[GHC] #14569: refutable let bindings produce "irrefutable pattern failed" errors

[GHC]

#14569: refutable let bindings produce "irrefutable pattern failed" errors
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           Reporter:  int-e          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In ghc-8.2.2 (probably since ghc-8.0.1), strict pattern matches in let
 bindings produce "Irrefutable pattern failed" errors.
 {{{#!hs
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XBangPatterns
 Prelude> let ![] = [1] in 1
 *** Exception: <interactive>:2:5-13: Irrefutable pattern failed for
 pattern []
 }}}

 In ghc-7.10.2, normal pattern match failures were produced.
 {{{#!hs
 GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XBangPatterns
 Prelude> let ![] = [1] in 1
 *** Exception: <interactive>:3:5-13: Non-exhaustive patterns in pattern
 binding
 }}}

 To my mind, the latter error makes much more sense; the effect of the ! is
 to make the pattern an ordinary, refutable one. Could the old behavior be
 restored?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14569>
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