[Haskell-cafe] Quasiquotation page on HaskellWiki needs updating

[adam vogt]

On Sat, Nov 24, 2012 at 1:32 AM, Erik de Castro Lopo
<mle+hs at mega-nerd.com> wrote:
> Hi all,
>
> It seems the Quasiquotation page on HaskellWiki
>
>     http://www.haskell.org/haskellwiki/Quasiquotation
>
> has fallen behind the actually Quasiquotation implementation that
> is in ghc-7.4.2 and later.
>
> Specifically, the QuasiQuoter constructor that the Wiki takes two
> parameters:
>
>     data QuasiQuoter
>         = QuasiQuoter
>         { quoteExp :: String -> Q Exp
>         , quotePat :: String -> Q Pat
>         }
>
> while the one in ghc-7.4 and later takes four:
>
>     data QuasiQuoter
>         = QuasiQuoter
>         { quoteExp :: String -> Q Exp
>         , quotePat :: String -> Q Pat
>         , quoteType :: String -> Q Type
>         , quoteDec :: String -> Q [Dec]
>         }
>
> I'm just starting out with quasquotation and am not yet qualified
> to update this page myself.
>
> Erik

Hi Erik,

I've made a tiny edit so that the code will work on either version.

The extra functions are only called if you put quasiquoters in places
that are Type or Dec. It's harder to come up with useful quasiquoters
for those locations, which is probably why quoteDec and quoteType were
added later. Here's a pretty useless example of using those fields:

in M1.hs

> {-# LANGUAGE TemplateHaskell #-}
> module M1 where
> import Language.Haskell.TH.Quote
>
> e = QuasiQuoter { quoteDec = \ _ -> [d| x = 1 |], quoteType = \ _ -> [t| Int |] }

Then in another module:

> {-# LANGUAGE QuasiQuotes #-}
> import M1
> [e| this text goes to (quoteDec e) and ends up as x = 1|]
> y = 1 :: [e|this text goes to (quoteType e) and ends up as if just Int was written here|]

--
Adam