From 767ff7cab7fc2d27b66cdd25d551ccf9e9e7c51d Mon Sep 17 00:00:00 2001
From: Ben Gamari <bgamari.foss@gmail.com>
Date: Fri, 11 Mar 2016 10:38:42 +0100
Subject: [PATCH] Document Quasi-quotes/list comprehension ambiguity
Test Plan: read it
Reviewers: austin, goldfire
Reviewed By: goldfire
Subscribers: hvr, thomie
Differential Revision: https://phabricator.haskell.org/D1981
GHC Trac Issues: #11679
---
docs/users_guide/glasgow_exts.rst | 28 +++++++++++++++++++++++++++-
1 file changed, 27 insertions(+), 1 deletion(-)
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 454c73e737c..dd714ece359 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -1873,7 +1873,15 @@ The following syntax is stolen:
Stolen by: :ghc-flag:`-XImplicitParams`
-``[|``, ``[e|``, ``[p|``, ``[d|``, ``[t|``, ``$(``, ``$$(``, ``[||``, ``[e||``, ``$varid``, ``$$varid``
+``[|``, ``[e|``, ``[p|``, ``[d|``, ``[t|``, ``[||``, ``[e||``
+ .. index::
+ single: Quasi-quotes
+
+ Stolen by: :ghc-flag:`-XQuasiQuotes`. Moreover, this introduces an ambiguity
+ with list comprehension syntax. See
+ :ref:`quasi-quotes-list-comprehension-ambiguity` for details.
+
+``$(``, ``$$(``, ``$varid``, ``$$varid``
.. index::
single: Template Haskell
@@ -9985,6 +9993,24 @@ Here are the salient features
quasi-quotes do not cause a declaration group break. See
:ref:`th-syntax` for more information.
+.. _quasi-quotes-list-comprehension-ambiguity:
+
+.. warning::
+
+ .. index::
+ single: quasi-quotes; ambiguity with list comprehensions
+ single: list comprehensions; ambiguity with quasi-quotes
+
+ :ghc-flag:`-XQuasiQuotes` introduces an unfortunate ambiguity with list
+ comprehension syntax. Consider the following, ::
+
+ let x = [v| v <- [0..10]]
+
+ Without :ghc-flag:`-XQuasiQuotes` this is parsed as a list comprehension.
+ With :ghc-flag:`-XQuasiQuotes` this is parsed as a quasi-quote; however,
+ this parse will fail due to the lack of a closing ``|]``. See
+ :ghc-ticket:`11679`.
+
The example below shows quasi-quotation in action. The quoter ``expr``
is bound to a value of type ``QuasiQuoter`` defined in module ``Expr``.
The example makes use of an antiquoted variable ``n``, indicated by the
--
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