On Thu, May 20, 2004 at 07:34:18PM +0100, MR K P SCHUPKE wrote:

that is not the case with "-fallow-undecidable-instances" ... as far as I understand it , ghc never considers the dependancies when selecting an instance. If you don't think so you will need to show me an example where it clearly does... as I haven't seen one yet (but just because I haven't seen it doesn't mean its not possible),,,

Am I to understand you're asserting that GHC "undecidable-instances" are in fact decidable? Hugs explicitly documents its type system as being undecidable, and I'd presumed this was the 'make GHC behave that way too' flag.

I mean, given two instances like:

instance a b c instance d e f

there is no way to tell between them... if you said choose a in preferenc 'a' would be chosen all the time.

If you were to indicate one instance declaration was preferred, and both _could otherwise yield a suitable instance_, then that would be prefered. (Though if one were to 'search' in the preference order of the rules, one wouldn't need to find more than one in any event.)