Make coroutine functions be forced to be created with the coroutine keyword.

Upsilon/BaseTypes/ScriptFunction/ScriptFunction.cs

@@ -6,6 +6,11 @@ namespace Upsilon.BaseTypes.ScriptFunction
 {
     public abstract class ScriptFunction : ScriptType
     {
+        protected ScriptFunction(bool isCoroutine)
+        {
+            IsCoroutine = isCoroutine;
+        }
+
         public override TypeContainer Type => BaseTypes.Type.Function;
         public override object ToCSharpObject()
         {
@@ -17,6 +22,8 @@ namespace Upsilon.BaseTypes.ScriptFunction
             return null;
         }
 
+        public bool IsCoroutine { get; }
+
         public abstract ScriptType Run(Diagnostics diagnostics, ScriptType[] variables, Script script, EvaluationScope scope, TextSpan span);
         public abstract IEnumerator RunCoroutine(Diagnostics diagnostics, ScriptType[] variables, Script script, EvaluationScope scope, TextSpan span);
     }

Upsilon/BaseTypes/ScriptFunction/ScriptMethodInfoFunction.cs

@@ -14,7 +14,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
     public class ScriptMethodInfoFunction : ScriptFunction
     {
         public ScriptMethodInfoFunction(UserDataMethod method, object o, bool directTypeManipulation,
-            bool passScriptReference = false, bool passScopeReference = false)
+            bool isCoroutine, bool passScriptReference = false, bool passScopeReference = false) : base(isCoroutine)
         {
             Method                 = method;
             _object                 = o;

Upsilon/BaseTypes/ScriptFunction/ScriptRuntimeFunction.cs

@@ -16,7 +16,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
     {
         public List<ScriptRuntimeFunctionOption> Options { get; }
 
-        public ScriptRuntimeFunction(List<ScriptRuntimeFunctionOption> options)
+        public ScriptRuntimeFunction(List<ScriptRuntimeFunctionOption> options, bool isCoroutine) : base(isCoroutine)
         {
             Options = options;
         }
@@ -95,7 +95,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
                             }
                         }
                     }
-                    else if (parameter.VariableSymbol is TableVariableSymbol tableVariableSymbol)
+                    else if (parameter.VariableSymbol is TableVariableSymbol)
                     {
                         if (parameterType.GetScriptType() != BaseTypes.Type.Table)
                         {

Upsilon/BaseTypes/UserData/UserDataType.cs

@@ -1,3 +1,4 @@
+using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
@@ -77,7 +78,8 @@ namespace Upsilon.BaseTypes.UserData
             }
             if (Methods.TryGetValue(member, out var method))
             {
-                return (new ScriptMethodInfoFunction(method, value, false), false, null);
+                var isCoroutine = typeof(IEnumerator).IsAssignableFrom(method.ReturnType);
+                return (new ScriptMethodInfoFunction(method, value, false, isCoroutine), false, null);
             }
 
             return (null, true, $"Can't find public member '{member}' on type '{Type}'.");

Upsilon/Binder/Binder.cs

@@ -691,7 +691,8 @@ namespace Upsilon.Binder
             }
             else
             {
-                var unbound = new UnboundFunctionExpression(parameters.ToImmutable(), e.Block, e.Span, innerScope, functionVariableSymbol);
+                var unbound = new UnboundFunctionExpression(parameters.ToImmutable(), e.Block, e.Span, innerScope,
+                    functionVariableSymbol, e.IsCoroutine);
                 _unboundFunctions.Add(unbound);
                 return unbound;
             }

Upsilon/Binder/BoundExpressions/BoundFunctionExpression.cs

@@ -40,7 +40,8 @@ namespace Upsilon.Binder
         internal override ScriptType Evaluate(EvaluationScope scope, Diagnostics diagnostics, ref EvaluationState state)
         {
             var option = new ScriptRuntimeFunction.ScriptRuntimeFunctionOption(Parameters, Block, scope);
-            var func   = new ScriptRuntimeFunction(new List<ScriptRuntimeFunction.ScriptRuntimeFunctionOption>(){option});
+            var func = new ScriptRuntimeFunction(new List<ScriptRuntimeFunction.ScriptRuntimeFunctionOption>() {option},
+                IsCoroutine);
             return func;
 
         }

Upsilon/Binder/BoundStatements/BoundYieldStatement.cs

@@ -42,6 +42,9 @@ namespace Upsilon.Binder
                 {
                     throw new EvaluationException(state.Script.FileName, $"Variable is not a function.", functionCall.Identifier.Span);
                 }
+
+                if (function.IsCoroutine)
+                {
                     var ls = new List<ScriptType>();
                     foreach (var t in functionCall.Parameters)
                     {
@@ -55,6 +58,7 @@ namespace Upsilon.Binder
                     }
                     yield break;
                 }
+            }
             var value = Expression.Evaluate(scope, diagnostics, ref state);
             yield return value;
         }

Upsilon/Binder/BoundStatements/UnboundFunctionExpression.cs

@@ -9,8 +9,8 @@ namespace Upsilon.Binder
     public class UnboundFunctionExpression : BoundFunctionExpression
     {
         public UnboundFunctionExpression(ImmutableArray<BoundVariableSymbol> parameters,
-            BlockStatementSyntax unboundBlock, TextSpan span, BoundScope scope, string name)
+            BlockStatementSyntax unboundBlock, TextSpan span, BoundScope scope, string name, bool isCoroutine)
-            : base(parameters, null, span, scope, BaseTypes.Type.Unknown, false)
+            : base(parameters, null, span, scope, BaseTypes.Type.Unknown, isCoroutine)
         {
             UnboundBlock = unboundBlock;
             Name = name;

Upsilon/Evaluator/Evaluator.cs

@@ -91,6 +91,11 @@ namespace Upsilon.Evaluator
                 throw new ArgumentException(($"Function '{functionName}' could not be found"));
             }
             var function = (ScriptRuntimeFunction) statement;
+            if (!function.IsCoroutine)
+            {
+                throw new EvaluationException(_script.FileName,
+                    $"Function with name '{functionName}' is not a coroutine, and can't be executed as such.", e.Span);
+            }
             var option   = function.GetValidOption(parameters);
             if (option == null)
             {