ApplicativeDo desugaring is lazier than standard desugaring

Suppose I write

import Data.Maybe (isJust)

bangy m = do
  (_,_) <- m
  return ()

main = print $ isJust (bangy (Just undefined))

If I compile this with ApplicativeDo, it prints True. Otherwise, it throws an exception. The basic problem is that the (correct) bangy function requires a Monad constraint, but ApplicativeDo replaces it with a lazier function that can get by with Functor. I believe it should desugar correctly, and impose a Monad constraint here. To get the Functor constraint should require an explicit lazy pattern match:

bangy m = do
  ~(_,_) <- m
  return ()

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture