[GHC] #14119: Refactor type patterns

[GHC]

#14119: Refactor type patterns
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12564, 13910,
                                     |  13938, 14038
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 It's true that there's no counterpart to `Expr`, but perhaps there could
 be. The problem is that it wouldn't pay its way: the only real usage for
 the pattern type would be in RULEs. In types, though, patterns come up
 more often, perhaps tilting the needle enough.

 Another way of examining this: pattern-matches in terms are elaborately
 desugared into nested cases. If they weren't, the case for an expression-
 level pattern type would be stronger. And in types, we don't do this
 desugaring.

 On the other hand, there is my sentiment in comment:1. I don't think I'd
 argue with "Well, that's a good idea, but not worth it since we'll be
 doing much heavier refactoring soon, anyway."

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14119#comment:7>
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