Allow redirect modifiers to throw exceptions

src/main/java/com/mojang/brigadier/CommandDispatcher.java

@@ -87,7 +87,7 @@ public class CommandDispatcher<S> {
             final CommandContextBuilder<S> child = context.getChild();
             if (child != null) {
                 if (!child.getNodes().isEmpty()) {
-                    final Function<CommandContext<S>, Collection<S>> modifier = Iterators.getLast(context.getNodes().keySet().iterator()).getRedirectModifier();
+                    final RedirectModifier<S> modifier = Iterators.getLast(context.getNodes().keySet().iterator()).getRedirectModifier();
                     for (final S source : modifier.apply(context.build())) {
                         contexts.add(child.copy().withSource(source));
                     }

src/main/java/com/mojang/brigadier/RedirectModifier.java

@@ -0,0 +1,11 @@
+package com.mojang.brigadier;
+
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.exceptions.CommandSyntaxException;
+
+import java.util.Collection;
+
+@FunctionalInterface
+public interface RedirectModifier<S> {
+    Collection<S> apply(CommandContext<S> context) throws CommandSyntaxException;
+}

src/main/java/com/mojang/brigadier/builder/ArgumentBuilder.java

@@ -1,6 +1,7 @@
 package com.mojang.brigadier.builder;
 
 import com.mojang.brigadier.Command;
+import com.mojang.brigadier.RedirectModifier;
 import com.mojang.brigadier.context.CommandContext;
 import com.mojang.brigadier.tree.CommandNode;
 import com.mojang.brigadier.tree.RootCommandNode;
@@ -15,7 +16,7 @@ public abstract class ArgumentBuilder<S, T extends ArgumentBuilder<S, T>> {
     private Command<S> command;
     private Predicate<S> requirement = s -> true;
     private CommandNode<S> target;
-    private Function<CommandContext<S>, Collection<S>> modifier = s -> Collections.singleton(s.getSource());
+    private RedirectModifier<S> modifier = s -> Collections.singleton(s.getSource());
 
     protected abstract T getThis();
 
@@ -53,7 +54,7 @@ public abstract class ArgumentBuilder<S, T extends ArgumentBuilder<S, T>> {
         return redirect(target, modifier);
     }
 
-    public T redirect(final CommandNode<S> target, final Function<CommandContext<S>, Collection<S>> modifier) {
+    public T redirect(final CommandNode<S> target, final RedirectModifier<S> modifier) {
         if (!arguments.getChildren().isEmpty()) {
             throw new IllegalStateException("Cannot redirect a node with children");
         }
@@ -66,7 +67,7 @@ public abstract class ArgumentBuilder<S, T extends ArgumentBuilder<S, T>> {
         return target;
     }
 
-    public Function<CommandContext<S>, Collection<S>> getRedirectModifier() {
+    public RedirectModifier<S> getRedirectModifier() {
         return modifier;
     }
 

src/main/java/com/mojang/brigadier/tree/ArgumentCommandNode.java

@@ -1,6 +1,7 @@
 package com.mojang.brigadier.tree;
 
 import com.mojang.brigadier.Command;
+import com.mojang.brigadier.RedirectModifier;
 import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.arguments.ArgumentType;
 import com.mojang.brigadier.builder.RequiredArgumentBuilder;
@@ -21,7 +22,7 @@ public class ArgumentCommandNode<S, T> extends CommandNode<S> {
     private final String name;
     private final ArgumentType<T> type;
 
-    public ArgumentCommandNode(final String name, final ArgumentType<T> type, final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final Function<CommandContext<S>, Collection<S>> modifier) {
+    public ArgumentCommandNode(final String name, final ArgumentType<T> type, final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final RedirectModifier<S> modifier) {
         super(command, requirement, redirect, modifier);
         this.name = name;
         this.type = type;

src/main/java/com/mojang/brigadier/tree/CommandNode.java

@@ -3,6 +3,7 @@ package com.mojang.brigadier.tree;
 import com.google.common.collect.ComparisonChain;
 import com.google.common.collect.Maps;
 import com.mojang.brigadier.Command;
+import com.mojang.brigadier.RedirectModifier;
 import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.builder.ArgumentBuilder;
 import com.mojang.brigadier.context.CommandContext;
@@ -21,10 +22,10 @@ public abstract class CommandNode<S> implements Comparable<CommandNode<S>> {
     private Map<Object, CommandNode<S>> children = Maps.newLinkedHashMap();
     private final Predicate<S> requirement;
     private final CommandNode<S> redirect;
-    private final Function<CommandContext<S>, Collection<S>> modifier;
+    private final RedirectModifier<S> modifier;
     private Command<S> command;
 
-    protected CommandNode(final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final Function<CommandContext<S>, Collection<S>> modifier) {
+    protected CommandNode(final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final RedirectModifier<S> modifier) {
         this.command = command;
         this.requirement = requirement;
         this.redirect = redirect;
@@ -43,7 +44,7 @@ public abstract class CommandNode<S> implements Comparable<CommandNode<S>> {
         return redirect;
     }
 
-    public Function<CommandContext<S>, Collection<S>> getRedirectModifier() {
+    public RedirectModifier<S> getRedirectModifier() {
         return modifier;
     }
 

src/main/java/com/mojang/brigadier/tree/LiteralCommandNode.java

@@ -1,6 +1,7 @@
 package com.mojang.brigadier.tree;
 
 import com.mojang.brigadier.Command;
+import com.mojang.brigadier.RedirectModifier;
 import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.builder.LiteralArgumentBuilder;
 import com.mojang.brigadier.context.CommandContext;
@@ -18,7 +19,7 @@ public class LiteralCommandNode<S> extends CommandNode<S> {
 
     private final String literal;
 
-    public LiteralCommandNode(final String literal, final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final Function<CommandContext<S>, Collection<S>> modifier) {
+    public LiteralCommandNode(final String literal, final Command<S> command, final Predicate<S> requirement, final CommandNode<S> redirect, final RedirectModifier<S> modifier) {
         super(command, requirement, redirect, modifier);
         this.literal = literal;
     }

src/test/java/com/mojang/brigadier/CommandDispatcherTest.java

@@ -254,7 +254,7 @@ public class CommandDispatcherTest {
     @SuppressWarnings("unchecked")
     @Test
     public void testExecuteRedirected() throws Exception {
-        final Function<CommandContext<Object>, Collection<Object>> modifier = mock(Function.class);
+        final RedirectModifier<Object> modifier = mock(RedirectModifier.class);
         final Object source1 = new Object();
         final Object source2 = new Object();