[GHC] #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`

[GHC]

#9127: Don't warn about pattern-bindings of the form `let !_ = rhs`
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        Reporter:  refold            |            Owner:  gzayas
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:  fixed             |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 I'd love to see this documented in the user manual, please.  Look at
 [http://www.haskell.org/ghc/docs/latest/html/users_guide/options-
 sanity.html sanity checking] and `-fwarn-unused-bindings`.

 There are really two things enabled by `-fwarn-unused_bindings`:
   * Warning about a named variable brought into scope but not used (unless
 exported or starting with underscore).  e.g.
 {{{
 let f x = rhs1 in True   -- Warning for unused f
 let (p,q) = rhs2 in p+1  -- Warning about unused q
 }}}

   * Warning about a pattern binding that doesn't bind anything; e.g.
 {{{
 Just _ = rhs1     -- Warning for unused binding
 (_, _) = rhs2     -- Warning for unused binding
 }}}
   But no warning is given for a lone wild-card pattern or banged wild-card
 pattern:
 {{{
 _  = rhs3  -- No warning
 !_ = rhs4  -- No warning; behaves like seq
 }}}
   The former is not very different from `_v = rhs3` which elicits no
 warning; and can be useful to add a type constraint, e.g. `_ = x::Int`.
 The latter is useful as an alternative way to force evaluation.

 Could you add all that to the manual?  Thanks

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9127#comment:11>
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