[Haskell-cafe] Either as a Monad instance

Brian Hulley brianh at metamilk.com
Tue Oct 3 10:45:13 EDT 2006

Ross Paterson wrote:
> On Tue, Oct 03, 2006 at 07:52:54AM -0400, Lennart Augustsson wrote:
>> Yes, having fail in the Monad is a horrible wart.
> (As far as I know, it's there for the translation of pattern match
> failure in do-expressions.)

I think it would be much less of a wart if the signature for fail was just:

    fail :: m a

because what's the point of the String argument anyway (other than trying to 
hack a kind of half-baked debug facility into the language)?

Regards, Brian.
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.


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