Recommend MonadFailDesugaring? about rio

[chrisdone]

By the same token we should recommend -Wincomplete-uni-patterns which is not enabled by -Wall.

stack --resolver ghc-8.4.1 exec ghci
GHCi, version 8.4.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/chris/.ghci
Prelude> :set -Wall
Prelude> let f = x where Just x = Nothing
Prelude> :set -fwarn-incomplete-uni-patterns
Prelude> let g = x where Just x = Nothing

<interactive>:5:17: warning: [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding: Patterns not matched: Nothing
Prelude> 

[akhra]

+1 to both of these.

It's unfortunate that -Wincomplete-uni-patterns doesn't catch the do-notation case, or I'd prefer just that by default. I think you're far more likely to have situations where you're stuck in Maybe but legitimately know it's a Just value in monadic code, due to the shape of other peoples' APIs; and the MonadFail error may not be clear to everyone, while the warning is nicely succinct. But it's better than nothing, and trivial to work around.