#12143: ApplicativeDo Fails to Desugar 'return True' -------------------------------------+------------------------------------- Reporter: MichaelBurge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by MichaelBurge: @@ -1,2 +1,2 @@ - If I bring in a spurious parameter, ApplicativeDo correctly infers - Functor. + If I bring in a spurious parameter, ApplicativeDo correctly infers Functor + if I bind an unused variable: @@ -11,5 +11,2 @@ - However, without the spurious parameter, ApplicativeDo assumes monad. - {{{ - Prelude> :t do { return True; } - do { return True; } :: Monad m => m Bool - }}} + It seems like if 'x' is unused, then 'x <- m' and 'm' should be + equivalent. @@ -17,5 +14,1 @@ - The first block in particular shows two examples that seem inconsistent - when placed next to each other. It seems like if 'x' is unused, then 'x <- - m' and 'm' should be equivalent. - - I would expect all of these to desugar to Functor or Applicative. And + I would expect the second example desugar to Functor or Applicative. And New description: If I bring in a spurious parameter, ApplicativeDo correctly infers Functor if I bind an unused variable: {{{ Prelude> :t \m -> do { x <- m; return True; } \m -> do { x <- m; return True; } :: Functor f => f t -> f Bool Prelude> :t \m -> do { m; return True; } \m -> do { m; return True; } :: Monad m => m a -> m Bool }}} It seems like if 'x' is unused, then 'x <- m' and 'm' should be equivalent. I would expect the second example desugar to Functor or Applicative. And maybe 'return True' should be Monad, but 'do { return True; }' should be Applicative? -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12143#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler