[Haskell-cafe] Either Monad and Laziness

[Eric Velten de Melo]

Thanks for all the tips! The iteratees seem worth checking out. I'll
see what I can do and will report back if I come up with something.

Eric

On 12 September 2012 03:03,  <oleg at okmij.org> wrote:
>
>> I am currently trying to rewrite the Graphics.Pgm library from hackage
>> to parse the PGM to a lazy array.
>
> Laziness and IO really do not mix.
>
>> The problem is that even using a lazy array structure, because the
>> parser returns an Either structure it is only possible to know if the
>> parser was successful or not after the whole file is read,
>
> That is one of the problems. Unexpected memory blowups could be
> another problem. The drawbacks of lazy IO are well documented by now.
>
>> The behaviour I want to achieve is like this: I want the program when
>> compiled to read from a file, parsing the PGM and at the same time
>> apply transformations to the entries as they are read and write them
>> back to another PGM file.
>
> Such problems are the main motivation for iteratees, conduits, pipes,
> etc. Every such library contains procedures for doing exactly what you
> want. Please check Hackage. John Lato's iteratee library, for example,
> has procedure for handling sound (AIFF) files -- which may be very
> big. IterateeM has the TIFF decoder -- which is incremental and
> strict. TIFF is much harder to parse than PGM.
>
>