On Mon, Dec 16, 2013 at 5:26 PM, David Luposchainsky
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/Cont.... Otherwise I suppose section "3.17.2" of the 2010 report covers this case. Regards, Adam