From e3dc71de7307d30f6063a8447b93e54f1551a041 Mon Sep 17 00:00:00 2001
From: Simon Peyton Jones <simonpj@microsoft.com>
Date: Wed, 5 Dec 2012 11:07:38 +0000
Subject: [PATCH] Allow existential data constructors to be promoted
This reverts the change in Trac #7347, which prevented promotion
of existential data constructors. Ones with constraints in
their types, or kind polymorphism, still can't be promoted.
---
compiler/basicTypes/DataCon.lhs | 8 +++++---
compiler/types/TyCon.lhs | 6 ++++--
2 files changed, 9 insertions(+), 5 deletions(-)
diff --git a/compiler/basicTypes/DataCon.lhs b/compiler/basicTypes/DataCon.lhs
index c82f018962cb..edb3627993d2 100644
--- a/compiler/basicTypes/DataCon.lhs
+++ b/compiler/basicTypes/DataCon.lhs
@@ -391,6 +391,7 @@ data DataCon
-- The actual fixity is stored elsewhere
dcPromoted :: Maybe TyCon -- The promoted TyCon if this DataCon is promotable
+ -- See Note [Promoted data constructors] in TyCon
}
deriving Data.Typeable.Typeable
@@ -559,9 +560,10 @@ mkDataCon name declared_infix
mkFunTys rep_arg_tys $
mkTyConApp rep_tycon (mkTyVarTys univ_tvs)
- mb_promoted
- | is_vanilla -- No existentials or context
- , all (isLiftedTypeKind . tyVarKind) univ_tvs
+ mb_promoted -- See Note [Promoted data constructors] in TyCon
+ | all (isLiftedTypeKind . tyVarKind) (univ_tvs ++ ex_tvs)
+ -- No kind polymorphism, and all of kind *
+ , null full_theta -- No constraints
, all isPromotableType orig_arg_tys
= Just (mkPromotedDataCon con name (getUnique name) prom_kind arity)
| otherwise
diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index 5398adc8f1fd..e90cdec24bda 100644
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -581,8 +581,10 @@ Note [Promoted data constructors]
A data constructor can be promoted to become a type constructor,
via the PromotedTyCon alternative in TyCon.
-* Only "vanilla" data constructors are promoted; ones with no GADT
- stuff, no existentials, etc. We might generalise this later.
+* Only data constructors with
+ (a) no kind polymorphism
+ (b) no constraints in its type (eg GADTs)
+ are promoted. Existentials are ok; see Trac #7347.
* The TyCon promoted from a DataCon has the *same* Name and Unique as
the DataCon. Eg. If the data constructor Data.Maybe.Just(unique 78,
--
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