Re: [Haskell-cafe] Metaprogramming in Haskell vs. Ocaml

On Mon, Apr 5, 2010 at 7:05 PM, Jason Dagit wrote:

On Mon, Apr 5, 2010 at 12:27 PM, Jacques Carette wrote:

...

2. people who care about types use a typed meta-language (like metaocaml) instead of an untyped template layer atop a (fantastic!) typed language.

Are you implying that template haskell is not typed?

Not to speak for Jacques, but my impression is that while TH itself is typed--it's just more Haskell after all--it doesn't do much to prevent you from generating code that is not well-typed. Or even well-formed, for that matter; my initial attempts to learn how to use TH produced quite a few "that's impossible!" errors from GHC (I do not think that word means what it thinks it means). There's also type-level metaprogramming, as in e.g. HList, which is almost completely untyped. I have some personal library code that implements a simple meta-type system and it's a huge, horrid, painful mess for something with an expressive power no better than System F. In contrast, MetaOCaml seems to be some variety of a multi-stage system where metaprogramming blends smoothly into "regular" programming with a single, consistent type ensuring type safety at all points. - C.