws ParserException
 	 *         If something went wrong during parsing.
 	 */
-	public abstract Tree<Token<K, V>> denote(Tree<Token<K, V>> operand, Token<K, V> operator,
+	public abstract CommandResult<K, V> denote(Tree<Token<K, V>> operand, Token<K, V> operator,
 			ParserContext<K, V, C> ctx) throws ParserException;
 
 	/**
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/BlockInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/BlockInitialCommand.java
index db11484..1d54996 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/BlockInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/BlockInitialCommand.java
@@ -3,6 +3,7 @@ package bjc.pratt.commands.impls;
 import bjc.pratt.ParserContext;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.utils.parserutils.ParserException;
@@ -35,7 +36,7 @@ public class BlockInitialCommand<K, V, C> extends AbstractInitialCommand<K, V, C
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
 		return blck.parse(ctx);
 	}
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/ChainCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/ChainCommand.java
index ed7a088..7311eb9 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/ChainCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/ChainCommand.java
@@ -4,6 +4,8 @@ import java.util.Set;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.commands.BinaryPostCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -48,10 +50,12 @@ public class ChainCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) throws ParserException {
-		final Tree<Token<K, V>> tree = ctx.parse.parseExpression(1 + leftBinding(), ctx.tokens, ctx.state,
+		CommandResult<K, V> resOuter = ctx.parse.parseExpression(1 + leftBinding(), ctx.tokens, ctx.state,
 				false);
+		if (resOuter.status != Status.SUCCESS) return resOuter;
+		final Tree<Token<K, V>> tree = resOuter.success();
 
 		final Tree<Token<K, V>> res = new SimpleTree<>(operator, operand, tree);
 
@@ -59,13 +63,16 @@ public class ChainCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 			final Token<K, V> tok = ctx.tokens.current();
 			ctx.tokens.next();
 
-			final Tree<Token<K, V>> other = denote(tree, tok,
+			CommandResult<K, V> resOther = denote(tree, tok,
 					new ParserContext<>(ctx.tokens, ctx.parse, ctx.state));
+			if (resOther.status != Status.SUCCESS) return resOther;
+			
+			final Tree<Token<K, V>> other = resOther.success();
 
-			return new SimpleTree<>(chain, res, other);
+			return CommandResult.success(new SimpleTree<>(chain, res, other));
 		}
 
-		return res;
+		return CommandResult.success(res);
 	}
 
 	@Override
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/ConstantCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/ConstantCommand.java
index 409adbb..657743c 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/ConstantCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/ConstantCommand.java
@@ -1,6 +1,7 @@
 package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -34,8 +35,8 @@ public class ConstantCommand<K, V, C> implements InitialCommand<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	public CommandResult<K, V> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		return val;
+		return CommandResult.success(val);
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultInitialCommand.java
index 296cb1c..2dfc576 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultInitialCommand.java
@@ -1,6 +1,7 @@
 package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -22,7 +23,7 @@ import bjc.utils.parserutils.ParserException;
  */
 public class DefaultInitialCommand<K, V, C> implements InitialCommand<K, V, C> {
 	@Override
-	public Tree<Token<K, V>> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	public CommandResult<K, V> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
 		throw new ParserException("Unexpected token " + operator);
 	}
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultNonInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultNonInitialCommand.java
index 797473a..2ae9fb7 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultNonInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/DefaultNonInitialCommand.java
@@ -1,6 +1,7 @@
 package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.commands.NonInitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -21,7 +22,7 @@ import bjc.data.Tree;
  */
 public class DefaultNonInitialCommand<K, V, C> extends NonInitialCommand<K, V, C> {
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) {
 		throw new UnsupportedOperationException("Default command has no left denotation");
 	}
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/DenestingCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/DenestingCommand.java
index fbcd35c..4935c81 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/DenestingCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/DenestingCommand.java
@@ -2,6 +2,8 @@ package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -39,8 +41,10 @@ public class DenestingCommand<K, V, C> extends AbstractInitialCommand<K, V, C> {
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		return wrapped.denote(operator, ctx).getChild(0);
+		CommandResult<K, V> res = wrapped.denote(operator, ctx);
+		if (res.status != Status.SUCCESS) return res;
+		return CommandResult.success(res.success().getChild(0));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/GroupingCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/GroupingCommand.java
index 1515359..44aa2c1 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/GroupingCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/GroupingCommand.java
@@ -3,6 +3,7 @@ package bjc.pratt.commands.impls;
 import bjc.pratt.ParserContext;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -43,10 +44,10 @@ public class GroupingCommand<K, V, C> extends AbstractInitialCommand<K, V, C> {
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		final Tree<Token<K, V>> opr = innerBlock.parse(ctx);
-
-		return new SimpleTree<>(mark, opr);
+		final CommandResult<K,V> resOpr = innerBlock.parse(ctx);
+		Tree<Token<K, V>> opr = resOpr.success();
+		return CommandResult.success(new SimpleTree<>(mark, opr));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/InitialCommands.java b/JPratt/src/main/java/bjc/pratt/commands/impls/InitialCommands.java
index 0cfec29..9801788 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/InitialCommands.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/InitialCommands.java
@@ -9,6 +9,7 @@ import java.util.function.UnaryOperator;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
+import bjc.utils.parserutils.ParserException;
 import bjc.data.SimpleTree;
 import bjc.data.Tree;
 
@@ -210,16 +211,6 @@ public class InitialCommands {
 	 * @return A command that implements a panfix operator
 	 */
 	public static <K, V, C> InitialCommand<K, V, C> panfix(final int precedence, final K term, final Token<K, V> marker) {
-		return (operator, ctx) -> {
-			Tree<Token<K,V>> leftSide = ctx.parse.parseExpression(precedence + 1, ctx.tokens, ctx.state, false);
-			ctx.tokens.expect(term);
-			ctx.tokens.next();
-			
-			Tree<Token<K,V>> rightSide = ctx.parse.parseExpression(precedence + 1, ctx.tokens, ctx.state, false);
-			ctx.tokens.expect(term);
-			ctx.tokens.next();
-			
-			return new SimpleTree<>(marker, leftSide, rightSide);
-		};
+		return new PanfixCommand<K, V, C>(marker, term, precedence);
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/LeafCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/LeafCommand.java
index 1223641..c702323 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/LeafCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/LeafCommand.java
@@ -1,6 +1,7 @@
 package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -23,8 +24,8 @@ import bjc.utils.parserutils.ParserException;
  */
 public class LeafCommand<K, V, C> implements InitialCommand<K, V, C> {
 	@Override
-	public Tree<Token<K, V>> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	public CommandResult<K, V> denote(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		return new SimpleTree<>(operator);
+		return CommandResult.success(new SimpleTree<>(operator));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/PanfixCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/PanfixCommand.java
new file mode 100644
index 0000000..06e69c1
--- /dev/null
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/PanfixCommand.java
@@ -0,0 +1,39 @@
+package bjc.pratt.commands.impls;
+
+import bjc.data.SimpleTree;
+import bjc.data.Tree;
+import bjc.pratt.ParserContext;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.InitialCommand;
+import bjc.pratt.commands.CommandResult.Status;
+import bjc.pratt.tokens.Token;
+import bjc.utils.parserutils.ParserException;
+
+public final class PanfixCommand<K, V, C> implements InitialCommand<K, V, C> {
+	private final Token<K, V> marker;
+	private final K term;
+	private final int precedence;
+
+	public PanfixCommand(Token<K, V> marker, K term, int precedence) {
+		this.marker = marker;
+		this.term = term;
+		this.precedence = precedence;
+	}
+
+	@Override
+	public CommandResult<K, V> denote(Token<K, V> operator, ParserContext<K, V, C> ctx) throws ParserException {
+		CommandResult<K,V> resLeftSide = ctx.parse.parseExpression(precedence + 1, ctx.tokens, ctx.state, false);
+		if (resLeftSide.status != Status.SUCCESS) return resLeftSide;
+		Tree<Token<K, V>> leftSide = resLeftSide.success();
+		ctx.tokens.expect(term);
+		ctx.tokens.next();
+		
+		CommandResult<K, V> resRightSide = ctx.parse.parseExpression(precedence + 1, ctx.tokens, ctx.state, false);
+		if (resLeftSide.status != Status.SUCCESS) return resRightSide;
+		Tree<Token<K,V>> rightSide = resRightSide.success();
+		ctx.tokens.expect(term);
+		ctx.tokens.next();
+		
+		return CommandResult.success(new SimpleTree<>(marker, leftSide, rightSide));
+	}
+}
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/PostCircumfixCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/PostCircumfixCommand.java
index 78ac1ef..ec2c8fb 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/PostCircumfixCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/PostCircumfixCommand.java
@@ -3,6 +3,8 @@ package bjc.pratt.commands.impls;
 import bjc.pratt.ParserContext;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.BinaryPostCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -50,10 +52,11 @@ public class PostCircumfixCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) throws ParserException {
-		final Tree<Token<K, V>> inside = innerBlock.parse(ctx);
-
-		return new SimpleTree<>(mark, operand, inside);
+		final CommandResult<K,V> insideRes = innerBlock.parse(ctx);
+		if (insideRes.status != Status.SUCCESS) return insideRes;
+		Tree<Token<K, V>> inside = insideRes.success();
+		return CommandResult.success(new SimpleTree<>(mark, operand, inside));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/PostfixCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/PostfixCommand.java
index da587c1..ff370d0 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/PostfixCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/PostfixCommand.java
@@ -2,6 +2,7 @@ package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.commands.BinaryPostCommand;
+import bjc.pratt.commands.CommandResult;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -33,8 +34,8 @@ public class PostfixCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) throws ParserException {
-		return new SimpleTree<>(operator, operand);
+		return CommandResult.success(new SimpleTree<>(operator, operand));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/PreTernaryCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/PreTernaryCommand.java
index e315804..5d5cbe1 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/PreTernaryCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/PreTernaryCommand.java
@@ -3,6 +3,8 @@ package bjc.pratt.commands.impls;
 import bjc.pratt.ParserContext;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -63,14 +65,19 @@ public class PreTernaryCommand<K, V, C> extends AbstractInitialCommand<K, V, C>
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		final Tree<Token<K, V>> cond = condBlock.parse(ctx);
+		final CommandResult<K,V> condRes = condBlock.parse(ctx);
+		if (condRes.status != Status.SUCCESS) return condRes;
+		Tree<Token<K, V>> cond = condRes.success();
+		
+		final CommandResult<K,V> op1Res = opblock1.parse(ctx);
+		if (op1Res.status != Status.SUCCESS) return op1Res;
+		Tree<Token<K, V>> op1 = op1Res.success();
 
-		final Tree<Token<K, V>> op1 = opblock1.parse(ctx);
-
-		final Tree<Token<K, V>> op2 = opblock2.parse(ctx);
-
-		return new SimpleTree<>(trm, cond, op1, op2);
+		final CommandResult<K,V> op2Res = opblock2.parse(ctx);
+		if (op2Res.status != Status.SUCCESS) return op2Res;
+		Tree<Token<K, V>> op2 = op2Res.success();
+		return CommandResult.success(new SimpleTree<>(trm, cond, op1, op2));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/TernaryCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/TernaryCommand.java
index 174f6fb..786dfec 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/TernaryCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/TernaryCommand.java
@@ -3,6 +3,8 @@ package bjc.pratt.commands.impls;
 import bjc.pratt.ParserContext;
 import bjc.pratt.blocks.ParseBlock;
 import bjc.pratt.commands.BinaryPostCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -13,14 +15,11 @@ import bjc.utils.parserutils.ParserException;
  *
  * @author bjculkin
  *
- * @param <K>
- *        The key type of the tokens.
+ * @param <K> The key type of the tokens.
  *
- * @param <V>
- *        The value type of the tokens.
+ * @param <V> The value type of the tokens.
  *
- * @param <C>
- *        The state type of the parser.
+ * @param <C> The state type of the parser.
  */
 public class TernaryCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	private final ParseBlock<K, V, C> innerBlck;
@@ -32,25 +31,22 @@ public class TernaryCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	/**
 	 * Create a new ternary command.
 	 *
-	 * @param precedence
-	 *        The precedence of this operator.
+	 * @param precedence The precedence of this operator.
 	 *
-	 * @param innerBlock
-	 *        The representation of the inner block of the expression.
+	 * @param innerBlock The representation of the inner block of the expression.
 	 *
-	 * @param marker
-	 *        The token to use as the root of the AST node.
+	 * @param marker     The token to use as the root of the AST node.
 	 *
-	 * @param isNonassoc
-	 *        Whether or not the conditional is associative.
+	 * @param isNonassoc Whether or not the conditional is associative.
 	 */
 	public TernaryCommand(final int precedence, final ParseBlock<K, V, C> innerBlock, final Token<K, V> marker,
 			final boolean isNonassoc) {
 		super(precedence);
 
-		if(innerBlock == null)
+		if (innerBlock == null)
 			throw new NullPointerException("Inner block must not be null");
-		else if(marker == null) throw new NullPointerException("Marker must not be null");
+		else if (marker == null)
+			throw new NullPointerException("Marker must not be null");
 
 		innerBlck = innerBlock;
 		mark = marker;
@@ -58,19 +54,22 @@ public class TernaryCommand<K, V, C> extends BinaryPostCommand<K, V, C> {
 	}
 
 	@Override
-	public Tree<Token<K, V>> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
+	public CommandResult<K, V> denote(final Tree<Token<K, V>> operand, final Token<K, V> operator,
 			final ParserContext<K, V, C> ctx) throws ParserException {
-		final Tree<Token<K, V>> inner = innerBlck.parse(ctx);
-
-		final Tree<Token<K, V>> outer = ctx.parse.parseExpression(1 + leftBinding(), ctx.tokens, ctx.state,
-				false);
-
-		return new SimpleTree<>(mark, inner, operand, outer);
+		final CommandResult<K, V> innerRes = innerBlck.parse(ctx);
+		if (innerRes.status != Status.SUCCESS) return innerRes;
+		Tree<Token<K, V>> inner = innerRes.success();
+		
+		final CommandResult<K,V> outerRes = ctx.parse.parseExpression(1 + leftBinding(), ctx.tokens, ctx.state, false);
+		if (outerRes.status != Status.SUCCESS) return outerRes;
+		Tree<Token<K, V>> outer = outerRes.success();
+		return CommandResult.success(new SimpleTree<>(mark, inner, operand, outer));
 	}
 
 	@Override
 	public int nextBinding() {
-		if(nonassoc) return leftBinding() - 1;
+		if (nonassoc)
+			return leftBinding() - 1;
 
 		return leftBinding();
 	}
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/TransformingInitialCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/TransformingInitialCommand.java
index d39ec90..36f881d 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/TransformingInitialCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/TransformingInitialCommand.java
@@ -4,6 +4,8 @@ import java.util.function.UnaryOperator;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.commands.InitialCommand;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
@@ -45,9 +47,12 @@ public class TransformingInitialCommand<K, V, C> extends AbstractInitialCommand<
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		return transfrm.apply(internl.denote(operator, ctx));
+		CommandResult<K,V> result = internl.denote(operator, ctx);
+		if (result.status != Status.SUCCESS) return result;
+		
+		return CommandResult.success(transfrm.apply(result.success()));
 	}
 
 }
diff --git a/JPratt/src/main/java/bjc/pratt/commands/impls/UnaryCommand.java b/JPratt/src/main/java/bjc/pratt/commands/impls/UnaryCommand.java
index 2e7365b..657c004 100644
--- a/JPratt/src/main/java/bjc/pratt/commands/impls/UnaryCommand.java
+++ b/JPratt/src/main/java/bjc/pratt/commands/impls/UnaryCommand.java
@@ -2,6 +2,8 @@ package bjc.pratt.commands.impls;
 
 import bjc.pratt.ParserContext;
 import bjc.pratt.commands.AbstractInitialCommand;
+import bjc.pratt.commands.CommandResult;
+import bjc.pratt.commands.CommandResult.Status;
 import bjc.pratt.tokens.Token;
 import bjc.data.Tree;
 import bjc.data.SimpleTree;
@@ -37,10 +39,12 @@ public class UnaryCommand<K, V, C> extends AbstractInitialCommand<K, V, C> {
 	}
 
 	@Override
-	protected Tree<Token<K, V>> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
+	protected CommandResult<K, V> intNullDenotation(final Token<K, V> operator, final ParserContext<K, V, C> ctx)
 			throws ParserException {
-		final Tree<Token<K, V>> opr = ctx.parse.parseExpression(nullPwer, ctx.tokens, ctx.state, false);
+		CommandResult<K,V> result = ctx.parse.parseExpression(nullPwer, ctx.tokens, ctx.state, false);
+		if (result.status != Status.SUCCESS) return result;
+		final Tree<Token<K, V>> opr = result.success();
 
-		return new SimpleTree<>(operator, opr);
+		return CommandResult.success(new SimpleTree<>(operator, opr));
 	}
 }
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/tokens/ExpectionNotMet.java b/JPratt/src/main/java/bjc/pratt/tokens/ExpectionNotMet.java
new file mode 100644
index 0000000..2bd45e2
--- /dev/null
+++ b/JPratt/src/main/java/bjc/pratt/tokens/ExpectionNotMet.java
@@ -0,0 +1,23 @@
+package bjc.pratt.tokens;
+
+import bjc.utils.parserutils.ParserException;
+
+/**
+ * The exception thrown when an expectation fails.
+ *
+ * @author EVE
+ *
+ */
+public class ExpectionNotMet extends ParserException {
+	private static final long serialVersionUID = 4299299480127680805L;
+
+	/**
+	 * Create a new exception with the specified message.
+	 *
+	 * @param msg
+	 *        The message of the exception.
+	 */
+	public ExpectionNotMet(final String msg) {
+		super(msg);
+	}
+}
\ No newline at end of file
diff --git a/JPratt/src/main/java/bjc/pratt/tokens/StringTokenStream.java b/JPratt/src/main/java/bjc/pratt/tokens/StringTokenStream.java
index 044d19a..8c7fcec 100644
--- a/JPratt/src/main/java/bjc/pratt/tokens/StringTokenStream.java
+++ b/JPratt/src/main/java/bjc/pratt/tokens/StringTokenStream.java
@@ -4,6 +4,8 @@ import static bjc.pratt.tokens.StringToken.litToken;
 
 import java.util.Iterator;
 
+import bjc.data.MarkListIterator;
+
 /**
  * Simple implementation of token stream for strings.
  *
@@ -14,8 +16,8 @@ import java.util.Iterator;
  *
  */
 public class StringTokenStream extends TokenStream<String, String> {
-	private final Iterator<Token<String, String>> iter;
-
+	private final MarkListIterator<Token<String, String>> iter;
+	
 	private Token<String, String> curr;
 
 	/**
@@ -26,8 +28,7 @@ public class StringTokenStream extends TokenStream<String, String> {
 	 *
 	 */
 	public StringTokenStream(final Iterator<Token<String, String>> itr) {
-		iter = itr;
-
+		iter = new MarkListIterator<>(itr);
 	}
 
 	@Override
@@ -50,4 +51,29 @@ public class StringTokenStream extends TokenStream<String, String> {
 	public boolean hasNext() {
 		return iter.hasNext();
 	}
+	
+	@Override
+	public void mark() {
+		iter.mark();
+	}
+	
+	@Override
+	public void commit() {
+		iter.commit();
+		
+		if (!iter.hasMark()) {
+			// No marks outstanding; we can release the previous state
+			iter.reset();
+		}
+	}
+	
+	@Override
+	public void rollback() {
+		iter.rollback();
+	}
+	
+	@Override
+	public boolean hasMark() {
+ 		return iter.hasMark();
+	}
 }
diff --git a/JPratt/src/main/java/bjc/pratt/tokens/TokenStream.java b/JPratt/src/main/java/bjc/pratt/tokens/TokenStream.java
index 1c550de..dc09bcc 100644
--- a/JPratt/src/main/java/bjc/pratt/tokens/TokenStream.java
+++ b/JPratt/src/main/java/bjc/pratt/tokens/TokenStream.java
@@ -6,7 +6,6 @@ import java.util.Iterator;
 import java.util.Set;
 
 import bjc.utils.funcutils.StringUtils;
-import bjc.utils.parserutils.ParserException;
 
 /**
  * A stream of tokens.
@@ -20,26 +19,6 @@ import bjc.utils.parserutils.ParserException;
  *        The value type of the token.
  */
 public abstract class TokenStream<K, V> implements Iterator<Token<K, V>> {
-	/**
-	 * The exception thrown when an expectation fails.
-	 *
-	 * @author EVE
-	 *
-	 */
-	public static class ExpectationException extends ParserException {
-		private static final long serialVersionUID = 4299299480127680805L;
-
-		/**
-		 * Create a new exception with the specified message.
-		 *
-		 * @param msg
-		 *        The message of the exception.
-		 */
-		public ExpectationException(final String msg) {
-			super(msg);
-		}
-	}
-
 	/**
 	 * Get the current token.
 	 *
@@ -53,6 +32,28 @@ public abstract class TokenStream<K, V> implements Iterator<Token<K, V>> {
 	@Override
 	public abstract boolean hasNext();
 
+	/**
+	 * Place a mark in the current stream, which can be either returned to or abandoned later on.
+	 */
+	public abstract void mark();
+	
+	/**
+	 * Reset the stream to the state it was in when the last mark was taken.
+	 */
+	public abstract void rollback();
+	
+	/**
+	 * Check if the stream has at least one mark.
+	 * 
+	 * @return Whether or not at least one mark exists.
+	 */
+	public abstract boolean hasMark();
+	
+	/**
+	 * Remove the last mark placed into the stream. This prevents returning to it later on.
+	 */
+	public abstract void commit();
+	
 	/**
 	 * Utility method for checking that the next token is one of a specific
 	 * set of types, and then consuming it.
@@ -60,16 +61,16 @@ public abstract class TokenStream<K, V> implements Iterator<Token<K, V>> {
 	 * @param expectedKeys
 	 *        The expected values
 	 *
-	 * @throws ExpectationException
+	 * @throws ExpectionNotMet
 	 *         If the token is not one of the expected types.
 	 */
-	public void expect(final Set<K> expectedKeys) throws ExpectationException {
+	public void expect(final Set<K> expectedKeys) throws ExpectionNotMet {
 		final K curKey = current().getKey();
 
 		if(!expectedKeys.contains(curKey)) {
 			final String expectedList = StringUtils.toEnglishList(expectedKeys.toArray(), false);
 
-			throw new ExpectationException("One of '" + expectedList + "' was expected, not " + curKey);
+			throw new ExpectionNotMet("One of '" + expectedList + "' was expected, not " + curKey);
 		}
 
 		next();
@@ -82,12 +83,13 @@ public abstract class TokenStream<K, V> implements Iterator<Token<K, V>> {
 	 * @param expectedKeys
 	 *        The expected values
 	 *
-	 * @throws ExpectationException
+	 * @throws ExpectionNotMet
 	 *         If the token is not one of the expected types.
 	 */
 	@SafeVarargs
-	public final void expect(final K... expectedKeys) throws ExpectationException {
-		expect(new HashSet<>(Arrays.asList(expectedKeys)));
+	public final void expect(final K... expectedKeys) throws ExpectionNotMet {
+		HashSet<K> keys = new HashSet<>(Arrays.asList(expectedKeys));
+		expect(keys);
 	}
 
 	/**
-- 
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