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

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Robin Green wrote:

On Tue, 29 May 2007 19:28:02 -0400 Isaac Dupree 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGXXryHgcxvIWYTTURAnEeAJ9PrQUQLxeoTuIhaG8GcHW5mN6T4QCeL6FT KCQeF43ye/GzLka4zFUK66s= =y7MZ -----END PGP SIGNATURE-----