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

[GHC]

#10746: No non-exhaustive pattern match warning given for empty case analysis
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        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):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Replying to [comment:15 gkaracha]:
 > Replying to [comment:14 dfeuer]:
 > All the related tickets give me the impression that it is expected for
 `f` to be non-exhaustive and `g` to be exhaustive, which is not in
 accordance with Haskell's operational semantics.
 > {{{#!hs
 > f :: Int -> a
 > f x = case x of {}
 >
 > g :: Void -> a
 > g x = case x of {}
 > }}}
 >
 > Unless you force `x` in another way, both `case x of {}` are equally
 non-exhaustive.

 It seems your argument may be valid, but your premise is false! Using GHCi
 7.10.3,

 {{{#!hs
 Prelude> :set -XEmptyCase -XEmptyDataDecls
 Prelude> data Void
 Prelude> let absurd :: Void -> a; absurd x = case x of
 Prelude> absurd (error "Really?")
 *** Exception: Really?
 }}}

 As you can see, the scrutinee is forced, producing the desired exception,
 ''not'' a pattern match failure. This is explained in the note on "Empty
 case alternatives" in `compiler/deSugar/Match.hs`:

 > The list of `EquationInfo` can be empty, arising from
 >
 >  `case x of {}`   or    `\case {}`
 >
 > In that situation we desugar to
 >
 > `case x of { _ -> error "pattern match failure" }`
 >
 > The ''desugarer'' isn't certain whether there really should be no
 > alternatives, so it adds a default case, as it always does.  A later
 > pass may remove it if it's inaccessible.  (See also Note [Empty case
 > alternatives] in `CoreSyn`.)
 >
 > We do ''not'' desugar simply to
 >
 > `error "empty case"`
 >
 > or some such, because `x` might be bound to `(error "hello")`, in which
 > case we want to see that `"hello"` exception, not `(error "empty
 case")`.

 Note that in the above text,

 > `case x of { _ -> error "pattern match failure" }`

 represents a ''Core'' `case` expression, not a ''Haskell'' case
 expression, so it always forces its scrutinee, regardless of what patterns
 it may or may not contain.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10746#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler