Re: [GHC] #10746: No non-exhaustive pattern match warning given for empty case analysis

13 May 2016

      ...
* Normalise `x`'s type, to get it to the form `T ty1 .. tyn`. (I don't
 understand the "bounded" bit.)  For this, we must reduce type families,
 but NOT newtypes.  For pattern matching purposes, newtypes behave just
#10746: No non-exhaustive pattern match warning given for empty case analysis
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7669, #11806     |  Differential Rev(s):  Phab:D2105
       Wiki Page:                    |
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Comment (by gkaracha):

 Replying to [comment:25 simonpj]:
 like data types.  So use `FamInstEnv.normaliseType`.

 This sounds wrong to me. Newtypes and data types do not behave the same
 when it comes to pattern matching. From [https://wiki.haskell.org/Newtype
 Newtype Wiki Page]:

 {{{#!hs
 data    Foo1 = Foo1 Int
 newtype Foo3 = Foo3 Int

 y1 = case undefined of
        Foo1 _ -> 1      -- undefined

 y3 = case undefined of
        Foo3 _ -> 1      -- 1
 }}}
 which is verified by my GHCi (7.10). Additionally, by bounded I mean that
 when we have a family
 {{{#!hs
 type family F (a :: *) :: *
 type instance F a = F a
 }}}
 we do not want to keep rewriting forever. Hence, I would bound
 normalization with a fixed number of maximum iterations. If we exceed
 that, we (playing on the safe side) assume that the type is inhabited and
 issue a warning.

 The rest you mentioned I agree with, but I missed a bit:
...
* If some of `T`'s data constructor are GADTs, then enumerate them all
 and recurse.
What do you mean by **recurse** in this case? Does `EmptyCase` imply deep
 evaluation (is it really strict pattern matching) or just in WHNF? Because
 I thought that it means the latter.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10746#comment:26
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