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

On Wed, Jun 10, 2015 at 2:14 PM, Johan Tibell wrote:

On Wed, Jun 10, 2015 at 1:46 PM, Roman Cheplyaka wrote:

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On 10/06/15 14:22, Johan Tibell wrote:

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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.

That would be an improvement, but only a small one. You'd have to turn off the warning if you really meant to use fail, and then if you changed the monad you're using, or you're writing a polymorphic function, nothing will warn you. I'm +1 on this change. The plan looks thorough, minimizing breakage. It'll bring many advantages. For example, I'll feel much safer when I use functions like time's parseTimeM, to give a concrete example. Regards, Erik