transformers: remove instance MonadFix ExceptT and ErrorT

[Levent Erkok]

Henning: When a computation fails, there's no value to tie the recursive
knot over. And in those cases, mfix is supposed to produce bottom. There's
actually even a theorem showing that the mfix as defined satisfies all the
required axioms of value-recursion if the underlying monad's mfix does
(Page 54, Proposition 4.9.1 of
https://sites.google.com/site/leventerkok/erkok-thesis.pdf)

-Levent.


On Tue, Aug 12, 2014 at 2:13 AM, Henning Thielemann <
schlepptop at henning-thielemann.de> wrote:

> I propose to remove the MonadFix instance for the ExceptT transformer,
> because it gives the illusion that it can handle exceptions, which it
> cannot.
>
>
> The current implementation is:
>
> instance (MonadFix m) => MonadFix (ExceptT e m) where
>     mfix f = ExceptT $ mfix $ \ a -> runExceptT $ f $ case a of
>         Right x -> x
>         Left _ -> error "mfix ExceptT: Left"
>
> You see, that it cannot handle the exception case. Whenever someone needs
> an mfix on ExceptT I strongly suggest that he first handles the exception,
> thus turning (ExceptT e m a) into plain (m a) and then calls 'mfix' only on
> (m a).
>
>
> I further propose to declare the MonadFix instance as non-implementable in
> a way suggested in [1]:
>
> class NoMonadFix m where
>
> instance (NoMonadFix m) => MonadFix (ExceptT e m) where
>    mfix = error "unimplementable"
>
> where no instance of NoMonadFix exist and NoMonadFix is not exported.
> Whenever someone tries to call 'mfix' on 'ExceptT e m' he will get the type
> error, that a NoMonadFix instance on 'm' is missing and he will not be able
> to add it.
>
>
>
> [1] http://ghc.haskell.org/trac/ghc/ticket/9334#comment:9
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