Make BranchFlag a new kind

this is resolving an old TODO comment

1 files changed, 13 insertions, 14 deletions

diff --git a/compiler/types/CoAxiom.hs b/compiler/types/CoAxiom.hs
index cd8607a4bb..9a85185cc6 100644
--- a/compiler/types/CoAxiom.hs
+++ b/compiler/types/CoAxiom.hs
@@ -1,12 +1,12 @@
 -- (c) The University of Glasgow 2012
 
-{-# LANGUAGE CPP, DeriveDataTypeable, GADTs, ScopedTypeVariables #-}
+{-# LANGUAGE CPP, DataKinds, DeriveDataTypeable, GADTs, KindSignatures, ScopedTypeVariables, StandaloneDeriving #-}
 
 -- | Module for coercion axioms, used to represent type family instances
 -- and newtypes
 
 module CoAxiom (
-       Branched, Unbranched, BranchIndex, BranchList(..),
+       BranchFlag, Branched, Unbranched, BranchIndex, BranchList(..),
        toBranchList, fromBranchList,
        toBranchedList, toUnbranchedList,
        brListLength, brListNth, brListMap, brListFoldr, brListMapM,
@@ -108,13 +108,6 @@ declaring whether it is known to be a singleton or not. The list of branches
 is stored using a special form of list, declared below, that ensures that the
 type variable is accurate.
 
-As of this writing (Dec 2012), it would not be appropriate to use a promoted
-type as the phantom type, so we use empty datatypes. We wish to have GHC
-remain compilable with GHC 7.2.1. If you are revising this code and GHC no
-longer needs to remain compatible with GHC 7.2.x, then please update this
-code to use promoted types.
-
-
 ************************************************************************
 *                                                                      *
                     Branch lists
@@ -125,11 +118,17 @@ code to use promoted types.
 type BranchIndex = Int  -- The index of the branch in the list of branches
                         -- Counting from zero
 
--- the phantom type labels
-data Unbranched deriving Typeable
-data Branched deriving Typeable
-
-data BranchList a br where
+-- promoted data type
+data BranchFlag = Branched | Unbranched
+type Branched = 'Branched
+deriving instance Typeable 'Branched
+type Unbranched = 'Unbranched
+deriving instance Typeable 'Unbranched
+-- By using type synonyms for the promoted constructors, we avoid needing
+-- DataKinds and the promotion quote in client modules. This also means that
+-- we don't need to export the term-level constructors, which should never be used.
+
+data BranchList a (br :: BranchFlag) where
   FirstBranch :: a -> BranchList a br
   NextBranch :: a -> BranchList a br -> BranchList a Branched