[Haskell-cafe] Typed TemplateHaskell?

[Simon Peyton-Jones]

Maybe take a look at
   http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-
| bounces at haskell.org] On Behalf Of Tillmann Rendel
| Sent: 23 May 2012 18:20
| To: Haskell Café
| Subject: Re: [Haskell-cafe] Typed TemplateHaskell?
| 
| Hi Ilya,
| 
| Ilya Portnov wrote:
| > As far as can I see, using features of last GHC one could write typed
| > TH library relatively easily, and saving backwards compatibility.
| >
| > For example, now we have Q monad and Exp type in "template-haskell"
| > package. Let's imagine some new package, say "typed-template-haskell",
| > with new TQ monad and new polymorphic type Exp :: * -> *. Using last
| > GHC's features, one will easily write something like "expr :: Exp
| > String", which will mean that "expr" represents a string expression.
| > And we will need a new function, say runTQ :: TQ a -> Q a (or some
| > more complicated type), which will turn TypedTemplateHaskell's
| > constructs into plain TH.
| 
| That would be a good thing to have. But it might be quite hard to
| implement. For example, I guess you might want to have functions like
| this one:
| 
|    apply :: Exp (a -> b) -> Exp a -> Exp b
| 
| This function takes two typed expressions and produces an application.
| The types ensure that the generated application will typecheck. Cool.
| 
| But can you do the same thing for lambdas? Lambdas create functions, so
| the type would be something like the following:
| 
|    lambda :: ... -> Exp (a -> b)
| 
| But what would you put instead of the ...?
| 
| I fear that overall, you would have to reimplement Haskell's type system
| in Haskell's type system. Which sounds like a cool thing to do, but
| maybe not so easily.
| 
|    Tillmann
| 
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