-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 MonadFail proposal update 1 =========================== Rendered version of this text: https://github.com/quchen/articles/blob/master/monad_fail_update1.md 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJVgDvGAAoJELrQsaT5WQUspmIIAJi9UVYIitHv2CKvWSmk1fg0 hYaPRXDJMnyFS21v57+JeTPhM/dnI4k0guUUrlIB9k5WPaySQ6MKIAnB51o5O9Gv zt87FII5/oYsJtVPruKgBtLPbJVhg6zGUXmNco1S2wvB5m5HdBooQsiBRY+qiFfZ MJOdzXpRCrYJk/0PeF7sglBOElSwsSmGq/klvJUo4VeVAdi8bU+lKRfET/AmAAM5 oqckAI0SEaFo+w6EXBLPiL/F5SoFBmKR50Nu4NKWRBcoNGq7AwvWEKDZeU0PvC3a dykqSnFTRtL5LeWZnByuZTVVqlDG3afjX6ZYkrUbMKQeE9rVf24Gx9jlRusxSds= =zUDu -----END PGP SIGNATURE-----