[GHC] #9637: Type level programming needs an error function

[GHC]

#9637: Type level programming needs an error function
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        Reporter:  augustss          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9180, #9636      |  Differential Rev(s):  Phab:D1236
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"2d1a563bf25a4e402088feb1cdcac3d7bc50c6d3/ghc" 2d1a563/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2d1a563bf25a4e402088feb1cdcac3d7bc50c6d3"
 Implement support for user-defined type errors.

 Implements Lennart's idea from the Haskell Symposium.
 Users may use the special type function `TypeError`, which is
 similar to `error` at the value level.

 See Trac ticket https://ghc.haskell.org/trac/ghc/ticket/9637, and wiki
 page https://ghc.haskell.org/trac/ghc/wiki/CustomTypeErros

 Test Plan: Included testcases

 Reviewers: simonpj, austin, hvr, goldfire, bgamari

 Reviewed By: goldfire, bgamari

 Subscribers: adamgundry, thomie

 Differential Revision: https://phabricator.haskell.org/D1236

 GHC Trac Issues: #9637
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9637#comment:11>
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