diff --git a/Expr.g.cs b/Expr.g.cs
index ffd5a41..963fd75 100644
--- a/Expr.g.cs
+++ b/Expr.g.cs
@@ -125,7 +125,7 @@ public partial record Let(Binding[] Bindings, Expr Body) : Expr()
 		return visitor.visitLetExpr(context, this);
 	}
 }
-public partial record When(Expr Head, Binding[] Cases) : Expr()
+public partial record When(Expr Head, VarBinding[] Cases) : Expr()
 {
 	public override TResult accept<TContext, TResult>(TContext context, IExprVisitor<TContext, TResult> visitor)
 	{
diff --git a/Interpreter.cs b/Interpreter.cs
index fd90310..d912686 100644
--- a/Interpreter.cs
+++ b/Interpreter.cs
@@ -595,6 +595,26 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitWhenExpr(Env env, AST.When expr)
 	{
-		throw new System.NotImplementedException();
+		var head = evaluate(env, expr.Head);
+		// TODO use real info
+		var tok = new Token(TokenType.When, "when", null, 1);
+		foreach (var c in expr.Cases)
+		{
+			try
+			{
+				var newEnv = new Env(env);
+				c.Pattern.accept((head, newEnv), new PatternBinder());
+				return evaluate(newEnv, c.Value);
+			}
+			catch (PatternTagMismatchException)
+			{
+				continue;
+			}
+			catch (PatternTypeMismatchException e)
+			{
+				throw new RuntimeError(tok, e.Message);
+			}
+		}
+		throw new RuntimeError(tok, "No matching patterns.");
 	}
 }
\ No newline at end of file
diff --git a/Parser.cs b/Parser.cs
index 6ab195a..0b75ffc 100644
--- a/Parser.cs
+++ b/Parser.cs
@@ -308,7 +308,7 @@ class Parser
 		Expr head = expression();
 		consume(TokenType.Is, "Expect 'is' after expression.");
 
-		List<Binding> cases = new List<Binding>();
+		List<VarBinding> cases = new List<VarBinding>();
 		cases.Add(parseCase());
 		while (match(TokenType.Comma))
 		{
@@ -316,7 +316,7 @@ class Parser
 		}
 		return new When(head, cases.ToArray());
 
-		Binding parseCase()
+		VarBinding parseCase()
 		{
 			Pattern pat = pattern();
 			consume(TokenType.DoubleArrow, "Expect '=>' after pattern.");
diff --git a/ast_classes.fsx b/ast_classes.fsx
index 170afb3..d6c7c19 100644
--- a/ast_classes.fsx
+++ b/ast_classes.fsx
@@ -45,7 +45,7 @@ let exprTypes =
               Name = "Bindings" }
             { Type = "Expr"; Name = "Body" } ] }
       { Name = "When"
-        Fields = [ { Type = "Expr"; Name = "Head" }; { Type = "Binding[]"; Name = "Cases" } ] } ]
+        Fields = [ { Type = "Expr"; Name = "Head" }; { Type = "VarBinding[]"; Name = "Cases" } ] } ]
 
 let patternTypes =
     [ { Name = "SimplePattern"