#4430: Better support for resolving infix expressions in template haskell
---------------------------------+------------------------------------------
Reporter: reinerp | Owner:
Type: feature request | Status: new
Priority: normal | Component: Template Haskell
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
Consider writing a quasiquoter to parse haskell (for example, the
{{{parseHaskell}}} quasiquoter mentioned in Part D of
[http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal
Simon's TH blog post]
How is the quasiquoter supposed to handle infix expressions, such as
{{{
foo = [parseHaskell| 3 * 5 + 4 |]
}}}
In order to parse this expression properly, the quasiquoter needs to know
the fixities of the operators mentioned, and this information is only
available at the call site. We could try to use {{{reify}}}, but according
to the description of "what reify sees" in the above-linked blog post,
this would fail if, for example if the fixity is defined later in the file
than the splice is done. Another obstacle to the {{{reify}}} approach is
#4429
For context, the Hackage package [http://hackage.haskell.org/package
/haskell-src-meta haskell-src-meta] provides a haskell parser which
produces TH, similar to the {{{parseHaskell}}} quasiquoter discussed
above. This package does not, in general, parse infix expressions
correctly.
One option for supporting fixities better is to improve reify. In
particular, reify could be made to return an operator's fixity even if
typechecking hasn't been performed.
A second option would be to move responsibility of fixity parsing away
from the quasiquoter and back to GHC, by adding the following constructor
(or similar) to template haskell's {{{Exp}}} datatype:
{{{
...
| UnresolvedInfixE [(Exp, Exp)] Exp
...
}}}
with the intention that {{{UnresolvedInfixE [("a","+"), ("b", "*")] "c"}}}
(here I take strings to be expressions) denote the (unparenthesised)
expression {{{ a + b * c }}} with the expectation that GHC will apply the
correct fixities when splicing.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4430>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
