MonadFail proposal (MFP): Moving fail out of Monad

[David Luposchainsky]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 10.06.2015 00:26, Johan Tibell wrote:
> "As a consequence, in current Haskell, you can not use Monad-polymorphic code 
> safely, because although it claims to work for all Monads, it might just crash
> on you. This kind of implicit non-totality baked into the class is terrible."
> 
> Is this actually a problem in practice? Is there any code we can point to
> that suffers because of the current state of affairs? Could it be included in
> the proposal?

I don't have hard evidence, but the Monad class being partial strikes me as
pretty backwards in a language where totality and no implicit failures are
important to the programmers. We try our best to advocate not using certain
functions like "head" carelessly, but do-notation comes with similar partiality.

One concrete example that I know is bimap, but that's something I stumbled upon
months ago by accident. Another is that Binary does not have a monomorphic "fail"
function and it hurts me a bit to use the Monad-"fail" and write a comment on how
that is safe to do in the current context.

I think there are two important consequences of MonadFail. First of all, we can
all safely write failable patterns if we so desire. Second, the compiler can
ensure other people's codebases do not lie to us (knowingly or unknowingly).

David/quchen
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iQEcBAEBAgAGBQJVd2vEAAoJELrQsaT5WQUs+m8IAOWA9Hd52MG1wZ6g6FoOcXd6
x64dRDlilmkVu2IRxHADzip75Oji254yKQ5VY9yMGjYpFajtgf0Q8LrmA0ePTzhg
E/oxdm1vyRoJab1C5TfdrzPM/voP+wHi7y2ak1j0hTNky+wETj4MKtJ/Jef225nd
APUq05t6nPwzEDCz37RitfbA6/nwwYShaVjNe0tRluPrJuxdBu0+aobFc2lzVL+s
J7egnV1kqEOhc7INOhWYsvAJPAJSiY950y/Nmxb2/r5orTfN3tsr98d1zwRxhCmq
UNXhUaj5xD7BK2Rn1Zy7VwUv1T8IRLZuOQrlZh3HWz4t1SI0tTu3tdS468s/B1g=
=4mEU
-----END PGP SIGNATURE-----