Re: [Haskell-cafe] Why were unfailable patterns removed and "fail" added to Monad?

On Fri, Jan 20, 2012 at 6:52 AM, Michael Snoyman wrote:

On Fri, Jan 20, 2012 at 6:41 AM, Edward Z. Yang wrote:

...

Aw, that is really suboptimal.  Have you filed a bug?

I think it's a feature, not a bug. When dealing with monads that provide nice[1] implementations of `fail`, you can (ab)use this to avoid writing a bunch of case expressions. I remember reading it in one of the first tutorials on Haskell I looked at (four years ago now? you can see how much this bothered me if I still remember that).

I admit that there are some use cases where the current behavior is convenient, but I think we're paying too steep a price. If we got rid of this feature entirely, we could (a) get rid of fail and (b) have the compiler warn us about a bunch of errors at compile time.

But maybe I should file a feature request: provide an extra warning flag (turned on by -Wall) that will warn when you match on a failable pattern. Essentially, I would want:

SomeConstr args <- someAction

to be interpreted as:

temp <- someAction case temp of    SomeConstr args ->

Michael

I've filed a feature request for this warning: http://hackage.haskell.org/trac/ghc/ticket/5813 Michael