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| author | Ben Gamari <bgamari.foss@gmail.com> | 2016-03-11 10:38:42 +0100 |
|---|---|---|
| committer | Ben Gamari <ben@smart-cactus.org> | 2016-03-11 13:20:17 +0100 |
| commit | 767ff7cab7fc2d27b66cdd25d551ccf9e9e7c51d (patch) | |
| tree | add5136a1b957c92443beecb47c6a9e8c2500175 /docs/users_guide | |
| parent | f3def7643d390db54d18b8c3d385c490fba58a41 (diff) | |
| download | haskell-767ff7cab7fc2d27b66cdd25d551ccf9e9e7c51d.tar.gz | |
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
Diffstat (limited to 'docs/users_guide')
| -rw-r--r-- | docs/users_guide/glasgow_exts.rst | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst index 454c73e737..dd714ece35 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 |