Variable-arity returns and function contracts.

src/main/java/ch/njol/skript/classes/data/DefaultFunctions.java

@@ -19,6 +19,7 @@
 package ch.njol.skript.classes.data;
 
 import ch.njol.skript.expressions.base.EventValueExpression;
+import ch.njol.skript.lang.Expression;
 import ch.njol.skript.lang.function.FunctionEvent;
 import ch.njol.skript.lang.function.Functions;
 import ch.njol.skript.lang.function.JavaFunction;
@@ -40,6 +41,7 @@
 import org.bukkit.entity.Player;
 import org.bukkit.util.Vector;
 import org.eclipse.jdt.annotation.Nullable;
+import ch.njol.skript.util.Contract;
 
 import java.math.BigDecimal;
 import java.math.RoundingMode;
@@ -309,11 +311,22 @@ public Number[] executeSimple(Object[][] params) {
 			.examples("min(1) = 1", "min(1, 2, 3, 4) = 1", "min({some list variable::*})")
 			.since("2.2"));
 
-		Functions.registerFunction(new SimpleJavaFunction<Number>("clamp", new Parameter[]{
-			new Parameter<>("values", DefaultClasses.NUMBER, false, null),
-			new Parameter<>("min", DefaultClasses.NUMBER, true, null),
-			new Parameter<>("max", DefaultClasses.NUMBER, true, null)
-		}, DefaultClasses.NUMBER, false) {
+		Functions.registerFunction(new SimpleJavaFunction<Number>("clamp", new Parameter[] {
+					 new Parameter<>("values", DefaultClasses.NUMBER, false, null),
+					 new Parameter<>("min", DefaultClasses.NUMBER, true, null),
+					 new Parameter<>("max", DefaultClasses.NUMBER, true, null)
+				 }, DefaultClasses.NUMBER, false, new Contract() {
+
+					 @Override
+					 public boolean isSingle(Expression<?>... arguments) {
+						 return arguments[0].isSingle();
+					 }
+
+					 @Override
+					 public Class<?> getReturnType(Expression<?>... arguments) {
+						 return Number.class;
+					 }
+				 }) {
 			@Override
 			public @Nullable Number[] executeSimple(Object[][] params) {
 				Number[] values = (Number[]) params[0];

src/main/java/ch/njol/skript/effects/EffChange.java

@@ -248,7 +248,7 @@ else if (mode == ChangeMode.SET)
 			assert x != null;
 			changer = ch = v;
 
-			if (!ch.isSingle() && single) {
+			if (!ch.canBeSingle() && single) {
 				if (mode == ChangeMode.SET)
 					Skript.error(changed + " can only be set to one " + Classes.getSuperClassInfo(x).getName() + ", not more", ErrorQuality.SEMANTIC_ERROR);
 				else

src/main/java/ch/njol/skript/lang/Expression.java

@@ -120,6 +120,20 @@ default Optional<T> getOptionalSingle(Event event) {
 	 */
 	boolean isSingle();
 
+	/**
+	 * Whether there's a possibility this could return a single value.
+	 * Designed for expressions that may return more than one value, but are equally appropriate to use where
+	 * only singular values are accepted, in which case a single value (out of all available values) will be returned.
+	 * An example would be functions that return results based on their inputs.
+	 * Ideally, this will return {@link #isSingle()} based on its known inputs at initialisation, but for some syntax
+	 * this may not be known (or a syntax may be intentionally vague in its permissible returns).
+	 * @return Whether this can be used by single changers
+	 * @see #getSingle(Event)
+	 */
+	default boolean canBeSingle() {
+		return this.isSingle();
+	}
+
 	/**
 	 * Checks this expression against the given checker. This is the normal version of this method and the one which must be used for simple checks,
 	 * or as the innermost check of nested checks.

src/main/java/ch/njol/skript/lang/function/FunctionReference.java

@@ -33,6 +33,7 @@
 import ch.njol.util.StringUtils;
 import org.bukkit.event.Event;
 import org.eclipse.jdt.annotation.Nullable;
+import ch.njol.skript.util.Contract;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -41,7 +42,7 @@
 /**
  * Reference to a Skript function.
  */
-public class FunctionReference<T> {
+public class FunctionReference<T> implements Contract {
 
 	/**
 	 * Name of function that is called, for logging purposes.
@@ -97,7 +98,13 @@ public class FunctionReference<T> {
 	 */
 	@Nullable
 	public final String script;
-
+
+	/**
+	 * The contract for this function (typically the function reference itself).
+	 * Used to determine input-based return types and simple behaviour.
+	 */
+	private Contract contract;
+
 	public FunctionReference(
 			String functionName, @Nullable Node node, @Nullable String script,
 			@Nullable Class<? extends T>[] returnTypes, Expression<?>[] params
@@ -106,7 +113,8 @@ public FunctionReference(
 		this.node = node;
 		this.script = script;
 		this.returnTypes = returnTypes;
-		parameters = params;
+		this.parameters = params;
+		this.contract = this;
 	}
 
 	public boolean validateParameterArity(boolean first) {
@@ -257,6 +265,10 @@ public boolean validateFunction(boolean first) {
 
 		signature = (Signature<? extends T>) sign;
 		sign.calls.add(this);
+
+		Contract contract = sign.getContract();
+		if (contract != null)
+			this.contract = contract;
 
 		return true;
 	}
@@ -310,21 +322,41 @@ protected T[] execute(Event e) {
 		// Execute the function
 		return function.execute(params);
 	}
-	
+
 	public boolean isSingle() {
+		return contract.isSingle(parameters);
+	}
+
+	@Override
+	public boolean isSingle(Expression<?>... arguments) {
 		return single;
 	}
-	
+
 	@Nullable
 	public Class<? extends T> getReturnType() {
+		//noinspection unchecked
+		return (Class<? extends T>) contract.getReturnType(parameters);
+	}
+
+	@Override
+	@Nullable
+	public Class<?> getReturnType(Expression<?>... arguments) {
 		if (signature == null)
 			throw new SkriptAPIException("Signature of function is null when return type is asked!");
-		
+
 		@SuppressWarnings("ConstantConditions")
 		ClassInfo<? extends T> ret = signature.returnType;
 		return ret == null ? null : ret.getC();
 	}
-
+
+	/**
+	 * The contract is used in preference to the function for determining return type, etc.
+	 * @return The contract determining this function's parse-time hints, potentially this reference
+	 */
+	public Contract getContract() {
+		return contract;
+	}
+
 	public String toString(@Nullable Event e, boolean debug) {
 		StringBuilder b = new StringBuilder(functionName + "(");
 		for (int i = 0; i < parameters.length; i++) {

src/main/java/ch/njol/skript/lang/function/JavaFunction.java

@@ -21,6 +21,7 @@
 import org.eclipse.jdt.annotation.Nullable;
 
 import ch.njol.skript.classes.ClassInfo;
+import ch.njol.skript.util.Contract;
 
 /**
  * @author Peter Güttinger
@@ -32,7 +33,11 @@ public JavaFunction(Signature<T> sign) {
 	}
 
 	public JavaFunction(String name, Parameter<?>[] parameters, ClassInfo<T> returnType, boolean single) {
-		this(new Signature<>("none", name, parameters, false, returnType, single, Thread.currentThread().getStackTrace()[3].getClassName()));
+		this(name, parameters, returnType, single, null);
+	}
+
+	public JavaFunction(String name, Parameter<?>[] parameters, ClassInfo<T> returnType, boolean single, @Nullable Contract contract) {
+		this(new Signature<>("none", name, parameters, false, returnType, single, Thread.currentThread().getStackTrace()[3].getClassName(), contract));
 	}
 
 	@Override

src/main/java/ch/njol/skript/lang/function/Signature.java

@@ -20,6 +20,7 @@
 
 import ch.njol.skript.classes.ClassInfo;
 import org.eclipse.jdt.annotation.Nullable;
+import ch.njol.skript.util.Contract;
 
 import java.util.Collection;
 import java.util.Collections;
@@ -75,23 +76,40 @@ public class Signature<T> {
 	@Nullable
 	final String originClassPath;
 
+	/**
+	 * An overriding contract for this function (e.g. to base its return on its arguments).
+	 */
+	@Nullable
+	final Contract contract;
+
 	public Signature(String script,
 					 String name,
 					 Parameter<?>[] parameters, boolean local,
 					 @Nullable ClassInfo<T> returnType,
 					 boolean single,
-					 @Nullable String originClassPath) {
+					 @Nullable String originClassPath,
+					 @Nullable Contract contract) {
 		this.script = script;
 		this.name = name;
 		this.parameters = parameters;
 		this.local = local;
 		this.returnType = returnType;
 		this.single = single;
 		this.originClassPath = originClassPath;
+		this.contract = contract;
 
 		calls = Collections.newSetFromMap(new WeakHashMap<>());
 	}
 
+	public Signature(String script,
+					 String name,
+					 Parameter<?>[] parameters, boolean local,
+					 @Nullable ClassInfo<T> returnType,
+					 boolean single,
+					 @Nullable String originClassPath) {
+		this(script, name, parameters, local, returnType, single, originClassPath, null);
+	}
+
 	public Signature(String script, String name, Parameter<?>[] parameters, boolean local, @Nullable ClassInfo<T> returnType, boolean single) {
 		this(script, name, parameters, local, returnType, single, null);
 	}
@@ -126,6 +144,11 @@ public String getOriginClassPath() {
 		return originClassPath;
 	}
 
+	@Nullable
+	public Contract getContract() {
+		return contract;
+	}
+
 	/**
 	 * Gets maximum number of parameters that the function described by this
 	 * signature is able to take.

src/main/java/ch/njol/skript/lang/function/SimpleJavaFunction.java

@@ -21,6 +21,7 @@
 import org.eclipse.jdt.annotation.Nullable;
 
 import ch.njol.skript.classes.ClassInfo;
+import ch.njol.skript.util.Contract;
 
 /**
  * A {@link JavaFunction} which doesn't make use of
@@ -36,6 +37,10 @@ public SimpleJavaFunction(Signature<T> sign) {
 	public SimpleJavaFunction(String name, Parameter<?>[] parameters, ClassInfo<T> returnType, boolean single) {
 		super(name, parameters, returnType, single);
 	}
+
+	public SimpleJavaFunction(String name, Parameter<?>[] parameters, ClassInfo<T> returnType, boolean single, Contract contract) {
+		super(name, parameters, returnType, single, contract);
+	}
 
 	@SuppressWarnings("ConstantConditions")
 	@Nullable

src/main/java/ch/njol/skript/util/Contract.java

@@ -0,0 +1,44 @@
+/**
+ *   This file is part of Skript.
+ *
+ *  Skript is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Skript is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with Skript.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright Peter Güttinger, SkriptLang team and contributors
+ */
+package ch.njol.skript.util;
+
+import ch.njol.skript.lang.Expression;
+import org.eclipse.jdt.annotation.Nullable;
+
+/**
+ * The 'contract' of a function or another callable.
+ * This is a non-exhaustive helper for type hints, singularity, etc. that may change based on the arguments
+ * passed to a callable, in order for it to make better judgements on correct use at parse time.
+ */
+public interface Contract {
+
+	/**
+	 * @return Whether, given these parameters, this will return a single value
+	 * @see Expression#isSingle()
+	 */
+	boolean isSingle(Expression<?>... arguments);
+
+	/**
+	 * @return What this will return, given these parameters
+	 * @see Expression#getReturnType()
+	 */
+	@Nullable
+	Class<?> getReturnType(Expression<?>... arguments);
+
+}

src/test/skript/tests/syntaxes/functions/clamp.sk

@@ -41,3 +41,15 @@ test "clamp numbers":
 	assert number within {_got::1} is not number within {_got::1} with "(edge cases list) NaN" # need within because the variables weren't cooperating
 	assert {_got::2} is {_expected::2} with "(edge cases list) -infinity"
 	assert {_got::3} is {_expected::3} with "(edge cases list) infinity"
+
+test "clamp numbers (single)":
+	set {_expected::*} to (1, 0.0, and 2.0)
+	set {_got::*} to clamp((1, -infinity value, infinity value), 0.0, 2.0)
+	assert size of {_got::*} is 3 with "(multiple) expected"
+	loop 3 times:
+		assert {_got::%loop-number%} is {_expected::%loop-number%} with "(plural) expected %{_expected::%loop-number%}% found %{_got::%loop-number%}%"
+
+	# single store
+	set {_expected} to 2
+	set {_got} to clamp(0, 2, 5)
+	assert {_got} is {_expected} with "(single) expected"