Re: [GHC] #7783: MultiWayIf should start a new layout

21 Mar 2013

      #7783: MultiWayIf should start a new layout
---------------------------------+------------------------------------------
    Reporter:  exbb2             |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:  #4359             |  
---------------------------------+------------------------------------------
Description changed by simonpj:

Old description:
...
Currently, MultiWayIf doesn't fully replace old {{{case () of _}}} form.
This outputs 3:
{{{
{-# LANGUAGE MultiWayIf #-}
x = case () of
    _ | False -> case () of
                _ | False -> 1
                  | False -> 2
      | True -> 3
main = print (x :: Int)
}}}
but this errors out:
{{{
{-# LANGUAGE MultiWayIf #-}
x = if | False -> if | False -> 1
                     | False -> 2
       | True -> 3
main = print (x :: Int)
}}}
because it parses like this:
{{{
x = if | False -> if | False -> 1
                     | False -> 2
                     | True -> 3
}}}
Every other syntax introducing guards starts a layout; MultiWayIf's
current behavior leads to subtle logic bugs for those unaware that
MultiWayIf is special.
New description:

 Currently, `MultiWayIf` doesn't fully replace old {{{case () of _}}} form.
 This outputs 3:

 {{{
 {-# LANGUAGE MultiWayIf #-}
 x = case () of
     _ | False -> case () of
                 _ | False -> 1
                   | False -> 2
       | True -> 3

 main = print (x :: Int)
 }}}

 but this errors out:

 {{{
 {-# LANGUAGE MultiWayIf #-}
 x = if | False -> if | False -> 1
                      | False -> 2
        | True -> 3

 main = print (x :: Int)
 }}}
 because it parses like this:
 {{{
 x = if | False -> if | False -> 1
                      | False -> 2
                      | True -> 3
 }}}
 Every other syntax introducing guards starts a layout; `MultiWayIf`'s
 current behavior leads to subtle logic bugs for those unaware that
 `MultiWayIf` is special.

--

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7783#comment:2
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