[Haskell-cafe] Pattern variables in Template Haskell

[Alejandro Serrano Mena]

I found some old docs saying that pattern splices are not completely
implemented due to this problem with binding.
At the end, I used quasi-quotation + haskell-src-exts to achieve the same
results. In this case, it seemed to work because quasi-quoters are run
before the renamer, instead of after it.

In any case, thanks very much for your help :)

2014-11-03 15:53 GMT+01:00 Richard Eisenberg <eir at cis.upenn.edu>:

> I just tried
>
> > foo $( [p| x |] ) = x
>
> and got that x is unbound on the RHS. This may be a GHC bug if no one here
> knows how this is supposed to work. I sure don't, but I've never explored
> pattern splices.
>
> Richard
>
> On Nov 3, 2014, at 3:07 AM, Alejandro Serrano Mena <trupill at gmail.com>
> wrote:
>
> > Dear Haskell-cafe,
> > I'm trying to use Template Haskell to generate patterns in Haskell code,
> but after a couple of days of trying, I have not come with a solution. I
> hope somebody could help me.
> >
> > My problem is basically that I want something like:
> >
> > example :: Maybe a -> a
> > example $(j "x") = x
> >
> > to work correctly. The idea of this simple quoter is to generate code
> similar to:
> >
> > example :: Maybe a -> a
> > example (Just x_123) = x_123
> >
> > that is, to create the "Just" pattern, and to make "x" bind to the same
> element. Right now my code reads:
> >
> > j :: String -> Q Pat
> > j s = return (ConP 'Just [VarP (mkName s)])
> >
> > which generated a new variable "x" each time, thus unable to match with
> the "x" in the RHS. I have also tried with lookupValueName.
> >
> > Note that my problem is a bit more complicated, with variable names
> coming from a different source. However, I think that if I solve this
> problem where I can make variables in a pattern and in an expression match,
> I should be able to use them in my complete solution.
> >
> > Thanks in advance.
> > _______________________________________________
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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
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