[GHC] #11316: Too many guards warning causes issues

30 Dec 2015

      #11316: Too many guards warning causes issues
-------------------------------------+-------------------------------------
           Reporter:  NeilMitchell   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           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:
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 GHC HEAD warns if you have too many guards, for example:

 {{{
 src/HSE/Bracket.hs:44:5: warning:
     Too many guards in an equation for ‘needBracket’
     Guard checking has been over-simplified
     (Use: -Wno-too-many-guards to suppress this warning
           -ffull-guard-reasoning
             to run the full checker (may increase
             compilation time and memory consumption))
 }}}

 The code in question is from
 https://github.com/ndmitchell/hlint/blob/master/src/HSE/Bracket.hs#L44 and
 reads:

 {{{#!hs
     needBracket i parent child
         | isAtom child = False
         | InfixApp{} <- parent, App{} <- child = False
         | isSection parent, App{} <- child = False
         | Let{} <- parent, App{} <- child = False
         | ListComp{} <- parent = False
         | List{} <- parent = False
         | Tuple{} <- parent = False
         | If{} <- parent, isAnyApp child = False
         | App{} <- parent, i == 0, App{} <- child = False
         | ExpTypeSig{} <- parent, i == 0, isApp child = False
         | Paren{} <- parent = False
         | isDotApp parent, isDotApp child, i == 1 = False
         | RecConstr{} <- parent = False
         | RecUpdate{} <- parent, i /= 0 = False
         | Case{} <- parent, i /= 0 || isAnyApp child = False
         | Lambda{} <- parent, i == length (universeBi parent :: [Pat_]) -
 1 = False -- watch out for PViewPat
         | Do{} <- parent = False
         | otherwise = True
 }}}

 For code that runs with the default flags, and likes to have zero warnings
 (or warnings as errors), and compile with multiple GHC versions, that is
 problematic. How can I suppress this warning?

 * Adding {{{-Wno-too-many-guards}}} will only work with GHC 8, so I need
 to either use CPP to version select my warning suppression, or
 conditionality outside the compiler select the flags.
 * Or I can give up on detecting warnings as errors, which is a bit sad, as
 it's generally a useful criteria.
 * Or I can modify my code to make it harder to read in order to satisfy a
 checker whose semantics might change in future.
 * Or GHC could not warn on too many guards, because it's really a warning
 that the compiler can't cope, not that too many guards is necessarily bad.

 I suggest the last option, because warnings should generally be about code
 issues, not compiler issues.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11316
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