diff --git a/Expr.g.cs b/Expr.g.cs
index 91b55a1..398355e 100644
--- a/Expr.g.cs
+++ b/Expr.g.cs
@@ -111,7 +111,7 @@ public partial record Indexer(Expr Left, Expr Index) : Expr(Left.Start)
 		return visitor.visitIndexerExpr(context, this);
 	}
 }
-public partial record Call(Expr Left, Expr?[] Arguments) : Expr(Left.Start)
+public partial record Call(Expr Left, Expr?[] Arguments, Token LParen) : Expr(Left.Start)
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
diff --git a/Interpreter.cs b/Interpreter.cs
index 6edb57d..bed3074 100644
--- a/Interpreter.cs
+++ b/Interpreter.cs
@@ -1,8 +1,11 @@
 using System;
+using System.Linq;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 
 using AST = Finn.AST;
+using System.Runtime.CompilerServices;
+using System.Net;
 
 namespace Finn;
 
@@ -23,7 +26,7 @@ public class Env
 
 	public Env()
 	{
-		this.enclosing = null;
+		enclosing = null;
 	}
 
 	public Env(Env enclosing)
@@ -31,17 +34,18 @@ public class Env
 		this.enclosing = enclosing;
 	}
 
-	public object this[Token identifier]
+	public object this[string name]
 	{
 		set
 		{
-			var name = (string)identifier.Literal!;
-			if (values.ContainsKey(name))
-			{
-				throw new RuntimeError(identifier, $"Cannot redefine variable {name} in same scope.");
-			}
+			// Redefinition within the same scope should never happen, but front
+			// end should catch it.
 			values[name] = value;
 		}
+	}
+
+	public object this[Token identifier]
+	{
 		get
 		{
 			var name = (string)identifier.Literal!;
@@ -61,6 +65,29 @@ public class Env
 	}
 }
 
+public interface Callable
+{
+	object Call(Interpreter interpreter, object[] arguments);
+}
+
+public record NativeFunction(Func<object[], object> Function) : Callable
+{
+	public object Call(Interpreter interpreter, object[] arguments)
+	{
+		return this.Function(arguments);
+	}
+
+	public override string ToString() => "<native func>";
+}
+
+public class UserFunction : Callable
+{
+	public object Call(Interpreter interpreter, object[] arguments)
+	{
+		throw new NotImplementedException();
+	}
+}
+
 public class Interpreter : AST.IExprVisitor<Env, object>
 {
 	private class PatternMismatchException : Exception
@@ -115,7 +142,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			{
 				return ValueTuple.Create();
 			}
-			env[pattern.Identifier] = obj;
+			env[(string)pattern.Identifier.Literal!] = obj;
 			return ValueTuple.Create();
 		}
 
@@ -272,11 +299,21 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 		}
 	}
 
+	private readonly Env globals = new Env();
+
+	public Interpreter()
+	{
+		globals["clock"] = new NativeFunction((args) =>
+		{
+			return (double)DateTimeOffset.Now.ToUnixTimeSeconds();
+		});
+	}
+
 	public void Interpret(AST.Expr expression)
 	{
 		try
 		{
-			var value = evaluate(new Env(), expression);
+			var value = evaluate(globals, expression);
 			Console.WriteLine(value);
 		}
 		catch (RuntimeError err)
@@ -387,7 +424,19 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitCallExpr(Env env, AST.Call expr)
 	{
-		throw new System.NotImplementedException();
+		var callee = evaluate(env, expr.Left);
+		object[] args = new object[expr.Arguments.Length];
+		for (int i = 0; i < args.Length; i++)
+		{
+			var argExpr = expr.Arguments[i];
+			if (argExpr is null)
+			{
+				throw new NotImplementedException("partial application not implemented");
+			}
+			args[i] = evaluate(env, argExpr);
+		}
+		Callable function = (Callable)callee;
+		return function.Call(this, args);
 	}
 
 	public object visitGroupingExpr(Env env, AST.Grouping expr)
diff --git a/Parser.cs b/Parser.cs
index 7d24687..0bfff10 100644
--- a/Parser.cs
+++ b/Parser.cs
@@ -335,29 +335,41 @@ class Parser
 
 		if (match(TokenType.LParen))
 		{
-			var args = new List<Expr?>();
-			while (!check(TokenType.RParen))
-			{
-				if (match(TokenType.Blank))
-				{
-					args.Add(null);
-				}
-				else
-				{
-					args.Add(expression());
-				}
-				if (!match(TokenType.Comma))
-				{
-					break;
-				}
-			}
-			consume("Expect ')' after arguments.", TokenType.RParen);
-			return new Call(expr, args.ToArray());
+			return finishCall(expr);
 		}
 
 		return expr;
 	}
 
+	private Expr finishCall(Expr callee)
+	{
+		var lParen = previous();
+		var args = new List<Expr?>();
+		while (!check(TokenType.RParen))
+		{
+			if (match(TokenType.Blank))
+			{
+				args.Add(null);
+			}
+			else
+			{
+				args.Add(expression());
+			}
+			if (!match(TokenType.Comma))
+			{
+				break;
+			}
+		}
+		consume("Expect ')' after arguments.", TokenType.RParen);
+
+		const int MaxArgs = 25;
+		if (args.Count > MaxArgs)
+		{
+			error(peek(), $"Can't have more than {MaxArgs} arguments.");
+		}
+		return new Call(callee, args.ToArray(), lParen);
+	}
+
 	private Expr operand()
 	{
 		if (match(TokenType.Number, TokenType.String))
diff --git a/TODO.txt b/TODO.txt
index 5106079..359602a 100644
--- a/TODO.txt
+++ b/TODO.txt
@@ -1,2 +1,8 @@
+Implement Finn functions
 Figure out multiple-binding let-expr semantics
-Inject error handling into parser and scanner
\ No newline at end of file
+Inject error handling into parser and scanner
+Think about a way to hide record fields
+	- Maybe a compile-time primitive for generating globally unique symbols?
+		Would also need a syntax to use those symbols.
+	- Use @ as an operator to convert a comptime-known string to an identifier
+Replace `with` keyword with `but`
\ No newline at end of file
diff --git a/ast_classes.fsx b/ast_classes.fsx
index 151153b..aa49c4f 100644
--- a/ast_classes.fsx
+++ b/ast_classes.fsx
@@ -1,5 +1,9 @@
 type Field = { Type: string; Name: string }
-type Type = { Name: string; Fields: list<Field>; Start: string }
+
+type Type =
+    { Name: string
+      Fields: list<Field>
+      Start: string }
 
 let exprTypes: Type list =
     [ { Name = "Sequence"
@@ -12,13 +16,14 @@ let exprTypes: Type list =
             { Type = "Expr"; Name = "Right" } ]
         Start = "Left.Start" }
       { Name = "Grouping"
-        Fields = [ { Type = "Expr"; Name = "Expression" } ;{Type="Token";Name="LParen"}]
-        Start = "LParen"}
+        Fields = [ { Type = "Expr"; Name = "Expression" }; { Type = "Token"; Name = "LParen" } ]
+        Start = "LParen" }
       { Name = "Literal"
         Fields =
           [ { Type = "System.Object"
-              Name = "Value" }; {Type="Token"; Name="Token"} ]
-        Start = "Token"}
+              Name = "Value" }
+            { Type = "Token"; Name = "Token" } ]
+        Start = "Token" }
       { Name = "Unary"
         Fields = [ { Type = "Token"; Name = "Op" }; { Type = "Expr"; Name = "Right" } ]
         Start = "Op" }
@@ -33,18 +38,23 @@ let exprTypes: Type list =
         Fields = [ { Type = "Token"; Name = "Value" } ]
         Start = "Value" }
       { Name = "List"
-        Fields = [ { Type = "Expr[]"; Name = "Elements" }; {Type="Token"; Name = "LBracket"} ]
+        Fields =
+          [ { Type = "Expr[]"; Name = "Elements" }
+            { Type = "Token"; Name = "LBracket" } ]
         Start = "LBracket" }
       { Name = "Variant"
-        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Expr?"; Name = "Argument" }; {Type="Token";Name="Backtick"}]
+        Fields =
+          [ { Type = "Token"; Name = "Tag" }
+            { Type = "Expr?"; Name = "Argument" }
+            { Type = "Token"; Name = "Backtick" } ]
         Start = "Backtick" }
       { Name = "Record"
         Fields =
           [ { Type = "Field[]"
               Name = "Extensions" }
             { Type = "BaseRecord?"; Name = "Base" }
-            { Type = "Token"; Name= "LBrace"} ]
-        Start = "LBrace"}
+            { Type = "Token"; Name = "LBrace" } ]
+        Start = "LBrace" }
       { Name = "Selector"
         Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Token"; Name = "FieldName" } ]
         Start = "Left.Start" }
@@ -52,29 +62,35 @@ let exprTypes: Type list =
         Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Index" } ]
         Start = "Left.Start" }
       { Name = "Call"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr?[]"; Name = "Arguments" } ]
+        Fields =
+          [ { Type = "Expr"; Name = "Left" }
+            { Type = "Expr?[]"; Name = "Arguments" }
+            { Type = "Token"; Name = "LParen" } ]
         Start = "Left.Start" }
       { Name = "Let"
         Fields =
           [ { Type = "Binding[]"
               Name = "Bindings" }
             { Type = "Expr"; Name = "Body" }
-            { Type = "Token"; Name = "LetToken"} ]
+            { Type = "Token"; Name = "LetToken" } ]
         Start = "LetToken" }
       { Name = "When"
         Fields =
           [ { Type = "Expr"; Name = "Head" }
             { Type = "VarBinding[]"
               Name = "Cases" }
-            { Type = "Token"; Name = "WhenToken"} ]
+            { Type = "Token"; Name = "WhenToken" } ]
         Start = "WhenToken" } ]
 
 let patternTypes =
     [ { Name = "SimplePattern"
-        Fields = [ { Type = "Token?"; Name = "Identifier" }; {Type = "Token"; Name = "Token"}]
+        Fields = [ { Type = "Token?"; Name = "Identifier" }; { Type = "Token"; Name = "Token" } ]
         Start = "Token" }
       { Name = "VariantPattern"
-        Fields = [ { Type = "Token"; Name = "Tag" }; { Type = "Pattern?"; Name = "Argument" }; {Type="Token"; Name = "Backtick"} ]
+        Fields =
+          [ { Type = "Token"; Name = "Tag" }
+            { Type = "Pattern?"; Name = "Argument" }
+            { Type = "Token"; Name = "Backtick" } ]
         Start = "Backtick" }
       { Name = "FieldPattern"
         Fields = [ { Type = "Token"; Name = "Name" }; { Type = "Pattern?"; Name = "Pattern" } ]
@@ -85,8 +101,7 @@ let patternTypes =
               Name = "Fields" }
             { Type = "SimplePattern?"
               Name = "Rest" }
-            { Type = "Token"
-              Name = "LBrace"} ]
+            { Type = "Token"; Name = "LBrace" } ]
         Start = "LBrace" } ]
 
 let visitorMethod baseName t = $"visit{t.Name}{baseName}"