New branch implementing typed/untyped Template Haskell

[Geoffrey Mainland]

No special magic. Just make sure you get the th-new branches of ghc,
libraries/template-haskell, and testsuite.

If you hit any snags let me know---happy to help you get running.

Geoff

On 05/17/2013 07:12 PM, Jacques Carette wrote:
> Fantastic!  I will definitely  be trying it out early next week.
 >
 > Are there any special instructions regarding how to compile this
 > branch?  I will work on adding test cases, which would be translations
 > of larger metaocaml programs.
 >
 > Jacques
 >
 > On 13-05-16 11:21 AM, Geoffrey Mainland wrote:
 >> I have pushed a new branch, th-new, that partially implements the
 >> proposal outlined in Simon PJ's "New directions for Template Haskell"
 >> post at:
 >>
 >> http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal
 >>
 >> The main missing features are top-level pattern splices and local
 >> declaration splices.
 >>
 >> Typed expression quotations and typed expression splices *are*
 >> implemented. Syntax is as in Simon's original proposal: [|| ||] for
 >> typed expression brackets and $$(...) for typed expression splices. One
 >> can now write:
 >>
 >> power :: Int -> TExp (Int -> Int)
 >> power n = [|| \x -> $$(go n [|| x ||]) ||]
 >> where
 >> go :: Int -> TExp Int -> TExp Int
 >> go 0 x = [|| 1 ||]
 >> go n x = [|| $$x * $$(go (n-1) x) ||]
 >>
 >> In fact, one can even write:
 >>
 >> power :: Num a => Int -> TExp (a -> a)
 >> power n = [|| \x -> $$(go n [|| x ||]) ||]
 >> where
 >> go :: Num a => Int -> TExp a -> TExp a
 >> go 0 x = [|| 1 ||]
 >> go n x = [|| $$x * $$(go (n-1) x) ||]
 >>
 >> Writing the following
 >>
 >> f :: TExp Char -> TExp Integer
 >> f x = [|| $$x * 3 ||]
 >>
 >> gives the error
 >>
 >> Main.hs:28:7:
 >> Couldn't match type ‛Char’ with ‛Integer’
 >> Expected type: TExp Integer
 >> Actual type: TExp Char
 >> In the Template Haskell quotation [|| $$x * 3 ||]
 >> In the expression: [|| $$x * 3 ||]
 >> In an equation for ‛f’: f x = [|| $$x * 3 ||]
 >>
 >> The th-new branch of ghc has accompanying branches for template-haskell
 >> and testsuite, both named th-new. These are mirrors of my local branch,
 >> and I reserve the right to rebase them :). That said, please do try it
 >> out!
 >>
 >> Geoff

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