[Haskell-cafe] Re: Frisby grammars that have context

[Isaac Dupree]

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Robin Green wrote:
> On Tue, 29 May 2007 19:28:02 -0400
> Isaac Dupree <isaacdupree at charter.net> wrote:
>> Luckily, Haskell's laziness means that doing an extra "postprocessing
>> pass" doesn't necessarily yield two traversals requiring the whole
>> file to be stored in memory, nor worse hacks.  (For grammars that
>> aren't too wild / sequential)
> 
> But the suggested code fragment on the frisby homepage:
> 
>   -- parse complete file, returning 'Nothing' if parse fails
>   fmap Just (myParser <<- eof) // unit Nothing
> 
> does require one traversal of the file all by itself. Obviously, in
> order to know whether the file was fully parsed without error, you need
> to read in the whole file, before you can write out anything. Hence
> you end up with *some* representation of the whole file in memory. So,
> yes, it doesn't necessarily yield two traversals, but you need to be
> careful if you want to avoid two traversals.

Yes, then the choices are being failable (using something like "error",
or whatever happens if you don't wrap your parser as suggested)
or better yet, a careful lazy datatype like
data ListOutput a = Nil | Cons a (ListOutput a) | Error (ErrorInfo)

Isaac
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