On Thu, Oct 27, 2005 at 07:06:12PM +0100, Joel Reymont wrote:
This requires
{-# OPTIONS_GHC -fallow-undecidable-instances #-}
but since I'm using -fglasgow-exts in a lot of places I'm wondering if adding undecidable instances would be a bad habit. I guess not... not until I shoot myself in the foot :-).
I would avoid them if at all possible. especially if you are still learning haskell, I have found they often hide real errors in ones design of a class hierarchy by making something compile that shouldn't. I had a particularly nasty bug in my regex library when I only had one operator (=~) that was not being caught because of undecidable instances being allowed, when I turned them off, I thought about the problem again and realized that separate monadic (=~~) and non monadic (=~) versions not only solved the issue but was actually a much nicer design with much clearer semantics. John -- John Meacham - ⑆repetae.net⑆john⑈