[GHC] #11387: Typecasting using type application syntax

[GHC]

#11387: Typecasting using type application syntax
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11350            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by adamgundry):

 * cc: adamgundry (added)


Comment:

 It would make sense to do something a bit like this.

 If `a :: *` is a type, then a `Typeable a` constraint is essentially
 an (invisible) singleton witness to `a`; the visible singleton is
 `TypeRep a`. I think the Right Thing to do would be to get rid of
 the singleton construction and add pi-types, so that we have
 {{{#!hs
 doubleWhenInt :: pi a . a -> a
 doubleWhenInt @Int n = n + n
 doubleWhenInt @_   x = x
 }}}

 The `pi a .` quantifier corresponds to `forall a . Typeable a =>`.
 The type argument is operationally relevant, can be pattern-matched
 and hence is non-parametric. The argument would be
 invisible-by-default (e.g. you might write `doubleWhenInt True` at a
 call site) but could be made visible like other type arguments
 (e.g. `doubleWhenInt @Int 42` should be allowed).

 (It's not completely obvious whether arguments to pi-quantified
 functions should be in the type namespace or the term namespace,
 because they exist at both levels.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11387#comment:1>
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