[Haskell-cafe] Haskell syntax inside QuasiQuote

[Reiner Pope]

I've tried it out and it looks good so far. I had to fiddle with
haskell-src-ext's .cabal file to get it to install with GHC 6.10,
which is surprising since it isn't listed as a broken package... ah
well.

I'm able to write code like this now:

> foo x = [$vec|sin x, myFunc x, 4*5|]

Since Haskell expressions are not the entire grammar, I'm actually
making a very simple parsec lexer/bracket-counter whose sole purpose
is to find where the haskell expression stops (at a comma). This lexer
then just passes the string verbatim onto parseExp.

Unfortunately, I've uncovered a problem in the parser. For instance,
with your module, [$hs|1+1*2|] evaluates to 4 rather than 3. This
seems to be a general problem with infix operators, which I believe
arises since haskell-src-exts isn't given the fixity declarations for
+ and *, so it doesn't know to bind (*) tighter than (+). I don't see
how this problem can even be resolved without modifying Template
Haskell: given that the operators reside in user code, there is no way
to find their fixity.

Cheers,
Reiner


On Mon, Oct 27, 2008 at 12:22 AM, Matt Morrow <mjm2002 at gmail.com> wrote:
> I've just uploaded an alpha version of the translation to hackage:
>
> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-meta-0.0.1
>
> (I starting thinking after I uploaded that maybe haskell-src-th is a
> better name..)
>
> Here's one strategy for a haskell QQ:
>
> ------------------------------------------------------------
> module HsQQ where
>
> import Language.Haskell.Meta
> import Language.Haskell.TH.Lib
> import Language.Haskell.TH.Quote
> import Language.Haskell.TH.Syntax
>
> -- |
> -- > ghci> [$hs|\x -> (x,x)|] 42
> -- > (42,42)
> -- > ghci> (\[$hs|a@(x,_)|] -> (a,x)) (42,88)
> -- > ((42,88),42)
> hs :: QuasiQuoter
> hs = QuasiQuoter
>      (either fail transformE . parseExp)
>      (either fail transformP . parsePat)
>
> transformE :: Exp -> ExpQ
> transformE = return
>
> transformP :: Pat -> PatQ
> transformP = return
> ------------------------------------------------------------
>
> I'll post updates as I add to the pkg over the next few days.
>
> Cheers,
> Matt
>
>
>
> On 10/21/08, Reiner Pope <reiner.pope at gmail.com> wrote:
>> It sounds like you're doing exactly what I'm looking for. I look forward to
>> more.
>>
>> Reiner
>>
>> On Tue, Oct 21, 2008 at 4:28 PM, Matt Morrow <mjm2002 at gmail.com> wrote:
>>
>>> > Is there a simple way to do this, i.e. using existing libraries?
>>>
>>> Yes indeed. I'll be traveling over the next two days, and am shooting
>>> for a fully functional hackage release by mid next week.
>>>
>>> > What I need is a Haskell expression parser which outputs values of type
>>> > Language.Haskell.TH.Syntax.QExp, but I can't see one available in the TH
>>> > libraries, or in the haskell-src(-exts) libraries.
>>>
>>> My strategy is to use the existing haskell-src-exts parser, then
>>> translate that AST to the TH AST.
>>>
>>> Once I've got settled in one place, I'll follow up with a brain dump :)
>>>
>>> > Cheers,
>>> > Reiner
>>>
>>> Matt
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
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>>>
>>
>