[Template-haskell] types.

[Sean Seefried]

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