[Haskell-cafe] Having trouble with instance context

Chris Smith cdsmith at gmail.com
Wed Feb 23 16:18:42 CET 2011

On Wed, 2011-02-23 at 08:42 -0600, Kurt Stutsman wrote:
> When I was reviewing the Haskell language specification on haskell.org, 
> it certainly looked like what I was doing was supported by the language. 
> I found some comments on GHC's site about the reasoning behind these 
> flags, but I couldn't tell if they were restrictions GHC had added to 
> their implementation or if they derive from the language spec. Is this 
> kind of instance allowed by the spec?

FlexibleInstances and UndecidableInstances are language extensions that
relax the rules in the Haskell Report.  If they are needed, then the
instances you are writing are not allowed by the spec.


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