[Haskell-cafe] Alex problem, help needed!!!!!!!!! ASAP :)

[Don Stewart]

del_31416no:
> Dear list,
>  
> I am trying to make a compiler and we are having a hard time getting Alex to
> work. We have succeded to work out Alex using older version, but with the the
> 2.2 version we keep getting this error and we havent been able to figure it
> out.
>  
> So this is our tokens definition:
>  
> {
>    module Lexico where
>     import Alex
> }
>  
> %wrapper "posn"
>  
> $digit = 0-9 -- digits
> $alpha = [a-zA-Z] -- alphabetic characters
>  
> tokens :-
>  
> $white+ ;                            --Los espacios en blanco los omito
> \/\/.* ;                               --Lo que venga despu s de dos barras
> omito
> \/\*.*\*\/ ;                         --Lo que est  entre las barras de
> comentario omito
> $digit+                                 { \p s -> TokenEntero p (read s) }
> \' [$alpha $digit \_]* \'            { \p s -> TokenString p (read s)}
> [$digit]+\.[$digit]+                 { \p s -> TokenDouble p (read s) }
> $alpha [$alpha $digit \_]*        { \p s -> TokenVar p (read s) }
>  
>  
> And when we call the alexScanTokens "hello" we get this error:
>  
> [TokenVar (AlexPn 0 1 1) "*** Exception: Prelude.read: no parse
>  
> So we are concerned about the "TokenVar p (read s)" . Would that be the way to
> read a string?

Use readMaybe rather than read, and check the error.

        maybeRead :: Read a => String -> Just a
        maybeRead s = case reads s of
                [(x, s')] | all isSpace s' -> Just x
                _                          -> Nothing