On Thu, Apr 05, 2007 at 02:47:21PM +0100, Joel Reymont wrote:
> Here's the output from -ddump-splices (thanks Saizan for the tip).
>
> It's returning a1 instead of a0.
>
> ghci -fth -e '$( _derive_print_instance makeFunParser '"''"'Foo )'
> baz.hs -ddump-splices
> baz.hs:1:0:
> baz.hs:1:0: Splicing declarations
> derive makeFunParser 'Foo
> ======>
> baz.hs:30:3-28
> instance {FunParser Main.Foo} where
> []
> { parse = choice
> [(>>)
> (reserved ['F', 'o', 'o'])
> ((>>)
> (char '(') ((>>=) parse (\ a0 -> (>>)
> (char ')') (return (Main.Foo a1)))))] }
>
> baz.hs:30:3: Not in scope: `a1'
Sorry for the late multiple reply, I just spent seven hours sleeping...
I am not the maintainer of Data.Derive, nor did I write the majority
of the nice code; Neil Mitchell did it, you can ask him "why replace
DrIFT". However, using abstract syntax trees WAS my idea.
First, _derive_print_instance will never give you a TH splice error,
since it always evaluates to an empty list of declarations. It uses
the TH 'runIO' facility such that type-checking a file using
_derive_print_instance will emit the instances to standard output as a
side effect. So the error is coming from the $(derive) in baz.hs, if
you have more errors try commenting it out. (you'll get bogus code on
stdout, but at least it will be completly haskell!)
_derive_print_instance was not intended to be a debugging aid,
although it certainly works well in that capacity. The intent is that
it will be used when the standalone driver is rewritten to use TH,
which I intend to do not long after I can (Neil is out of
communication for a week with intent to continue hacking Derive; I'm
taking this as a repository lock).
Yes, we do use type classes to implement recursion across types. This
seems to be a very standard idiom in Haskell, used by Show, Read, Eq,
Ord, NFData, Arbitrary, and doubtless many more.
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