Re: [GHC] #10843: Allow do blocks without dollar signs as arguments

5 Sep 2015

      #10843: Allow do blocks without dollar signs as arguments
-------------------------------------+-------------------------------------
        Reporter:  agibiansky        |                   Owner:  agibiansky
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
  (Parser)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by agibiansky:

Old description:
...
I would like the following to be valid Haskell code:
{{{#!hs
main = when True do
  putStrLn "Hello!"
}}}
Instead of requiring a dollar sign before the "do". This would parse as
{{{#!hs
main = when True (do
  putStrLn "Hello!")
}}}

...
Similarly, allow lambdas in the same way
{{{#!hs
main = forM values \value ->
  print value
}}}
parses as
{{{#!hs
main = forM values (\value ->
  print value)
}}}
One possible question: does this also do the same thing for LambdaCase?
It's an option but I would say no.
New description:

 I would like the following to be valid Haskell code:

 {{{#!hs
 main = when True do
   putStrLn "Hello!"
 }}}

 Instead of requiring a dollar sign before the "do". This would parse as

 {{{#!hs
 main = when True (do
   putStrLn "Hello!")
 }}}

 Similarly, allow lambdas in the same way
 {{{#!hs
 main = forM values \value ->
   print value
 }}}

 parses as

 {{{#!hs
 main = forM values (\value ->
   print value)
 }}}

 One possible question: does this also do the same thing for LambdaCase? I
 think that since people expect lambda case to just be a simple desugaring
 it should also work, so then

 {{{#!hs
 main = forM values \case
     Just x -> print x
     Nothing -> print y
 }}}

 parses as

 {{{#!hs
 main = forM values (\case
     Just x -> print x
     Nothing -> print y)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10843#comment:4
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