Language level heads-up warnings

[David Luposchainsky]

Hello GHC-Devs,

The discussion on the libraries mailing list brought up the issue of
breaking compatbility in favour of language evolution/sanitation again;
topics mentioned specifically were Prelude's monomorphic
Foldable/Traversable functions, and the Applicative/Monad issue.

The main arguments of both sides are these:

(+) We've been aware of certain unfortunate features of Base for a long
time, and we understand very well how to fix this, and what the
consequences are.

(-) Changing core language features is something you can never forsee
the consequences of. Breaking compatibility should be avoided at all costs.

(There's also the argument about beginner friendliness, which is not
backed by evidence, so I'll leave it out here.)

These two seem contradictory, however they have one almost obvious
common ground: we can already write code right now that will be
unaffected by moving Foldable/Traversable/Applicative+Monad to the
Prelude. Suppose all code is written with this in mind, the change is
trivial.

Which brings us to the actual problem, namely that code is not written
with this in mind. It is easy to forget about the issue in a practical
setting, regardless of how small the required effort is. Therefore, I'd
like to bring forward the idea of "language level heads-up warnings"
(suggestions for better names appreciated).

Right now, we can only deprecate functions. When used, GHC informs us
"hey, this will work for now, but better use that in the future".
However, this mechanism is too specialized to tackle the problem above:
for example, there is no way to warn a user about a Monad that is not an
Applicative.

If the community agrees on making one of the "big" changes, what would
you think of adding warnings about language usage? I'm sure over the
course of a couple of GHC releases, the great majority of libraries
would be modified to rectify these warnings again. If you want an
example: "Warning: Monad without Applicative. This will be enforced in a
future language standard."

These warnings would have to be hardcoded into GHC, but keep in mind
that this is only for very few, relatively high impact changes, and can
be removed once everything is done. In my opinion, this may be a far
better transitional feature than the alternative (adding compatibility
modules, convincing thousands of developers to listen to the people in
favour of the change, ...).

Greetings,
David