Removing MonadFail from Monad

[adam vogt]

On Mon, Dec 16, 2013 at 5:26 PM, David Luposchainsky
<dluposchainsky at googlemail.com> wrote:
> Desugaring is then changed to the following:
>
> ```haskell
> -- Explicitly irrefutable pattern: do not add MonadFail constraint
> do ~pat <- computation     >>>     let f pat = more
>    more                    >>>     in  computation >>= f
>
> -- Only one data constructor: do not add MonadFail constraint
> do (Only x) <- computation     >>>     let f (Only x) = more
>    more                        >>>     in  computation >>= f
>
> -- Otherwise: add MonadFail constraint
> do pat <- computation     >>>     let f pat = more
>    more                   >>>         f _   = fail "..."
>                           >>>     in  computation >>= f
> ```

Hello David,

GHC can already do this for you. Only `f' below has no MonadFail in
the inferred type:

{-# LANGUAGE RebindableSyntax #-}
import Prelude hiding (fail)
class MonadFail m where fail :: String -> m a
f x = do x <- x; x
g x = do Just y <- return Nothing; x
h x = do (a, Just b) <- x; a

A specification for "pattern can fail given input that is fully
defined" outside of ghc might be
<http://hackage.haskell.org/package/applicative-quoters-0.1.0.8/docs/src/Control-Applicative-QQ-ADo.html#failingPattern>.
Otherwise I suppose section "3.17.2" of the 2010 report covers this
case.

Regards,
Adam