Tighten up what it means to be an "enumeration data constructor"

[ghc-hetmet.git] / compiler / types / TyCon.lhs

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index 2350973..2958107 100644 (file)
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -582,7 +582,7 @@ data CoTyConDesc
 
 Note [Enumeration types]
 ~~~~~~~~~~~~~~~~~~~~~~~~
-We define datatypes with no constructors to not be
+We define datatypes with no constructors to *not* be
 enumerations; this fixes trac #2578,  Otherwise we
 end up generating an empty table for
   <mod>_<type>_closure_tbl
@@ -590,6 +590,16 @@ which is used by tagToEnum# to map Int# to constructors
 in an enumeration. The empty table apparently upset
 the linker.
 
+Moreover, all the data constructor must be enumerations, meaning
+they have type  (forall abc. T a b c).  GADTs are not enumerations.
+For example consider
+    data T a where
+      T1 :: T Int
+      T2 :: T Bool
+      T3 :: T a
+What would [T1 ..] be?  [T1,T3] :: T Int? Easiest thing is to exclude them.
+See Trac #4528.
+
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)