[Haskell-cafe] Why were unfailable patterns removed and "fail" added to Monad?
2haskell at pkturner.org
Fri Jan 20 14:02:34 CET 2012
On 2012-01-19 23:52, Michael Snoyman wrote:
> 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
I fully agree if it's IO, so that a failed pattern match leads to an
exception. The "nice" implementations of fail in the List and Maybe
monads are a different story.
Ideally one would want to be able to turn on a warning whenever IO is
used in a way which could generate a pattern match exception. This
would call for a type distinction, as you said, "reinstate the MonadZero
Here's an idea that might address SPJ's "killer".
b) if you add an extra constructor to a single-constructor
type then pattern matches on the original constructor
suddenly become failable
Another binding operator might be introduced so that the code would show
the intention either to have a failable or non-failable pattern match.
do (x,y) <- pair failable, requires MonadZero
do (x,y) <=- pair requires non-failable pattern
supports Monads that should not fail
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