From b876c1bb5c8ccd73a203c0f94bac3cbb9c7e2d65 Mon Sep 17 00:00:00 2001
From: Ben Gamari <bgamari.foss@gmail.com>
Date: Sun, 27 May 2018 11:50:21 -0400
Subject: [PATCH] users-guide: Point out GNTD may require additional extensions
As noted in #15073, GeneralizedNewtypeDeriving may produce code that
uses extensions that do not directly appear in the code written by the
user. Make this clear in the users guide.
[skip ci]
Test Plan: Read it
Reviewers: RyanGlScott
Reviewed By: RyanGlScott
Subscribers: fosskers, rwbarton, thomie, carter
GHC Trac Issues: #15073
Differential Revision: https://phabricator.haskell.org/D4701
---
docs/users_guide/glasgow_exts.rst | 39 ++++++++++++++++++++++++++++++-
1 file changed, 38 insertions(+), 1 deletion(-)
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 0de1a7a74c..01d65be73f 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -4666,7 +4666,7 @@ In this case the derived instance declaration is of the form ::
instance Monad (State [tok] (Failure m)) => Monad (Parser tok m)
Notice that, since ``Monad`` is a constructor class, the instance is a
-*partial application* of the new type, not the entire left hand side. We
+*partial application* of the newtype, not the entire left hand side. We
can imagine that the type declaration is "eta-converted" to generate the
context of the instance declaration.
@@ -4694,6 +4694,43 @@ declarations are treated uniformly (and implemented just by reusing the
dictionary for the representation type), *except* ``Show`` and ``Read``,
which really behave differently for the newtype and its representation.
+.. note::
+
+ It is sometimes necessary to enable additional language extensions when
+ deriving instances via :extension:`GeneralizedNewtypeDeriving`. For instance,
+ consider a simple class and instance using :extension:`UnboxedTuples`
+ syntax: ::
+
+ {-# LANGUAGE UnboxedTuples #-}
+
+ module Lib where
+
+ class AClass a where
+ aMethod :: a -> (# Int, a #)
+
+ instance AClass Int where
+ aMethod x = (# x, x #)
+
+ The following will fail with an "Illegal unboxed tuple" error, since the
+ derived instance produced by the compiler makes use of unboxed tuple syntax,
+ ::
+
+ {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+ import Lib
+
+ newtype Int' = Int' Int
+ deriving (AClass)
+
+ However, enabling the :extension:`UnboxedTuples` extension allows the module
+ to compile. Similar errors may occur with a variety of extensions,
+ including:
+
+ * :extension:`UnboxedTuples`
+ * :extension:`TypeInType`
+ * :extension:`MultiParamTypeClasses`
+ * :extension:`FlexibleContexts`
+
.. _precise-gnd-specification:
A more precise specification
--
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