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

[GHC]

#9637: Type level programming needs an error function
-------------------------------------+-------------------------------------
       Reporter:  augustss           |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 When doing type level programming you often need a way to report errors.
 I suggest a closed type family called Error with special semantics.
 It is kinded like this Error :: Symbol -> k1 -> k2

 If this function has to be reduced during constraint solving it will
 simply generate a type error with the given string and printing the type
 as extra information (this kind can be generalized a lot if we want to).

 I've found that having this function gives more accurate and earlier error
 messages.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9637>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler