diff --git a/AST.cs b/AST.cs
index d9d85b1..1b2e2bb 100644
--- a/AST.cs
+++ b/AST.cs
@@ -42,9 +42,9 @@ public partial record SimplePattern
 	}
 }
 
-public abstract record Binding(Expr Value);
-public record VarBinding(Pattern Pattern, Expr Value) : Binding(Value);
-public record FuncBinding(Token Name, Pattern[] Params, Expr Value) : Binding(Value)
+public abstract record Binding(Expr Value, Token Start);
+public record VarBinding(Pattern Pattern, Expr Value) : Binding(Value, Pattern.Start);
+public record FuncBinding(Token Name, Pattern[] Params, Expr Value) : Binding(Value, Name)
 {
 	public override string ToString()
 	{
diff --git a/Interpreter.cs b/Interpreter.cs
index adb19b7..a1ba45a 100644
--- a/Interpreter.cs
+++ b/Interpreter.cs
@@ -38,7 +38,6 @@ public class Env
 			var name = (string)identifier.Literal!;
 			if (values.ContainsKey(name))
 			{
-				// TODO use real location info
 				throw new RuntimeError(identifier, $"Cannot redefine variable {name} in same scope.");
 			}
 			values[name] = value;
@@ -56,7 +55,6 @@ public class Env
 				{
 					return enclosing[identifier];
 				}
-				// TODO use real location info
 				throw new RuntimeError(identifier, $"Undefined variable {name}.");
 			}
 		}
@@ -75,12 +73,19 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 		}
 	}
 
-	private class PatternTagMismatchException : Exception
+	private class PatternMismatchException : Exception
 	{
-		public PatternTagMismatchException(string patternTag, string valueTag) : base($"Pattern tag {patternTag} does not match value tag {valueTag}.") { }
+		public readonly Token Start;
+		public PatternMismatchException(Token start, string message) : base(message)
+		{
+			Start = start;
+		}
 	}
 
-	private class PatternTypeMismatchException : Exception { }
+	private class VariantTagMismatchException : PatternMismatchException
+	{
+		public VariantTagMismatchException(Token start, string patternTag, string valueTag) : base(start, $"Pattern tag {patternTag} does not match value tag {valueTag}.") { }
+	}
 
 	class PatternBinder : AST.IPatternVisitor<(object, Env), ValueTuple>
 	{
@@ -110,7 +115,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 					}
 					return ValueTuple.Create();
 			}
-			throw new PatternTypeMismatchException();
+			throw new PatternMismatchException(pattern.Start, "Matched value {obj} is not a record.");
 		}
 
 		public ValueTuple visitSimplePatternPattern((object, Env) context, AST.SimplePattern pattern)
@@ -133,7 +138,7 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 					var tag = (string)pattern.Tag.Literal!;
 					if (v.Tag != tag)
 					{
-						throw new PatternTagMismatchException(tag, v.Tag);
+						throw new VariantTagMismatchException(pattern.Start, tag, v.Tag);
 					}
 					if (v.Value == null && pattern.Argument == null)
 					{
@@ -143,10 +148,9 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 					{
 						return pattern.Argument.accept((v.Value, env), this);
 					}
-					throw new PatternTypeMismatchException();
+					throw new PatternMismatchException(pattern.Start, "Variant pattern arity does not match variant value.");
 			}
-			// TODO throw a better exception
-			throw new Exception($"Not a variant.");
+			throw new PatternMismatchException(pattern.Start, $"Matched value {obj} is not a variant.");
 		}
 	}
 
@@ -448,9 +452,10 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 					}
 					catch (Exception e)
 					{
-						// TODO use real info
-						var tok = new Token(TokenType.Let, "let", null, new(0, 1, 1), new(0, 1, 1));
-						throw new RuntimeError(tok, e.Message);
+						var start = e is PatternMismatchException ?
+							((PatternMismatchException)e).Start :
+							binding.Start;
+						throw new RuntimeError(start, e.Message);
 					}
 					break;
 				default:
@@ -593,8 +598,6 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 	public object visitWhenExpr(Env env, AST.When expr)
 	{
 		var head = evaluate(env, expr.Head);
-		// TODO use real info
-		var tok = new Token(TokenType.When, "when", null, new(0, 1, 1), new(0, 1, 1));
 		foreach (var c in expr.Cases)
 		{
 			try
@@ -603,15 +606,15 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 				c.Pattern.accept((head, newEnv), new PatternBinder());
 				return evaluate(newEnv, c.Value);
 			}
-			catch (PatternTagMismatchException)
+			catch (VariantTagMismatchException)
 			{
 				continue;
 			}
-			catch (PatternTypeMismatchException e)
+			catch (PatternMismatchException e)
 			{
-				throw new RuntimeError(tok, e.Message);
+				throw new RuntimeError(e.Start, e.Message);
 			}
 		}
-		throw new RuntimeError(tok, "No matching patterns.");
+		throw new RuntimeError(expr.Start, "No matching patterns.");
 	}
 }
\ No newline at end of file