[GHC] #10708: Rejection of constant functions defined using conditional pattern matching

[GHC]

#10708: Rejection of constant functions defined using conditional pattern matching
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        Reporter:  HubertGaravel     |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:  invalid           |                Keywords:
Operating System:  Linux             |            Architecture:  x86
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by HubertGaravel):

 Thanks for the insight. If I try to reformulate, the definition of d3 is
 allowed by the syntax but forbidden by the static semantics.

 Perhaps the static semantics rule is too rigid and could be relaxed to
 make Haskell more regular. The example below shows that regularity is
 broken for arity 0 (empty pattern list). But I can live with this
 bizarre exception and will modify my Haskell generator to handle this
 corner case.

 Thanks again!
 Hubert

 {{{#!hs

 -- valid for arity 2
 f2 x y | x == 0 = x
 f2 x y | x /= 0 = x + 1

 -- valid for arity 1
 f1 x   | x == 0 = x
 f1 x   | x /= 0 = x + 1

 -- invalid for arity 0
 f0     | 0 == 0 = 0
 f0     | 0 /= 0 = 1

 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10708#comment:5>
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