[Haskell-cafe] Haskell syntax inside QuasiQuote

[Reiner Pope]

On Tue, Oct 28, 2008 at 9:44 PM, Niklas Broberg
<niklas.broberg at gmail.com> wrote:
> On Tue, Oct 28, 2008 at 7:56 AM, Reiner Pope <reiner.pope at gmail.com> 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