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MonadFail proposal update 1

(Original MFP: https://github.com/quchen/articles/blob/master/monad_fail.md)

Short summary

A week has passed since I posted the MFP, and the initial discussion is mostly over. Here are my observations:

  • Everyone agrees that fail should not be in Monad.
  • Almost everyone agrees that it should be thrown out of it.
  • Some would prefer to see the special desugaring be gone entirely.
  • The name MonadFail is controversial, because of a potential Applicative constraint.
  • We're still unsure about whether IO should get a MonadFail instance, but the bias seems to be towards "yes".

New ideas worth thinking about

Special desugaring or not

Johann suggested an optional warning whenever something desugars to use fail. I think that's an idea we should think about. It is easily implemented in the compiler, and would probably behave similar to -fwarn-unused-do-binds in practice: notation that is not wrong, but might not be what the programmer intended.

Errors vs. Exceptions

Henning is concerned about the confusion between exceptions and programming errors. In his words,

We should clearly decide what "fail" is intended for - for programming errors or for exceptions.

What I see clashing with his point is backwards compatibility. Removing the String argument breaks all explicit invocations of fail. Unfortunately, we're not in a position to break very much. If someone has a better idea I'd love to hear about it though.

ApplicativeDo

ApplicativeDo is somewhere out there on the horizon, and we're not sure yet how much fail makes sense in the context of Applicative. An Applicative computation is statically determined in its shape, so it either always or never fails. Depending on previous results would introduce the Monad constraint anyway.

Probing status

Henning has started to look at the impact of the proposal when explicit invocations of fail are considered as well, something I have not done in my original survey. Luckily, things don't look too bad, Lens and its forest of dependencies can be fixed in around ten trivial changes, for example.

Greetings, David/quchen