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  • Glasgow Haskell Compiler
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  • #13875
Closed
Open
Created Jun 26, 2017 by David Feuer@treeowlReporter

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
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CC simonmar
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