[Haskell-cafe] Typed TH

[Edward Amsden]

Relevant paper to typed quotation:
http://repository.cmu.edu/cgi/viewcontent.cgi?article=2969&context=compsci

I'm very interested in metaprogramming in typed languages, and this paper
seems to suggest that it is a Hard Problem™.

On Thu Dec 18 2014 at 1:41:07 PM adam vogt <vogt.adam at gmail.com> wrote:

> On Thu, Dec 18, 2014 at 12:43 PM, Tom Ellis
> <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> > This is a great idea and looks very useful, which raises the following
> > question: Why hasn't this already been done?  Is there some fundamental
> > obstruction?  Perhaps someone who has tried this has some wisdom to
> share.
>
> Sometimes I found myself using:
>
>  foldl appE :: ExpQ -> [ExpQ] -> ExpQ
>
> If you try that with appE_, you end up with a type error "Occurs
> check: cannot construct the infinite type: t1 ~ t -> t1". It would
> type check if it was a foldl over a hlist, but I don't think that
> would be a preferred solution. Since appE_ doesn't work when you don't
> know how many times it will be used, I think people would just inline
> one possible definition:
>
>  appE_ :: Q (TExp (a -> b)) -> Q (TExp a) -> Q (TExp b)
>  appE_ f x = [|| $$f $$x ||]
>
> Secondly, it looks like you can only make a value of type "L String"
> by using stringL_, and only consume them with litE_, so I think that
> intermediate type does not really help. In other words, I think it is
> better to just use the following:
>
>  liftString_ :: String -> Q (TExp String)
>  liftString_ = litE_ . stringL_
>
> or just inline the following more general function:
>
>  lift_ :: Lift t => t -> Q (TExp t)
>  lift_ x = [|| x ||]
>
> I don't see those extra functions providing something that the typed
> bracket / quotation syntax doesn't already provide in a prettier way.
>
>
> Regards,
> Adam
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