From cf735db883b2ff83e61f9848a39f1066d3288a43 Mon Sep 17 00:00:00 2001
From: Andrei Borzenkov <andreyborzenkov2002@gmail.com>
Date: Mon, 3 Jul 2023 16:45:57 +0400
Subject: [PATCH] Add Note about why we need forall in Code to be on the right
---
.../Language/Haskell/TH/Syntax.hs | 20 +++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 42d01ffd581c..b3b730ee83c9 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -380,8 +380,7 @@ The splice will evaluate to (MkAge 3) and you can't add that to
-- Code constructor
#if __GLASGOW_HASKELL__ >= 909
type Code :: (Kind.Type -> Kind.Type) -> forall r. TYPE r -> Kind.Type
- -- The nested `forall` makes it possible to assign the arity of 0 to
- -- type CodeQ = Code Q
+ -- See Note [Foralls to the right in Code]
#else
type Code :: (Kind.Type -> Kind.Type) -> TYPE r -> Kind.Type
#endif
@@ -424,6 +423,23 @@ newtype Code m a = Code
-- In the Template Haskell splice $$([|| "foo" ||])
+{- Note [Foralls to the right in Code]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Code has the following type signature:
+ type Code :: (Kind.Type -> Kind.Type) -> forall r. TYPE r -> Kind.Type
+
+This allows us to write
+ data T (f :: forall r . (TYPE r) -> Type) = MkT (f Int) (f Int#)
+
+ tcodeq :: T (Code Q)
+ tcodeq = MkT [||5||] [||5#||]
+
+If we used the slightly more straightforward signature
+ type Code :: foral r. (Kind.Type -> Kind.Type) -> TYPE r -> Kind.Type
+
+then the example above would become ill-typed. (See #23592 for some discussion.)
+-}
+
-- | Unsafely convert an untyped code representation into a typed code
-- representation.
unsafeCodeCoerce :: forall (r :: RuntimeRep) (a :: TYPE r) m .
--
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