[Haskell-cafe] Multi-parameter Type Class[MESSAGE NOT SCANNED]

[Daniel Fischer]

Am Donnerstag, 11. Dezember 2008 16:52 schrieb Paul Keir:
> I took your suggestion and it worked exactly as I had hoped. Thankyou.
>
> GHCI (6.8.2) was though a little concerned, and told me
> I had an: Illegal instance declaration for `ZOT [x] [y]'
> and recommended I use -fallow-undecidable-instances. I did,
> and it worked. What have I done though? The word "undecidable"
> scares me a little :)
>
> ---SNIP---

It's not dangerous. ghci told you that the Coverage Condition failed for the 
functional dependency, read section 8.6.3 of the user's guide for more info, 
so instance inference is not guaranteed to terminate, that's what 
'undecidable' means. The flag says go ahead and try until either the question 
is decided or the recursion stack is exhausted, whichever happens first.