Re: [Haskell-cafe] Haskell syntax inside QuasiQuote

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 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-...

(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 wrote:

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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 wrote:

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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.

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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 :)

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Cheers, Reiner

Matt _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe