Functor (Either a)

Bryan O'Sullivan bos at
Sun Jul 15 18:16:17 EDT 2007

Ian Lynagh wrote:

> It's in Control.Monad.Instances:
> Prelude> :m + Control.Monad.Instances
> Prelude Control.Monad.Instances> fmap not (Right True)
> Right False
> I don't know if there's a way of deducing that from the haddock
> documentation.

There doesn't seem to be.  I didn't discover that module until I finally 
sat down and spent 15 minutes trying to answer the question "why is 
importing, or omitting, some completely unrelated mtl module influencing 
my program's ability to compile?"

Wonderful as haddock is, there's no substitute for having the GHC 
sources and half of hackage unpacked on your hard disk, to make possible 
a few rounds of "grep -r".


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