[Haskell-cafe] a novice Alex question

[Robert Dockins]

On Aug 25, 2006, at 6:27 AM, Xiong Yingfei wrote:

> Hi,
>
> I am trying out Alex. I copied the calculator specification file  
> from Alex's official document and changed the wrapper type from  
> "basic" to "monad". However, after I generated the ".hs" file from  
> the lexical specification and compiled the ".hs" file, I got the  
> message "Variable not in scope: `alexEOF'". I cannot find  
> explanation about this 'alexEOF' function in the document. Can any  
> body be kindly enough to tell me what this function is? Should I  
> write it myself or not? My lexical code is listed as the below.  
> Thanks a lot.

You should provide alexEOF.  The idea is that it is a special token  
representing the end of input.  This is necessary because the monad  
wrapper doesn't deliver a list of tokens like the basic wrapper, so  
it needs some way to signal the end of input.  The easiest thing to  
do is add a constructor to your token datatype, and then just set  
alexEOF to that constructor:

data Token =
    ....
    | EOFToken


alexEOF = EOFToken




> {
> module Lex where
>
> }
>
> %wrapper "monad"
>
> $digit = 0-9   -- digits
> $alpha = [a-zA-Z]  -- alphabetic characters
>
> tokens :-
>
>   $white+    ;
>   "--".*    ;
>   let     { \s -> Let }
>   in     { \s -> In }
>   $digit+    { \s -> Int (read s) }
>   [\=\+\-\*\/\(\)]   { \s -> Sym (head s) }
>   $alpha [$alpha $digit \_ \']*  { \s -> Var s }
>
> {
> -- Each action has type :: String -> Token
>
> -- The token type:
> data Token =
>  Let   |
>  In    |
>  Sym Char |
>  Var String |
>  Int Int
>  deriving (Eq,Show)
> }
>
> --
> Xiong, Yingfei (熊英飞)
> Ph.D. Student
> Institute of Software
> School of Electronics Engineering and Computer Science
> Peking University
> Beijing, 100871, PRC.
> Web: http:// 
> xiong.yingfei.googlepages.com_________________________________________ 
> ______


Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG