[Haskell-cafe] alex + happy parser problem

[Sean McLaughlin]

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

}