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[Ashley Yakeley]

At 2002-12-06 15:57, Wolfgang Jeltsch wrote:

>I cannot see how this relatively simple representation can be used to 
>describe 
>certain parsers. If I would use Identity for baseMonad, I would have a type 
>for simple parsers. MyParser Identity would essentially be token -> output. 

You can't use Identity for baseMonad. You would need to use something 
that managed a stream. The idea is that your source is something of type 
"baseMonad token", and by repeatedly calling source, you pull off 
individual tokens.

>Which function of this type should be used to describe, for instance,
>    (a) a parser reading no tokens and returning ()

  MkMyParser (\_ -> return ())

>    (b) a parser reading two tokens and returning the pair of these tokens?

  MkMyParser (\source -> do
    {
    a <- source;
    b <- source;
    return (a,b);
    })

You can then use parsers of type MyParser with all kinds of sources, such 
as those that read items from a file, or a state transformer that pops 
items off a list, or whatever.

-- 
Ashley Yakeley, Seattle WA