Hi, that's a bug in GHC---it erroneously accepts polymorphic instances which violate the FD of a class. -Iavor On Fri, Mar 18, 2011 at 7:08 AM, Daniel Fischer < daniel.is.fischer@googlemail.com> wrote:
On Friday 18 March 2011 14:40:40, JP Moresmau wrote:
Thanks to you all, I think I understand better. instance Search Id Id where search _ _ i = only (FoundId i)
Is too restrictive on the first type, so declaring instead: instance Search id Id where search _ _ i = only (FoundId i)
Not sure what GHC does with that, but at least in spirit that violates the FunDep of
class Search id a | a -> id where ...
Even if it works now, it may well not work in the future. I'd go for a more stable solution respecting the intent of FunDeps (i.e. there should only be one type t with an instance Search t Id).
Fixed the issue!! Now the initial "id" is not Id and everybody is happy (and the code still seems to work as intended)
thanks again
JP
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