On Wednesday 13 February 2002 06:36 am, C.Reinke wrote:
Does anybody with their elbows in the code think variable interpolation and/or multi-line strings are good/doable ideas? Would this be the sort of change one could make without a lot of previous familiarity with the implementation of Hugs/Ghc?
Unlike my rough proposal, one should aim for a combination of (mostly) in-Haskell implementation and (some) pre-processing. As Thomas Nordin has pointed out to me in private email, Hugs (Dec 2001) already supports this (module lib/hugs/Quote.hs and flag +H).
The real trick is to have the same support in all implementations..
I use here docs quite a bit. They are wonderful for writing error messages that are also readable in the code ;-) The point about same support in all implementations is of course a good one. Thomas and I are the culprits who put here docs in hugs in the first place. However, it is just as easy to support here docs using a pre-processor. I have a medium sized project that uses here docs, and can be used under both hugs and ghc. With hugs, I use the builtin feature, of course. With GHC, we just use a pre-processor. This is a bit awkward with GHC 5.02 and earlier versions, but starting with 5.03, GHC now has a proper interface for hooking in a pre-processor (don't know the details, bug Sigbjorn says it's in there). A convenient feature of here docs that makes it easy to implement as a pre-processor is that you can do the unhere doc translation so that it preserves line numbers. The only drawback to using a pre-processor is that it probably won't work with ghci (but then you probably don't need to write here docs at the command line either!). --Jeff