[GHC] #12048: Allow CustomTypeErrors in type synonyms (+ evaluate nested type family?)

[GHC]

#12048: Allow CustomTypeErrors in type synonyms (+ evaluate nested type family?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  diatchki
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by andrewthad):

 Just to add information about my current workaround, the example given can
 be rewritten to use a type family instead:

 {{{#!hs

 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}

 import GHC.TypeLits
 import Data.Kind

 -- Works just like a type synonym
 type family Fail where
    Fail = TypeError (Text "It failed")

 type family Bad a where
    Bad a = TypeError (Text "It messed up on " :<>: Text (Whoami a))

 type family
   Whoami (ty :: Type) :: Symbol where
   Whoami Int   = "a number"
   Whoami Float = "a number"
   Whoami [_]   = "a list of things"
   Whoami _     = "something else"

 expr :: Bad [Bool]
 expr = 5

 main :: IO ()
 main = putStrLn "Hello"

 }}}

 Everyone on this ticket is probably already aware of this, but I just
 wanted to document it for passersby like myself who may be less familiar
 with the issue.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12048#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler