Re: [GHC] #15607: RebindableSyntax warns `return` is not in scope when trying to call `pure`, but does not desugar between the two

5 Sep 2018

      #15607: RebindableSyntax warns `return` is not in scope when trying to call `pure`,
but does not desugar between the two
-------------------------------------+-------------------------------------
        Reporter:  isovector         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
                                     |  RebindableSyntax
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 isovector:

Old description:
...
The following (incorrect) program fails to compile:
{{{#!hs
{-# LANGUAGE RebindableSyntax #-}
import Prelude hiding (pure, return)
t = do
  pure 5
}}}
{{{
    Not in scope: ‘return’
    Perhaps you want to remove ‘return’ from the explicit hiding list
    in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-40).
  |
6 |   pure 5
  |
}}}
Changing the import of Prelude to no longer hide `return` now gives the
correct error:
{{{#!hs
import Prelude hiding (pure)
}}}
{{{
    • Variable not in scope: pure :: Integer -> t
    • Perhaps you want to remove ‘pure’ from the explicit hiding list
      in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-28).
  |
6 |   pure 5
  |   ^^^^
}}}

...
This is particularly confusing when `pure` is in scope and would
typecheck but `return` is not. The error suggests that there is
desugaring going on behind the scenes to turn `pure` into `return` via
rebindable syntax.
New description:

 The following (incorrect) program fails to compile:

 {{{#!hs
 {-# LANGUAGE RebindableSyntax #-}

 import Prelude hiding (pure, return)

 t = do
   pure 5
 }}}

 {{{
     Not in scope: ‘return’
     Perhaps you want to remove ‘return’ from the explicit hiding list
     in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-40).
   |
 6 |   pure 5
   |
 }}}

 Notice that this error mentions `return` not being in scope, despite
 trying to call `pure`. This suggests to the user that there is some
 desugaring going on to transform pure into return (but there isn't!).

 Changing the import of Prelude to no longer hide `return` now gives the
 correct error:

 {{{#!hs
 import Prelude hiding (pure)
 }}}

 {{{
     • Variable not in scope: pure :: Integer -> t
     • Perhaps you want to remove ‘pure’ from the explicit hiding list
       in the import of ‘Prelude’ (/home/sandy/Bug.hs:3:1-28).
   |
 6 |   pure 5
   |   ^^^^
 }}}

 This is particularly confusing when `pure` is in scope and would typecheck
 but `return` is not. The error suggests that there is desugaring going on
 behind the scenes to turn `pure` into `return` via rebindable syntax.

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15607#comment:1
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