neil:

Hi,

Firstly my apologies if this is an outrageously newbie question.

I am trying to write a binary protocol parser using Data.ByteString. I have created a module "ByteParser" containing my parsing utilities, which imports ByteString as:

import qualified Data.ByteString as B

In my Main module, where all the IO happens, I want to use lazy ByteStrings so I ask for the following imports:

import ByteParser import qualified Data.ByteString.Lazy as L

The problem is that when I try to call a function in ByteParser with an L.ByteString, the compiler complains that:

Couldn't match expected type `Data.ByteString.Base.ByteString' against inferred type `L.ByteString'

Does this mean that the lazy version of ByteString is a completely separate, incompatible type from the base version? Obviously I can work

Yes, they're completely different. One is a strict, unboxed array of bytes. The other is a lazy list of cache-sized array chunks. You can convert between the two. But usually its best to pick one and stick to it.

around this problem by making ByteParser import Data.ByteString.Lazy instead of Data.ByteString, but then ByteParser would not be able to work with strict ByteStrings. Ideally I would like ByteParser to be agnostic about the use of lazy versus strict ByteStrings. Is this a sensible and/or possible thing to do?

You could make your binary parser polymorphic on the string type, and used a String class that both bytestrings are instances of. -- Don