On 9/7/10 3:10 PM, Ben Millwood wrote:
So I wonder what people think of the use of CPP in Haskell code, what alternatives people can propose, or what people hope to see in future to make conditional compilation of Haskell code more elegant and simple?
The only thing I ever use CPP for in Haskell is top-level conditional definitions.
* That is, say I have a function foo which has a vanilla Haskell definition, but also has a special definition for GHC performance hackery, or which needs a special definition on some compilers in order to correct compiler-specific bugs. I'll use #ifdef here to give the different versions. I'll also occasionally do this for things like choosing whether to use the FFI vs a native definition, for debugging purposes.
* Another example is when using GeneralizedNewtypeDeriving in GHC, but still wanting to give normal definitions for other compilers to use.
* The only other example I can think of is when defining Applicative instances, since I only want to do that when linking against versions of base which are new enough to have it. Occasionally you can get similar issues re ByteString vs base.
In general, I think using CPP for actual macro processing is extremely poor style and can easily make code inscrutable (and no doubt bug-prone). If the Haskell spec were to add support for this sort of top-level compiler/compiletime-flag conditional definition, I'd switch over.
This matches the style in most of the code I've looked at. And it also means that the incompatibilities are localized and hidden from most client code. Depending on the nature of your library API conflict, if you can localize things into a few definitions of the core functions you use in the rest of your code, then that'd be best. But that's not always possible. I've yet to run into the case where I really need to support incompatible versions of a library when it's that closely integrated, so I don't have much advice there.
-- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
