Hi, To learn alex and happy, I'm trying to write a parser for a simple expression language. When I wrote my own lexer and just used happy, it was fine. When I used the basic wrapper of alex it was also fine. However, when I use the posn wrapper to get position information, I get a strange exception when the parse error occurs at the end of the input. For example, parsing "1 + " yields "Internal Happy error" rather than something like "Parse error at line 1, column 5" The lexer and parser are attached. Can anyone see what I'm doing wrong? calling parse "1+" yields a "Internal Happy error" instead of a parse error as I would expect. Thanks, Sean ------------------ -- Lexer ------------------ { module ExprLexer ( Token(..), AlexPosn(..), alexScanTokens, tokenPosn ) where } %wrapper "posn" $digit = 0-9 tokens :- $digit+ { (\p s -> Int p (read s)) } [\+] { (\p s -> Sym p (head s)) } { data Token = Sym AlexPosn Char | Int AlexPosn Int deriving (Eq, Show) tokenPosn (Sym p _) = p tokenPosn (Int p _) = p } -------------------------- --- Parser -------------------------- { module ExprParser where import ExprLexer (Token(..), alexScanTokens, tokenPosn, AlexPosn(..)) } %name parseExp %tokentype { Token } %token int { Int _ $$ } '+' { Sym _ '+' } %right '+' %% Exp : Exp '+' Exp { Add $1 $3 } | int { Const $1 } { data Expr = Const Int | Add Expr Expr deriving Show parse :: String -> Expr parse = parseExp . alexScanTokens happyError :: [Token] -> a happyError tks = error ("Parse error at " ++ lcn ++ "\n") where lcn = case tks of [] -> "end of file" tk:_ -> "line " ++ show l ++ ", column " ++ show c where AlexPn _ l c = tokenPosn tk }