[GHC] #8459: Role of TExp

[GHC]

#8459: Role of TExp
-------------------------------+-------------------------------------------
        Reporter:  monoidal    |            Owner:  monoidal
            Type:  bug         |           Status:  new
        Priority:  normal      |        Milestone:
       Component:  Template    |          Version:  7.7
  Haskell                      |         Keywords:
      Resolution:              |     Architecture:  Unknown/Multiple
Operating System:              |       Difficulty:  Easy (less than 1 hour)
  Unknown/Multiple             |       Blocked By:
 Type of failure:  Other       |  Related Tickets:
       Test Case:              |
        Blocking:              |
-------------------------------+-------------------------------------------

Comment (by simonpj):

 Richard is right.  Consider
 {{{
 e :: TExp Age
 e = MkAge 3

 foo = $(coerce e) + 4::Int
 }}}
 The splice will evaluate to `(MkAge 3)` and you can't add that to
 `4::Int`. So you can't `coerce` a `(TExp Age)` to a `(TExp Int)`.

 Nominal!  I'll change it, via a role signature in `TH.Syntax`.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8459#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler