I agree with Eric. Aside from the practically important concerns he raised,
your proposed instance is not strictly lawful, which I find troubling for
such an essential wired-in type.
pure id <*> undefined
= (mempty, id) <*> undefined
= (mempty <> fst undefined, id undefined)
= (undefined, undefined)
But according to the Applicative identity law,
pure id <*> undefined = undefined
That's similar, but not the same.
Let me add that not only the Monad instance but also the Functor one is
incompatible, because we require
fmap f x = pure f <*> x
On Wed, Jun 26, 2019, 3:58 PM Tristan Wibberley
Dear Haskell Libraries mailing list members,
I've submitted a merge request for an increase in the laziness of the writer monad: https://gitlab.haskell.org/ghc/ghc/merge_requests/1262 and it requires discussion via this mailing list.
The link above includes an example of the motivation for this change and I hope you can all extrapolate to see the importance of this change, sufficient to not only accept this but to be moved to add a language flag, available within the content of a module's source file, to match unique constructors lazily throughout a module that uses it, lifting _|_ to the next least-defined value.
The core problem is that for the most handy and practical library interfaces, we need function results to be as well-defined as possible before a strict pattern match is required to choose between two constructors/values. There are several parts to providing that but one important part is that elements of patterns that cannot be unmatched should be lazily matched so the function's definition can be used without evaluating the function's arguments first. This avoids having to train users to always avoid patterns except after a let binding which would otherwise be necessary to establish a standard habit to lazily pattern match so the user base encourages each other to make their functions maximally useful.
-- Tristan _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries