Re: [Haskell-cafe] Haskell syntax inside QuasiQuote

On Tue, Oct 28, 2008 at 9:44 PM, Niklas Broberg wrote:

On Tue, Oct 28, 2008 at 7:56 AM, Reiner Pope wrote:

...

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.

Care to tell me what the problem is? I have no problem installing it with GHC 6.10, in fact the 0.3.9 version on hackage was uploaded solely to have it compile with both old and new GHC.

Running "cabal install haskell-src-exts" gave

Resolving dependencies... 'haskell-src-exts-0.3.9' is cached. Configuring haskell-src-exts-0.3.9... Preprocessing library haskell-src-exts-0.3.9... Building haskell-src-exts-0.3.9...

Language/Haskell/Exts/Syntax.hs:102:7: Could not find module `Data.Data': it is a member of package base, which is hidden cabal: Error: some packages failed to install: haskell-src-exts-0.3.9 failed during the building phase. The exception was: exit: ExitFailure 1

I am using GHC 6.10.0.20081007. Modifying the dependencies in the haskell-src-exts.cabal file to

if flag(splitBase) Build-Depends: base == 4.*, array >= 0.1, pretty >= 1.0

allows it to compile.

...

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.

Yes, you're right, haskell-src(-exts) does not handle fixity of operators for you. You need to collect the fixities yourself and make a second pass over expressions to get them right. This is definitely functionality I would like to see available in haskell-src-exts (though not on by default), so if anyone were to implement it I would gladly accept patches.

The problem in the case of quasi-quoting is worse than this, though. I create my quasiquoter library, and it will parse Haskell syntax including infix operators. A user may then write, eg

(*+*) = undefined infixr *+*

foo = [$hs|5*+*4+3|]

The hs quasiquoter has no way of knowing *+*'s fixity, and template haskell provides no way to find out, as far as I know. The quasiquoter's parser only has access to the string, "5*+*4+3". Cheers, Reiner