MonadFail proposal (MFP): Moving fail out of Monad

[Johan Tibell]

On Wed, Jun 10, 2015 at 1:46 PM, Roman Cheplyaka <roma at ro-che.info> wrote:

> On 10/06/15 14:22, Johan Tibell wrote:
> > On Wed, Jun 10, 2015 at 12:42 AM, David Luposchainsky
> > <dluposchainsky at googlemail.com <mailto:dluposchainsky at 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.
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