[Haskell-cafe] a novice Alex question

[ivan gomez rodriguez]

Robert Dockins wrote:
>
> 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
>
>
>
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>
I think that you also need add a token definition like :

eof  {\s -> EOFToken}