[Template-haskell] types.

[Keean Schupke]

Ahh, I am doing a form of source to source translation for Haskell. I 
don't want to maintain the parser for this so TH seemed to be ideal.

I would be happy with the restriction of no splices within the meta-quotes.

If TH is already type checking (it is, as it will catch type errors in 
the meta-quoted code) then surely this type info is available, it just 
needs to be reified?

Who maintains that part of the TH code in GHC?

    Keean



Sean Seefried wrote:

>
> On 19/02/2005, at 2:33 AM, Keean Schupke wrote:
>
>> Last mail seemed to get lost (think I forgot to CC it to the list):
>>
>> would this work with:
>>
>>    $(something [d| f a = a + 1 |])
>>
>> Such that I can reference the reified type of 'f' from inside 
>> 'something'.
>> 'something' would be imported from another file. I am using ghc-6.4
>
>
> As far as I know you *can't* do this.  It is true that TH will type 
> check things that are inside quasi quotes but it will only complete 
> this process if there are no inner splices. There's a good reason for 
> this.
>
> TH tries to be as inclusive of meta-programs as it can by not type 
> checking programs at compile-time (unlike, say, MetaML).  In fact, 
> unless one heavily restricts the allowable meta-programs type checking 
> them statically is undecidable. So what TH does it wait until all the 
> meta-programming has been done (i.e. all the code has been generated 
> and spliced in) before type checking.
>
> The type checking I referred to in the first paragraph seems to be 
> just an extra check to catch some errors earlier. It's not necessary.
> For a while I was interested in using TH for the transformation of 
> programs. I wasn't interested in generation at all.  I started work on 
> a type checking extension to TH that would type check stuff within in 
> quasi-quotes that a) had everything in scope and b) contained no inner 
> splices.
>
> However, I never got around to finishing it since I'm doing 
> source-to-source transformation another way now.
>
> Cheers,
>
> Sean
>