The very first RFC created (https://github.com/haskell/rfcs/pull/1) the
Applicative/Monad Proposal, has now reached the Last Call stage. In
order to ground the discussion, I have taken some time to update the
Prelude and the text of the Haskell Report with its effects before the
call. The rendered report is available at
https://github.com/blamario/rfcs/blob/amp/report/report/haskell.pdf for
your review.
TL;DR:
The proposed changes to the report add the latest design of the
Applicative and Alternative classes, but otherwise are intentionally
minimal. Any further modifications, like the MonadFail proposal or
moving return out of the Monad class, should be relegated to new RFCs.
In some more detail, the changes are:
1. Applicative has been added as a subclass of Functor and superclass of
Monad, its methods and laws as currently defined in the base library.
The class and all its methods (pure, (<*>), (<*), (*>), and liftA2) are
exported from Prelude, but no other Applicative-related functions (like
liftA3) are.
2. The Functor class definition has been moved from module Control.Monad
to Control.Applicative in order to avoid circular imports. Note that
neither module is a part of the language specification.
3. The Monad class has been left unmodified, apart from making
Applicative its superclass and adding return a == pure a as a law.
4. Alternative has been added to the Control.Applicative module, but not
to Prelude. This is the same treatment already meted to MonadPlus. I'm
unsure why MonadPlus even exists in the report, as it has no relevance
to the language specification, and I would gladly remove both classes.
Please take some time within the following three weeks (including some
extra allowance for the upcoming holiday breaks) to vote for or against
the proposal, or to leave a comment with suggestions for its improvement.
