[GHC] #12491: ghc-iserv pattern match failure

[GHC]

#12491: ghc-iserv pattern match failure
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While trying to debug the issue in
 https://ghc.haskell.org/trac/ghc/ticket/11981#comment:10 I found that that
 `ghc-iserv` executable run without any arguments fails with a pattern
 match failure:

 {{{
 $ inplace/lib/bin/ghc-iserv
 ghc-iserv: user error (Pattern match failure in do expression at
     iserv/src/Main.hs:28:3-18)
 }}}

 Would be far nicer if it printed out an error message saying what
 arguments it expected, or how to run it to avoid this pattern match
 failure.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12491>
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