[GHC] #14490: TTG Snags

[GHC]

#14490: TTG Snags
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        Reporter:  alanz             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14482            |  Differential Rev(s):
       Wiki Page:                    |
  ImplementingTreesThatGrow          |
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Comment (by RyanGlScott):

 Replying to [comment:41 alanz]:
 > Is this a weakness in the deriving process? Because as I understand it,
 there is a concrete value for every `(GhcPass p)`, given the way `GhcPass`
 is defined as being indexed by `Pass`.

 This is not how type families work. Type families only reduce when you
 provide arguments that match an instance in scope. `XOverlit (GhcPass p)`
 does not match any instances, and thus it doesn't reduce. If you want this
 to compile, you'll need something like this:

 {{{#!hs
 {-# LANGUAGE TypeFamilies, GADTs, FlexibleInstances, StandaloneDeriving,
 DeriveDataTypeable #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Oddity where

 import Data.Data

 data GhcPass (p :: Pass)
 data Pass = Parsed | Renamed | Typechecked
          deriving (Data)

 deriving instance Typeable p => Data (GhcPass p)

 type family XOverLit x

 type instance XOverLit (GhcPass 'Parsed)      = Int
 type instance XOverLit (GhcPass 'Renamed)     = Int
 type instance XOverLit (GhcPass 'Typechecked) = Char

 data Foo p = Foo (XOverLit p)
 deriving instance (Typeable p, Data (XOverLit (GhcPass p))) => Data (Foo
 (GhcPass p))
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14490#comment:43>
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