From 023cc940331caffe6710691f16fcca92e6ad5bfa Mon Sep 17 00:00:00 2001
From: Ben Culkin <scorpress@gmail.com>
Date: Sat, 21 Nov 2020 11:09:20 -0500
Subject: Add a functional interface for functions that throw an exception.

This allows you to be able to throw one type of checked exception from a
normal function

It also has some methods to allow you to attempt to recover from an
exception and hide the fact that it was thrown
---
 src/main/java/bjc/functypes/ThrowFunction.java | 88 ++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 src/main/java/bjc/functypes/ThrowFunction.java

(limited to 'src/main/java')

diff --git a/src/main/java/bjc/functypes/ThrowFunction.java b/src/main/java/bjc/functypes/ThrowFunction.java
new file mode 100644
index 0000000..b72d735
--- /dev/null
+++ b/src/main/java/bjc/functypes/ThrowFunction.java
@@ -0,0 +1,88 @@
+package bjc.functypes;
+
+import java.util.function.*;
+
+/**
+ * An instance of {@link Function} that can throw an exception.
+ * 
+ * @author Ben Culkin
+ *
+ * @param <InputType> The input to the function.
+ * @param <ReturnType> The output to the function.
+ * @param <ExType> The type of exception thrown.
+ */
+public interface ThrowFunction<InputType, ReturnType, ExType extends Throwable> {
+	/**
+	 * Does the possibly throwing computation embodied by this function.
+	 * 
+	 * @param arg The input to the function.
+	 * 
+	 * @return The result of the function.
+	 * 
+	 * @throws ExType If something went wrong with the function.
+	 */
+	public ReturnType apply(InputType arg) throws ExType;
+	
+	/**
+	 * Converts this into a {@link Function} by handling any thrown exceptions.
+	 * 
+	 * @param clasz The class of the handled exception. This needs to be provided
+	 *              because you can't catch a generic exception, and we want to
+	 *              make sure that we aren't catching any types of exception that
+	 *              we aren't supposed to.
+	 *              
+	 * @param handler The handler to use.
+	 * 
+	 * @return A function which will either return its proper value, or the result
+	 *         of invoking the handler.
+	 */
+	@SuppressWarnings("unchecked")
+	default Function<InputType, ReturnType> swallow(
+			Class<ExType> clasz, Function<ExType, ReturnType> handler) {
+		return (inp) -> {
+			try {
+				return this.apply(inp);
+			} catch (Throwable ex) {
+				if (clasz.isInstance(ex)) {
+					// Swallow this
+					return handler.apply((ExType) ex);
+				} else {
+					String msg = "Exception of incorrect type to be handled, only "
+											 + clasz.getName()
+											 + " are handled";
+					
+					throw new RuntimeException(msg, ex);
+				}
+			}
+		};
+	}
+	
+	/**
+	 * Convert this to a function which will attempt to recover from the thrown exception.
+	 * 
+	 * @param clasz The class of the exception. Needed for type-safety reasons.
+	 * @param rescue The function to use to convert an exception into a safe input value.
+	 * 
+	 * @return A function which attempts to recover from a exception.
+	 */
+	@SuppressWarnings("unchecked")
+	default Function<InputType, ReturnType> recover(
+			Class<ExType> clasz, BiFunction<InputType, ExType, InputType> rescue) {
+		return Fixpoints.fix((arg, self) -> {
+			try {
+				return this.apply(arg);
+			} catch (Throwable ex) {
+				if (clasz.isInstance(ex)) {
+					// Swallow this
+					return self.apply(rescue.apply(arg, (ExType) ex));
+				} else {
+					String msg = "Exception of incorrect type to be handled, only "
+											 + clasz.getName()
+											 + " are handled";
+					
+					throw new RuntimeException(msg, ex);
+				}
+			}
+		});
+	}
+}
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