#2399: Template Haskell: support for view patterns
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Reporter: fons | Owner: igloo
Type: feature request | Status: new
Priority: low | Milestone: _|_
Component: Template Haskell | Version: 6.11
Keywords: | Testcase:
Blockedby: | Difficulty: Unknown
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Changes (by reinerp):
* owner: => igloo
Comment:
I've attached patches (to ghc, template-haskell, and the testsuite) which
implement and test the request. The {{{Pat}}} datatype in template-haskell
now has a constructor {{{ViewP Exp Pat}}}, which is recognised when
splicing. Also, GHC no longer complains about "exotic patterns" when
writing things like {{{[p|(id -> x)|]}}}.
My thought about view patterns versus Simon's "compositional pattern
guards": view patterns are simpler. In particular, an important motivation
seems to be for pattern quasiquoters. The main feature such quasiquoters
require is being able to do computation before performing the pattern
match, and the {{{ViewP}}} constructor expresses exactly that, and no
more.
In my patch, I have left the {{{ViewP}}} case of
{{{Language.Haskell.TH.Lib.rename}}} as a {{{fail}}}. To implement this
case would seem to require extending {{{rename}}} to expressions as well
as just patterns; and in any case, I am not sure of the purpose of this
function.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2399#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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