[Haskell-cafe] Defining instance needs allow-undecidable-instance?

Daniel McAllansmith dagda at xtra.co.nz
Mon Mar 27 21:25:51 EST 2006

On Tuesday 28 March 2006 11:12, Daniel McAllansmith wrote:
> Hi, folks.
> I've got a working class and instances thereof.  I would now like to change
> the class such that error behaviour is specified by MonadError, for the
> moment throwing String errors.  When I try to add MonadError into the types
> I eventually hit the requirement for allow-undecidable-instances.
> Is there some way I can I avoid having to use this extension?

I've received a link[1] off-list to a patch to GHC changing this behaviour, so 
it looks like in the future I'll be able to remove the 
allow-undecidable-instances flag and rely on just glasgow-exts.

> instance (Num i, Bounded i, Monad m, MonadError String m)
>         => MonadSource i (SourceT i m) where
>     ...

That aside, is what I have done, including a type in the context, considered 
an acceptable solution for this sort of problem?


[1] http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/13500

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