[GHC] #14773: MultiWayIf makes it easy to write partial programs that are not catched by -Wall

7 Feb 2018

      #14773: MultiWayIf makes it easy to write partial programs that are not catched by
-Wall
-------------------------------------+-------------------------------------
           Reporter:  SimonHengel    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following partial function will pass {{{-Wall}}}

 {{{#!haskell
 -- program 1
 {-# LANGUAGE MultiWayIf #-}
 foo :: Bool -> Int
 foo b = if | b -> 23
 }}}

 While the following two alternatives will not:

 {{{#!haskell
 -- program 2
 foo :: Bool -> Int
 foo b | b = 23
 }}}

 {{{#!haskell
 -- program 3
 foo :: Bool -> Int
 foo b = case () of
   () | b -> 23
 }}}

 Note that the GHC User's Guide states that "program 1" and "program 3" are
 equivalent.

 1. Is this a bug or by design?
 2. I guess at the very least we would want to update the User's Guide

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14773
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler