Re: MonadFail proposal (MFP): Moving fail out of Monad

10 Jun 2015


      On Wed, Jun 10, 2015 at 1:46 PM, Roman Cheplyaka  wrote:
...
On 10/06/15 14:22, Johan Tibell wrote:
...
On Wed, Jun 10, 2015 at 12:42 AM, David Luposchainsky
mailto:dluposchainsky@googlemail.com>
wrote:
I think there are two important consequences of MonadFail. First of
    all, we can
    all safely write failable patterns if we so desire. Second, the
    compiler can
    ensure other people's codebases do not lie to us (knowingly or
    unknowingly).
The second is a bit overstated I think. Any function you call can still
have partial pattern matches in all the other places Haskell allows them
and you wouldn't know from the type.
For most of them, at least you get a warning from GHC (not for patterns
inside lambda, sadly, although that should be fixable). But for
do
  Just x <- a
  ...
it's not possible in principle to give a warning, because it's not clear
whether the implicit call to fail is intended.
That's a good point. An alternative to changing fail would to add a warning
for partial matches even in do-notation.