#2742: The -> in ViewPatterns binds more weakly than infix data constructors.
---------------------------------+------------------------------------------
Reporter: guest | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonpj):
* difficulty: => Unknown
* type: bug => feature request
* milestone: => _|_
Comment:
I agree with you. Here's why it's tricky. Haskell allows a pattern like
this
{{{
case x of
a:as -> rhs
}}}
If view patterns bind more tightly than infix ops, then presumably this
would be ok
{{{
case x of
a : f as -> bs -> rhs
}}}
and now we get an awkward ambiguity between the two uses of `->`.
The change I made to the parser was this:
* Remove this production from `texp`:
{{{
| fexp '->' exp { LL $ EViewPat $1 $3 }
}}}
* Add the same production to `exp10`.
That added a new shift/reduce conflict, described above, which gets
resolved the wrong way.
If someone wants to investigate alternatives, that'd be great, so I'll
leave this open as a feature request. It's a pure parser question so it
does not require deep GHC knowledge.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2742#comment:2>
GHC <http://www.haskell.org/ghc/>
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