diff --git a/Upsilon/BaseTypes/ScriptFunction/ScriptMethodInfoFunction.cs b/Upsilon/BaseTypes/ScriptFunction/ScriptMethodInfoFunction.cs
index f5104d2..93f6613 100644
--- a/Upsilon/BaseTypes/ScriptFunction/ScriptMethodInfoFunction.cs
+++ b/Upsilon/BaseTypes/ScriptFunction/ScriptMethodInfoFunction.cs
@@ -15,13 +15,13 @@ namespace Upsilon.BaseTypes.ScriptFunction
         public ScriptMethodInfoFunction(UserDataMethod method, object o, bool directTypeManipulation,
             bool passScriptReference = false, bool passScopeReference = false)
         {
-            _method                 = method;
+            Method                 = method;
             _object                 = o;
             _directTypeManipulation = directTypeManipulation;
             _passScriptReference = passScriptReference;
             _passScopeReference = passScopeReference;
 
-            ReturnType              = _method.ReturnType;
+            ReturnType              = Method.ReturnType;
 
             if (method.GetMethods().First().Attribute != null)
             {
@@ -33,7 +33,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
             }
         }
 
-        private readonly UserDataMethod _method;
+        public UserDataMethod Method { get; }
         private readonly object         _object;
         private readonly bool           _directTypeManipulation;
         private readonly bool           _passScriptReference;
@@ -42,7 +42,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
 
         public IEnumerable<UserDataMethod.UserDataMethodParameter> GetParameterInfo()
         {
-            return _method.GetMethods().First().Parameters;
+            return Method.GetMethods().First().Parameters;
         }
 
         public override ScriptType Run(Diagnostics diagnostics, ScriptType[] variables, Script script, EvaluationScope scope, TextSpan span)
@@ -61,7 +61,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
                 {
                     // grab the parameters, and just invoke it
                     var array = objects.ToArray();
-                    var methodInfo = _method.GetMethod(ref array);
+                    var methodInfo = Method.GetMethod(ref array);
                     result = methodInfo.Invoke(null, array);
                 }
                 else
@@ -90,7 +90,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
                     foreach (var match in methods)
                     {
                         // if the method does not have the name we're looking for, continue
-                        if (!string.Equals(match.Name, _method.Name)) continue;
+                        if (!string.Equals(match.Name, Method.Name)) continue;
                         // Use the type binder to check if the match is allowed
                         if (UpsilonBinder.Default.IsValidMatch(match, convertedTypes))
                         {
@@ -108,7 +108,7 @@ namespace Upsilon.BaseTypes.ScriptFunction
                     if (method == null)
                     {
                         throw new ScriptRuntimeException(
-                            $"Can't find method {_method.Name} with parameter types {string.Join(", ", convertedTypes.Select(x => x.Name))} on type {_object.GetType().Name}",
+                            $"Can't find method {Method.Name} with parameter types {string.Join(", ", convertedTypes.Select(x => x.Name))} on type {_object.GetType().Name}",
                             span, diagnostics.ScriptString.GetSpan(span));
                     }
                     // get the method parameters
diff --git a/Upsilon/BaseTypes/TypeContainer.cs b/Upsilon/BaseTypes/TypeContainer.cs
new file mode 100644
index 0000000..900d050
--- /dev/null
+++ b/Upsilon/BaseTypes/TypeContainer.cs
@@ -0,0 +1,65 @@
+using System.Collections.Immutable;
+
+namespace Upsilon.BaseTypes
+{
+    public class TypeContainer
+    {
+        public Type Type { get; }
+
+        protected TypeContainer(Type t)
+        {
+            Type = t;
+        }
+
+        public static implicit operator TypeContainer (Type type)
+        {
+            return new TypeContainer(type);
+        }
+
+        public static implicit operator Type(TypeContainer t)
+        {
+            return t.Type;
+        }
+
+        protected bool Equals(TypeContainer other)
+        {
+            return Type == other.Type;
+        }
+
+        public override bool Equals(object obj)
+        {
+            if (ReferenceEquals(null, obj)) return false;
+            if (ReferenceEquals(this, obj)) return true;
+            if (obj is Type t)
+            {
+                return t == Type;
+            }
+            if (obj.GetType() != this.GetType()) return false;
+            return Equals((TypeContainer) obj);
+        }
+
+        public override int GetHashCode()
+        {
+            return (int) Type;
+        }
+
+        public static bool operator == (TypeContainer a, object b)
+        {
+            return Equals(a, b);
+        }
+
+        public static bool operator != (TypeContainer a, object b)
+        {
+            return !Equals(a, b);
+        }
+    }
+
+    public class CompositeTypeContainer : TypeContainer
+    {
+        public ImmutableArray<Type> Types { get; set; }
+
+        public CompositeTypeContainer() : base(Type.Table)
+        {
+        }
+    }
+}
\ No newline at end of file
diff --git a/Upsilon/Binder/Binder.cs b/Upsilon/Binder/Binder.cs
index 580398c..7dcee03 100644
--- a/Upsilon/Binder/Binder.cs
+++ b/Upsilon/Binder/Binder.cs
@@ -55,7 +55,7 @@ namespace Upsilon.Binder
                 foreach (var valueParameter in unboundFunctionStatement.Value.Parameters)
                 {
                     Scope.AssignToNearest(valueParameter.VariableSymbol);
-                    if (valueParameter.VariableSymbol.Type == Type.Unknown)
+                    if (valueParameter.VariableSymbol.TypeContainer == Type.Unknown)
                         _diagnostics.LogUnknownVariableType(valueParameter.VariableSymbol.Name, valueParameter.Span);
                 }
                 unboundFunctionStatement.Value.Block =
@@ -220,13 +220,14 @@ namespace Upsilon.Binder
                 parameters.Add(bound);
             }
 
-            var returnType = Type.Unknown;
+            TypeContainer returnType = Type.Unknown;
             var resolved = ResolveVariable(expression, _diagnostics);
             if (resolved == null)
             {
                 _diagnostics.LogError("Can't resolve variable", expression.Span);
             }
-            if (resolved is FunctionVariableSymbol function)
+
+            else if (resolved is FunctionVariableSymbol function)
             {
                 if (function is ScriptFunctionVariableSymbol scriptFunction)
                 {
@@ -237,7 +238,7 @@ namespace Upsilon.Binder
                         {
                             var functionVariable = scriptFunction.Parameters[index];
                             var callingVariable  = parameters[index];
-                            functionVariable.Type = callingVariable.Type;
+                            functionVariable.TypeContainer = callingVariable.Type;
                             Scope.DefineLocalVariable(functionVariable);
                         }
 
@@ -280,10 +281,16 @@ namespace Upsilon.Binder
                             }
                         }
                     }
+                    if (resolved is InternalFunctionVariableSymbol internalFunction)
+                    {
+                        returnType = internalFunction.GetResultType(parameters.ToArray());
+                    }
+                    else
+                    {
+                        returnType = function.ResultType;
+                    }
                 }
 
-                returnType = function.ResultType;
-
                 var (isValid, error, wrongParameter) = function.ValidateParameters(parameters.ToImmutable());
                 if (!isValid)
                 {
@@ -331,11 +338,11 @@ namespace Upsilon.Binder
                 {
                     diagnostics?.LogError("Can't resolve variable", expression.Span);
                 }
-                else if (indexerVariable.Type == Type.Table)
+                else if (indexerVariable.TypeContainer == Type.Table)
                 {
                     return ((TableVariableSymbol)indexerVariable).Variables[fullStopIndexExpression.Index];
                 }
-                else if (indexerVariable.Type == Type.UserData)
+                else if (indexerVariable.TypeContainer == Type.UserData)
                 {
                     var parent =
                         (UserDataBoundTypeDefinition) ((UserDataVariableSymbol) indexerVariable).BoundTypeDefinition;
@@ -349,7 +356,7 @@ namespace Upsilon.Binder
                         }
                     }
                 }
-                else if (indexerVariable.Type == Type.Unknown)
+                else if (indexerVariable.TypeContainer == Type.Unknown)
                 {
                     if (indexerVariable is UserDataVariableSymbol funcSymbol)
                     {
@@ -405,17 +412,18 @@ namespace Upsilon.Binder
                 {
                     if (assignment.Kind == BoundKind.BoundTableExpression)
                     {
-                        variable = new TableVariableSymbol(name, isLocal, ((BoundTableExpression)assignment).Expressions);
+                        variable = new TableVariableSymbol(name, isLocal,
+                            ((BoundTableExpression) assignment).Expressions, assignment.Type);
                     }
                     else if (assignment.Kind == BoundKind.VariableExpression)
                     {
                         variable = new TableVariableSymbol(name, isLocal,
                             ((TableVariableSymbol) ((BoundVariableExpression) assignment).Variable.VariableSymbol)
-                            .Variables);
+                            .Variables, assignment.Type);
                     }
                     else
                     {
-                        variable = new TableVariableSymbol(name, isLocal);
+                        variable = new TableVariableSymbol(name, isLocal, assignment.Type);
                     }
                 }
                 else
@@ -435,26 +443,26 @@ namespace Upsilon.Binder
             else
             {
                 // don't allow assigning different typed variables to a variable, unless either of them is nil, allow assigning nil to all variables
-                if (assignment.Type != variable.Type)
+                if (assignment.Type != variable.TypeContainer)
                 {
-                    if (variable.Type == Type.Nil || assignment.Type == Type.Nil)
+                    if (variable.TypeContainer == Type.Nil || assignment.Type == Type.Nil)
                     {
-                        variable.Type = assignment.Type;
+                        variable.TypeContainer = assignment.Type;
                     }
-                    else if (variable.Type == Type.Unknown)
+                    else if (variable.TypeContainer == Type.Unknown)
                     {
-                        variable.Type = assignment.Type;
+                        variable.TypeContainer = assignment.Type;
                     }
                     else if (assignment.Type == Type.Unknown && assignment is BoundVariableExpression v)
                     {
-                        v.Variable.VariableSymbol.Type = variable.Type;
+                        v.Variable.VariableSymbol.TypeContainer = variable.TypeContainer;
                     }
                     else if (assignment.Type == Type.Unknown)
                     {
                     }
                     else
                     {
-                        _diagnostics.LogCannotConvert(assignment.Type, variable.Type, assignment.Span);
+                        _diagnostics.LogCannotConvert(assignment.Type, variable.TypeContainer, assignment.Span);
                         return (null, false);
                     }
                 }
@@ -474,7 +482,7 @@ namespace Upsilon.Binder
                 var (symbol, isCreation) = TryBindVariable(name, isLocal, boundExpression, e.CommentData);
                 if (symbol != null)
                 {
-                    if (symbol.Type == Type.Unknown)
+                    if (symbol.TypeContainer == Type.Unknown)
                     {
                         _diagnostics.LogUnknownVariableType(symbol.Name, variableExpression.Span);
                     }
@@ -496,7 +504,7 @@ namespace Upsilon.Binder
             foreach (var identifierToken in s.Identifiers)
             {
                 var boundVariable = TryBindVariable(identifierToken.Name, isLocal, assignment, null);
-                if (boundVariable.Symbol.Type == Type.Unknown)
+                if (boundVariable.Symbol.TypeContainer == Type.Unknown)
                 {
                     _diagnostics.LogUnknownVariableType(boundVariable.Symbol.Name, identifierToken.Span);
                 }
@@ -610,7 +618,7 @@ namespace Upsilon.Binder
 
             foreach (var parameter in func.Parameters)
             {
-                var vari = new VariableSymbol(parameter.VariableSymbol.Name, parameter.VariableSymbol.Type, true);
+                var vari = new VariableSymbol(parameter.VariableSymbol.Name, parameter.VariableSymbol.TypeContainer, true);
                 parameters.Add(vari);
                 innerScope.DefineLocalVariable(vari);
             }
@@ -637,15 +645,15 @@ namespace Upsilon.Binder
             else
             {
                 // don't allow assigning different typed variables to a variable, unless either of them is nil, allow assigning nil to all variables
-                if (variable.Type != Type.Function)
+                if (variable.TypeContainer != Type.Function)
                 {
-                    if (variable.Type == Type.Nil || variable.Type == Type.Unknown)
+                    if (variable.TypeContainer == Type.Nil || variable.TypeContainer == Type.Unknown)
                     {
-                        variable.Type = Type.Function;
+                        variable.TypeContainer = Type.Function;
                     }
                     else
                     {
-                        _diagnostics.LogCannotConvert(Type.Function, variable.Type, e.Span);
+                        _diagnostics.LogCannotConvert(Type.Function, variable.TypeContainer, e.Span);
                         return new BoundExpressionStatement(new BoundBadExpression(e.Span), e.Span);
                     }
                 }
@@ -712,7 +720,7 @@ namespace Upsilon.Binder
                 _diagnostics.LogInvalidIndexExpression(expression.Type, index.Type, e.Span);
                 return new BoundBadExpression(e.Span);
             }
-            switch (expression.Type)
+            switch (expression.Type.Type)
             {
                 case Type.Table:
                     if (isAssignment)
@@ -726,7 +734,7 @@ namespace Upsilon.Binder
                         var variableDic = table.Expressions;
                         if (variableDic.TryGetValue(realIndex.Value.ToString(), out var variable))
                         {
-                            return new BoundIndexExpression(expression, index, variable.Type, e.Span);
+                            return new BoundIndexExpression(expression, index, variable.TypeContainer, e.Span);
                         }
 
                         _diagnostics.LogError($"No variable '{realIndex.Value}' found in table.",
@@ -746,7 +754,7 @@ namespace Upsilon.Binder
                         var variableDic = tableSymbol.Variables;
                         if (variableDic.TryGetValue(realIndex.Value.ToString(), out var variable))
                         {
-                            return new BoundIndexExpression(expression, index, variable.Type, e.Span);
+                            return new BoundIndexExpression(expression, index, variable.TypeContainer, e.Span);
                         }
 
                         _diagnostics.LogError($"No variable '{realIndex.Value}' found in table '{realTable.VariableSymbol.Name}'.",
@@ -768,7 +776,7 @@ namespace Upsilon.Binder
         {
             var expression = BindExpression(e.Expression);
             var index = e.Index.Name;
-            switch (expression.Type)
+            switch (expression.Type.Type)
             {
                 case Type.Table:
                     if (isAssignment)
@@ -780,7 +788,7 @@ namespace Upsilon.Binder
                         var table     = (BoundTableExpression)expression;
                         if (table.Expressions.TryGetValue(index, out var variable))
                         {
-                            return new BoundFullStopIndexExpression(expression, index, variable.Type, e.Span);
+                            return new BoundFullStopIndexExpression(expression, index, variable.TypeContainer, e.Span);
                         }
                         _diagnostics.LogError($"No variable '{index}' found in table.", e.Span);
                     }
@@ -791,7 +799,7 @@ namespace Upsilon.Binder
                         var variableDic = ((TableVariableSymbol) realTable.VariableSymbol).Variables;
                         if (variableDic.TryGetValue(index, out var variable))
                         {
-                            return new BoundFullStopIndexExpression(expression, index, variable.Type, e.Span);
+                            return new BoundFullStopIndexExpression(expression, index, variable.TypeContainer, e.Span);
                         }
 
                         _diagnostics.LogError($"No variable '{index}' found in table '{realTable.VariableSymbol.Name}'.",
@@ -897,13 +905,29 @@ namespace Upsilon.Binder
         {
             Scope = new BoundScope(Scope);
             var array = ImmutableArray.CreateBuilder<BoundVariableSymbol>();
-            foreach (var variableIdentifier in e.Variables)
-            {
-                var variable = new VariableSymbol(variableIdentifier.Name, Type.Unknown, true);
-                Scope.DefineLocalVariable(variable);
-                array.Add(new BoundVariableSymbol(variable, true, variableIdentifier.Span));
-            }
+
+            var keyVar = e.Variables[0];
+            var keyVariable = new VariableSymbol(keyVar.Name, Type.String, true);
+            Scope.DefineLocalVariable(keyVariable);
+            array.Add(new BoundVariableSymbol(keyVariable, true, keyVar.Span));
+
             var boundEnumerableExpression = BindExpression(e.EnumerableExpression);
+
+            var valueVar = e.Variables[1];
+            VariableSymbol valueVariable;
+            if (boundEnumerableExpression.Type is CompositeTypeContainer composite && composite.Types.Length == 2)
+            {
+                valueVariable = new VariableSymbol(valueVar.Name, composite.Types[1], true);
+            }
+            else
+            {
+                throw new Exception();
+                valueVariable = new VariableSymbol(valueVar.Name, Type.Unknown, true);
+            }
+            Scope.DefineLocalVariable(valueVariable);
+            array.Add(new BoundVariableSymbol(valueVariable, true, valueVar.Span));
+
+
             var block = BindBlockStatement(e.Block);
 
             return new BoundGenericForStatement(array.ToImmutable(), boundEnumerableExpression, block, e.Span);
diff --git a/Upsilon/Binder/BoundExpressions/BoundBadExpression.cs b/Upsilon/Binder/BoundExpressions/BoundBadExpression.cs
index de7c972..be0b7a9 100644
--- a/Upsilon/Binder/BoundExpressions/BoundBadExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundBadExpression.cs
@@ -17,6 +17,6 @@ namespace Upsilon.Binder
             yield break;
         }
 
-        public override Type Type => Type.Nil;
+        public override TypeContainer Type => BaseTypes.Type.Nil;
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundExpressions/BoundBinaryExpression.cs b/Upsilon/Binder/BoundExpressions/BoundBinaryExpression.cs
index 28b736c..188055b 100644
--- a/Upsilon/Binder/BoundExpressions/BoundBinaryExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundBinaryExpression.cs
@@ -23,7 +23,7 @@ namespace Upsilon.Binder
             yield return RightExpression;
         }
 
-        public override Type Type { get; }
+        public override TypeContainer Type { get; }
 
         public BoundBinaryOperator Operator { get; }
         public BoundExpression LeftExpression { get; }
diff --git a/Upsilon/Binder/BoundExpressions/BoundExpression.cs b/Upsilon/Binder/BoundExpressions/BoundExpression.cs
index 1fc0bde..645589e 100644
--- a/Upsilon/Binder/BoundExpressions/BoundExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundExpression.cs
@@ -9,6 +9,6 @@ namespace Upsilon.Binder
         {
         }
 
-        public abstract Type Type { get; }
+        public abstract TypeContainer Type { get; }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundExpressions/BoundFunctionCallExpression.cs b/Upsilon/Binder/BoundExpressions/BoundFunctionCallExpression.cs
index c98b876..290b071 100644
--- a/Upsilon/Binder/BoundExpressions/BoundFunctionCallExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundFunctionCallExpression.cs
@@ -11,7 +11,7 @@ namespace Upsilon.Binder
         public ImmutableArray<BoundExpression> Parameters { get; }
 
         public BoundFunctionCallExpression(BoundExpression identifier, ImmutableArray<BoundExpression> parameters,
-            TextSpan span, Type type) : base(span)
+            TextSpan span, TypeContainer type) : base(span)
         {
             Identifier = identifier;
             Parameters = parameters;
@@ -29,6 +29,6 @@ namespace Upsilon.Binder
             }
         }
 
-        public override Type Type { get; }
+        public override TypeContainer Type { get; }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundExpressions/BoundFunctionExpression.cs b/Upsilon/Binder/BoundExpressions/BoundFunctionExpression.cs
index 853fbb3..6bf1e75 100644
--- a/Upsilon/Binder/BoundExpressions/BoundFunctionExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundFunctionExpression.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using Upsilon.BaseTypes;
 using Upsilon.Text;
 using Type = Upsilon.BaseTypes.Type;
 
@@ -31,7 +32,7 @@ namespace Upsilon.Binder
             yield return Block;
         }
 
-        public override Type Type => Type.Function;
+        public override TypeContainer Type => BaseTypes.Type.Function;
         public BoundScope Scope { get; set; }
         public Type ReturnType { get; }
     }
diff --git a/Upsilon/Binder/BoundExpressions/BoundIndexExpression.cs b/Upsilon/Binder/BoundExpressions/BoundIndexExpression.cs
index 4fe3ae3..7828ba4 100644
--- a/Upsilon/Binder/BoundExpressions/BoundIndexExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundIndexExpression.cs
@@ -25,7 +25,7 @@ namespace Upsilon.Binder
             yield return Index;
         }
 
-        public override Type Type { get; }
+        public override TypeContainer Type { get; }
     }
 
     public class BoundFullStopIndexExpression : BoundExpression
@@ -47,6 +47,6 @@ namespace Upsilon.Binder
             yield return Expression;
         }
 
-        public override Type Type { get; }
+        public override TypeContainer Type { get; }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundExpressions/BoundLiteralExpression.cs b/Upsilon/Binder/BoundExpressions/BoundLiteralExpression.cs
index b6d1978..6904e22 100644
--- a/Upsilon/Binder/BoundExpressions/BoundLiteralExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundLiteralExpression.cs
@@ -18,7 +18,7 @@ namespace Upsilon.Binder
             yield break;
         }
 
-        public override Type Type => Value.Type;
+        public override TypeContainer Type => Value.Type;
         public ScriptType Value { get; }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundExpressions/BoundTableExpression.cs b/Upsilon/Binder/BoundExpressions/BoundTableExpression.cs
index d6bc072..a5561dc 100644
--- a/Upsilon/Binder/BoundExpressions/BoundTableExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundTableExpression.cs
@@ -1,6 +1,6 @@
-using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using Upsilon.BaseTypes;
 using Upsilon.Binder.VariableSymbols;
 using Upsilon.Text;
 using Type = Upsilon.BaseTypes.Type;
@@ -24,7 +24,39 @@ namespace Upsilon.Binder
             return Statements;
         }
 
-        public override Type Type => Type.Table;
+        public override TypeContainer Type
+        {
+            get
+            {
+                Type? valueType = null;
+                foreach (var statement in Statements)
+                {
+                    if (!(statement is BoundExpressionStatement exp))
+                    {
+                        valueType = BaseTypes.Type.Unknown;
+                        break;
+                    }
+                    if (!valueType.HasValue)
+                    {
+                        valueType = exp.Expression.Type;
+                        continue;
+                    }
+                    if (valueType == exp.Expression.Type) continue;
+                    valueType = BaseTypes.Type.Unknown;
+                    break;
+                }
+
+                var valueRealType = BaseTypes.Type.Unknown;
+                if (valueType.HasValue)
+                    valueRealType = valueType.Value;
+
+                var arr = new[] {BaseTypes.Type.String, valueRealType};
+                return new CompositeTypeContainer()
+                {
+                    Types = arr.ToImmutableArray()
+                };
+            }
+        }
 
 
         public Dictionary<string, VariableSymbol> Expressions { get; }
diff --git a/Upsilon/Binder/BoundExpressions/BoundUnaryExpression.cs b/Upsilon/Binder/BoundExpressions/BoundUnaryExpression.cs
index 6e167ca..69370d5 100644
--- a/Upsilon/Binder/BoundExpressions/BoundUnaryExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundUnaryExpression.cs
@@ -13,7 +13,7 @@ namespace Upsilon.Binder
             yield return InExpression;
         }
 
-        public override Type Type { get; }
+        public override TypeContainer Type { get; }
 
         public BoundUnaryExpression(BoundUnaryOperator op, BoundExpression inExpression, Type type, TextSpan span) : base(span)
         {
diff --git a/Upsilon/Binder/BoundExpressions/BoundVariableExpression.cs b/Upsilon/Binder/BoundExpressions/BoundVariableExpression.cs
index 19937e0..172cd48 100644
--- a/Upsilon/Binder/BoundExpressions/BoundVariableExpression.cs
+++ b/Upsilon/Binder/BoundExpressions/BoundVariableExpression.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Upsilon.BaseTypes;
 using Upsilon.Text;
 using Type = Upsilon.BaseTypes.Type;
 
@@ -20,6 +21,6 @@ namespace Upsilon.Binder
             yield break;
         }
 
-        public override Type      Type => Variable.Type;
+        public override TypeContainer      Type => Variable.Type;
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/BoundStatements/UnboundFunctionExpression.cs b/Upsilon/Binder/BoundStatements/UnboundFunctionExpression.cs
index 6c1532e..ecf2927 100644
--- a/Upsilon/Binder/BoundStatements/UnboundFunctionExpression.cs
+++ b/Upsilon/Binder/BoundStatements/UnboundFunctionExpression.cs
@@ -10,7 +10,7 @@ namespace Upsilon.Binder
     {
         public UnboundFunctionExpression(ImmutableArray<BoundVariableSymbol> parameters,
             BlockStatementSyntax unboundBlock, TextSpan span, BoundScope scope)
-            : base(parameters, null, span, scope, Type.Unknown)
+            : base(parameters, null, span, scope, BaseTypes.Type.Unknown)
         {
             UnboundBlock = unboundBlock;
         }
diff --git a/Upsilon/Binder/BoundVariableSymbol.cs b/Upsilon/Binder/BoundVariableSymbol.cs
index dab8704..e3533b8 100644
--- a/Upsilon/Binder/BoundVariableSymbol.cs
+++ b/Upsilon/Binder/BoundVariableSymbol.cs
@@ -27,6 +27,6 @@ namespace Upsilon.Binder
             yield break;
         }
 
-        public override Type Type => VariableSymbol.Type;
+        public override TypeContainer Type => VariableSymbol.TypeContainer;
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/VariableSymbols/FunctionVariableSymbol.cs b/Upsilon/Binder/VariableSymbols/FunctionVariableSymbol.cs
index 13953c7..9955092 100644
--- a/Upsilon/Binder/VariableSymbols/FunctionVariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/FunctionVariableSymbol.cs
@@ -8,18 +8,12 @@ namespace Upsilon.Binder.VariableSymbols
         public         Type                           ResultType { get; internal set; }
 
         public FunctionVariableSymbol(string name, bool local, Type resultType)
-            : base(name, Type.Function, local)
+            : base(name, BaseTypes.Type.Function, local)
         {
             ResultType = resultType;
         }
 
         public abstract (bool IsValid, string Error, BoundExpression WrongParameter) ValidateParameters(
             ImmutableArray<BoundExpression> callingParameters);
-
-        public override Type Type
-        {
-            get => Type.Function;
-            set{}
-        }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/VariableSymbols/InternalFunctionVariableSymbol.cs b/Upsilon/Binder/VariableSymbols/InternalFunctionVariableSymbol.cs
index 177219a..78f56e8 100644
--- a/Upsilon/Binder/VariableSymbols/InternalFunctionVariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/InternalFunctionVariableSymbol.cs
@@ -1,6 +1,7 @@
-using System;
 using System.Collections.Immutable;
 using System.Linq;
+using System.Reflection;
+using Upsilon.BaseTypes;
 using Upsilon.BoundTypes;
 using Type = Upsilon.BaseTypes.Type;
 
@@ -8,37 +9,17 @@ namespace Upsilon.Binder.VariableSymbols
 {
     public class InternalFunctionVariableSymbol : FunctionVariableSymbol
     {
-        public class InternalFunctionParameter
-        {
-            public InternalFunctionParameter(string name, Type type, bool isOptional)
-            {
-                ValidTypes = type;
-                IsOptional = isOptional;
-                Name = name;
-            }
-
-            public InternalFunctionParameter(string name, Type type, string[] expectedUserData, bool isOptional)
-            {
-                ValidTypes = type;
-                ExpectedUserData = expectedUserData;
-                IsOptional = isOptional;
-                Name = name;
-            }
-
-            public string Name { get; }
-            public Type ValidTypes { get; }
-            public string[] ExpectedUserData { get; }
-            public bool IsOptional { get; }
-        }
-
-        public InternalFunctionParameter[] FunctionParameters { get; }
+        private InternalFunctionParameter[] FunctionParameters { get; }
         private int MinimalParametersRequired { get; }
+        private MethodInfo OverrideResultType { get; }
 
-        public InternalFunctionVariableSymbol(string name, bool local, Type resultType, InternalFunctionParameter[] functionParameters)
+        public InternalFunctionVariableSymbol(string name, bool local, Type resultType,
+            InternalFunctionParameter[] functionParameters, MethodInfo overrideResultType)
             : base(name, local, resultType)
         {
             FunctionParameters = functionParameters;
             MinimalParametersRequired = functionParameters.Count(x => !x.IsOptional);
+            OverrideResultType = overrideResultType;
         }
 
         public override (bool IsValid, string Error,
@@ -70,7 +51,7 @@ namespace Upsilon.Binder.VariableSymbols
                 if (functionParameter.ValidTypes.HasFlag(Type.UserData))
                 {
                     var variable = Binder.ResolveVariable(callingParameter, null);
-                    if (variable != null && variable.Type == Type.UserData)
+                    if (variable != null && variable.TypeContainer == Type.UserData)
                     {
                         var parent =
                             (UserDataBoundTypeDefinition) ((UserDataVariableSymbol) variable).BoundTypeDefinition;
@@ -87,5 +68,35 @@ namespace Upsilon.Binder.VariableSymbols
 
             return (true, null, null);
         }
+
+        public TypeContainer GetResultType(BoundExpression[] parameters)
+        {
+            if (OverrideResultType == null)
+                return ResultType;
+            return (TypeContainer) OverrideResultType.Invoke(null, new object[] {parameters});
+        }
+
+        public class InternalFunctionParameter
+        {
+            public InternalFunctionParameter(string name, TypeContainer type, bool isOptional)
+            {
+                ValidTypes = type;
+                IsOptional = isOptional;
+                Name       = name;
+            }
+
+            public InternalFunctionParameter(string name, TypeContainer type, string[] expectedUserData, bool isOptional)
+            {
+                ValidTypes       = type;
+                ExpectedUserData = expectedUserData;
+                IsOptional       = isOptional;
+                Name             = name;
+            }
+
+            public string   Name             { get; }
+            public Type     ValidTypes       { get; }
+            public string[] ExpectedUserData { get; }
+            public bool     IsOptional       { get; }
+        }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/VariableSymbols/ScriptFunctionVariableSymbol.cs b/Upsilon/Binder/VariableSymbols/ScriptFunctionVariableSymbol.cs
index 671780a..5d77abe 100644
--- a/Upsilon/Binder/VariableSymbols/ScriptFunctionVariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/ScriptFunctionVariableSymbol.cs
@@ -28,13 +28,13 @@ namespace Upsilon.Binder.VariableSymbols
             {
                 var functionParameter = Parameters[i];
                 var callingParameter  = callingParameters[i];
-                if (functionParameter.Type != Type.Unknown &&
-                    callingParameter.Type  != Type.Unknown && callingParameter.Type != Type.Nil)
+                if (functionParameter.TypeContainer != BaseTypes.Type.Unknown &&
+                    callingParameter.Type  != BaseTypes.Type.Unknown && callingParameter.Type != BaseTypes.Type.Nil)
                 {
-                    if (callingParameter.Type != functionParameter.Type)
+                    if (callingParameter.Type != functionParameter.TypeContainer)
                     {
                         return (false, $"Invalid type for function '{Name}' at parameter '{functionParameter.Name}'. " +
-                                       $"Expected type '{functionParameter.Type}', got '{callingParameter.Type}'",
+                                       $"Expected type '{functionParameter.TypeContainer}', got '{callingParameter.Type}'",
                             callingParameter);
                     }
                 }
diff --git a/Upsilon/Binder/VariableSymbols/TableVariableSymbol.cs b/Upsilon/Binder/VariableSymbols/TableVariableSymbol.cs
index ae1f7f0..13fc16d 100644
--- a/Upsilon/Binder/VariableSymbols/TableVariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/TableVariableSymbol.cs
@@ -7,15 +7,16 @@ namespace Upsilon.Binder.VariableSymbols
     {
         public Dictionary<string, VariableSymbol> Variables { get; }
         public bool ContentAware { get; }
-        public TableVariableSymbol(string name, bool local, Dictionary<string, VariableSymbol> variables)
-            :base (name, Type.Table, local)
+        public TableVariableSymbol(string name, bool local, Dictionary<string, VariableSymbol> variables,
+            TypeContainer type)
+            :base (name, type, local)
         {
             Variables    = variables;
             ContentAware = true;
         }
 
-        public TableVariableSymbol(string name, bool local)
-            :base (name, Type.Table, local)
+        public TableVariableSymbol(string name, bool local, TypeContainer type)
+            :base (name, type, local)
         {
             Variables    = new Dictionary<string, VariableSymbol>();
             ContentAware = false;
diff --git a/Upsilon/Binder/VariableSymbols/UserDataVariableSymbol.cs b/Upsilon/Binder/VariableSymbols/UserDataVariableSymbol.cs
index ad71603..7103443 100644
--- a/Upsilon/Binder/VariableSymbols/UserDataVariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/UserDataVariableSymbol.cs
@@ -8,9 +8,9 @@ namespace Upsilon.Binder.VariableSymbols
         public BoundTypeDefinition BoundTypeDefinition { get; }
         public UserDataBoundTypeDefinition Parent { get; }
 
-        public UserDataVariableSymbol(string name, Type type) : base(name, type, true)
+        public UserDataVariableSymbol(string name, Type typeContainer) : base(name, typeContainer, true)
         {
-            _type = type;
+            _typeContainer = typeContainer;
         }
 
         public UserDataVariableSymbol(string name, BoundTypeDefinition type, UserDataBoundTypeDefinition parent = null)
@@ -20,11 +20,11 @@ namespace Upsilon.Binder.VariableSymbols
             Parent = parent;
         }
 
-        private Type _type;
-        public override Type Type
+        private TypeContainer _typeContainer;
+        public override TypeContainer TypeContainer
         {
-            get => BoundTypeDefinition?.ScriptType ?? _type;
-            set => _type = Type;
+            get => BoundTypeDefinition?.ScriptType ?? _typeContainer;
+            set => _typeContainer = value;
         }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/Binder/VariableSymbols/VariableSymbol.cs b/Upsilon/Binder/VariableSymbols/VariableSymbol.cs
index 6cb86b0..335c88f 100644
--- a/Upsilon/Binder/VariableSymbols/VariableSymbol.cs
+++ b/Upsilon/Binder/VariableSymbols/VariableSymbol.cs
@@ -4,14 +4,14 @@ namespace Upsilon.Binder.VariableSymbols
 {
     public class VariableSymbol
     {
-        public VariableSymbol(string name, Type type, bool local)
+        public VariableSymbol(string name, TypeContainer typeContainer, bool local)
         {
-            Type = type;
+            TypeContainer = typeContainer;
             Local = local;
             Name = name;
         }
 
-        public virtual Type Type { get; set; } = Type.Unknown;
+        public virtual TypeContainer TypeContainer { get; set; }
         public bool Local { get; }
         public string Name { get; }
         public string[] CommentValue { get; set; }
diff --git a/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs b/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
index c4e91a0..9fd8458 100644
--- a/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
+++ b/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
@@ -191,7 +191,7 @@ namespace Upsilon.BoundTypes
                 if (functionParameter.Type.HasFlag(Type.UserData))
                 {
                     var variable = Binder.Binder.ResolveVariable(callingParameter, null);
-                    if (variable != null && variable.Type == Type.UserData)
+                    if (variable != null && variable.TypeContainer == Type.UserData)
                     {
                         var parent =
                             (UserDataBoundTypeDefinition) ((UserDataVariableSymbol) variable).BoundTypeDefinition;
diff --git a/Upsilon/StandardLibraries/BasicFunctions.cs b/Upsilon/StandardLibraries/BasicFunctions.cs
index 3eff75a..a5c662f 100644
--- a/Upsilon/StandardLibraries/BasicFunctions.cs
+++ b/Upsilon/StandardLibraries/BasicFunctions.cs
@@ -1,9 +1,9 @@
 using System;
-using System.Collections.Generic;
 using Upsilon.BaseTypes;
 using Upsilon.BaseTypes.Number;
 using Upsilon.BaseTypes.ScriptTypeInterfaces;
 using Upsilon.Binder;
+using Upsilon.Binder.VariableSymbols;
 using Upsilon.Evaluator;
 
 // ReSharper disable UnusedMember.Global
@@ -39,19 +39,27 @@ namespace Upsilon.StandardLibraries
         }
 
         [ScriptFunction("ipairs", "Iterates over an iterable variable, like a table, until it encounters a nil value.",
-            directScriptManipulation: true)]
-        public IIterable UpTillNullPairs(IIterable table)
+            directScriptManipulation: true, overrideReturnType: typeof(BasicFunctions), overrideReturnMethod: nameof(PairsBindHandler))]
+        public UpTillNullPairsScriptIterator UpTillNullPairs(IIterable table)
         {
             return new UpTillNullPairsScriptIterator(table);
         }
 
         [ScriptFunction("pairs", "Iterates over an iterable variable, like a table, skipping all nil values.",
-            directScriptManipulation: true)]
-        public IIterable Pairs(IIterable table)
+            directScriptManipulation: true, overrideReturnType: typeof(BasicFunctions), overrideReturnMethod: nameof(PairsBindHandler))]
+        public PairsScriptIterator Pairs(IIterable table)
         {
             return new PairsScriptIterator(table);
         }
 
+        private static TypeContainer PairsBindHandler(BoundExpression[] variableSymbols)
+        {
+            if (variableSymbols.Length != 1)
+                return BaseTypes.Type.Unknown;
+            var parameter = variableSymbols[0];
+            return parameter.Type;
+        }
+
         [ScriptFunction("print", "Prints a message to the action given in the script options", passScriptReference: true,
             directScriptManipulation: true)]
         public void Print(Script script, ScriptType message)
diff --git a/Upsilon/StandardLibraries/ScriptFunctionAttribute.cs b/Upsilon/StandardLibraries/ScriptFunctionAttribute.cs
index 2b5fe70..4de231c 100644
--- a/Upsilon/StandardLibraries/ScriptFunctionAttribute.cs
+++ b/Upsilon/StandardLibraries/ScriptFunctionAttribute.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Reflection;
 
 namespace Upsilon.StandardLibraries
 {
@@ -6,13 +7,20 @@ namespace Upsilon.StandardLibraries
     public class ScriptFunctionAttribute : Attribute
     {
         public ScriptFunctionAttribute(string name, string comment = null,
-            bool directScriptManipulation = false, bool passScriptReference = false, bool passScopeReference = false)
+            bool directScriptManipulation = false, bool passScriptReference = false, bool passScopeReference = false,
+            Type overrideReturnType = null, string overrideReturnMethod = null)
         {
             Name = name;
             Comment = comment;
             DirectScriptManipulation = directScriptManipulation;
             PassScriptReference = passScriptReference;
             PassScopeReference = passScopeReference;
+            if (overrideReturnType != null && overrideReturnMethod != null)
+            {
+                var method = overrideReturnType.GetMethod(overrideReturnMethod,
+                    BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static);
+                OverrideReturnType = method;
+            }
         }
 
         public string Name { get; }
@@ -20,5 +28,6 @@ namespace Upsilon.StandardLibraries
         public bool DirectScriptManipulation { get; }
         public bool PassScriptReference { get; }
         public bool PassScopeReference { get; }
+        public MethodInfo OverrideReturnType { get; }
     }
 }
\ No newline at end of file
diff --git a/Upsilon/StandardLibraries/StaticScope.cs b/Upsilon/StandardLibraries/StaticScope.cs
index e55e56c..763596f 100644
--- a/Upsilon/StandardLibraries/StaticScope.cs
+++ b/Upsilon/StandardLibraries/StaticScope.cs
@@ -65,7 +65,7 @@ namespace Upsilon.StandardLibraries
                         var derivedType = DeriveValidTypes(typeInfo.Type);
                         return new InternalFunctionVariableSymbol.InternalFunctionParameter(func.Key, derivedType,
                             typeInfo.IsOptional);
-                    }).ToArray())
+                    }).ToArray(), func.Value.MethodInfoFunction.Method.GetMethods()[0].Attribute.OverrideReturnType)
                 {
                     CommentValue = func.Value.CommentValue?.Split('\n')
                 };
@@ -128,7 +128,7 @@ namespace Upsilon.StandardLibraries
             }
 
             var result = genericParameters[genericParameters.Length - 1].GetScriptType();
-            return new InternalFunctionVariableSymbol(name, true, result, parameters.ToArray());
+            return new InternalFunctionVariableSymbol(name, true, result, parameters.ToArray(), null);
         }
 
         private static VariableSymbol BuildActionVariableSymbol(string name, System.Type type)
@@ -136,10 +136,10 @@ namespace Upsilon.StandardLibraries
             var genericParameters = type.GetGenericArguments();
             return new InternalFunctionVariableSymbol(name, true, Type.Nil,
                 genericParameters.Select(DeriveValidTypes).Select(t =>
-                    new InternalFunctionVariableSymbol.InternalFunctionParameter(name, t, false)).ToArray());
+                    new InternalFunctionVariableSymbol.InternalFunctionParameter(name, t, false)).ToArray(), null);
         }
 
-        public static Type DeriveValidTypes(System.Type type)
+        public static TypeContainer DeriveValidTypes(System.Type type)
         {
             if (type.IsEnum)
                 return Type.UserData | Type.Number;
diff --git a/Upsilon/Text/TextSpan.cs b/Upsilon/Text/TextSpan.cs
index 744af4a..02d16ab 100644
--- a/Upsilon/Text/TextSpan.cs
+++ b/Upsilon/Text/TextSpan.cs
@@ -29,9 +29,7 @@ namespace Upsilon.Text
         {
             if (StartLine == EndLine && linePosition == StartLine)
             {
-                if (characterPosition >= StartPosition && characterPosition <= EndPosition)
-                    return true;
-                return false;
+                return characterPosition >= StartPosition && characterPosition <= EndPosition;
             }
             if (StartLine == linePosition && StartPosition <= characterPosition) return true;
             if (StartLine < linePosition && EndLine > linePosition) return true;
diff --git a/UpsilonTests/GeneralTests/ForLoopTests.cs b/UpsilonTests/GeneralTests/ForLoopTests.cs
index 2521438..a4a24ef 100644
--- a/UpsilonTests/GeneralTests/ForLoopTests.cs
+++ b/UpsilonTests/GeneralTests/ForLoopTests.cs
@@ -1,4 +1,5 @@
 using Upsilon;
+using Upsilon.BaseTypes;
 using Upsilon.Evaluator;
 using Xunit;
 
@@ -98,5 +99,21 @@ return value
             var actual = Executor.EvaluateScript<long>(input, Options);
             Assert.Equal(6, actual);
         }
+
+        [Fact]
+        public void GenericForLoopValueTypeBindTest()
+        {
+            const string input = @"
+arr = {100, 56, 28}
+for key, val in ipairs(arr) do
+    a = val
+    break
+end
+";
+            var script = Executor.ParseInputAndEvaluate(input, Options);
+            Assert.True(script.Bind().Scope.TryGetVariable("a", true, out var variable));
+            Assert.Equal(Type.Number, variable.TypeContainer.Type);
+        }
+
     }
 }
\ No newline at end of file