From: Heinrich Apfelmus
* Meta-programming / partial evaluation. When designing a DSL, it is often the case that you know how to write an optimizing compiler for your DSL because it's usually a first-order language. However, trying to squeeze that into GHC rules is hopeless. Having some way of compiling code at run-time would solve that. Examples: ** Conal Elliott's image description language Pan ** Henning Thielemann's synthesizer-llvm
I've been thinking about this, and I suspect that meta-programming in Haskell may not be that far off. Suppose you have a Meta monad data Meta a = Meta { runMeta :: a} with the magical property that the compiler will optimize/recompile expressions of type `Meta a` when they are run via `runMeta`. That would provide usable metaprogramming, and I think it would have all the desired type properties etc. Of course no compiler currently has that facility, but we could use a different monad, perhaps something like this: data ThMeta a = ThMeta { runThMeta :: Language.Haskell.TH.ExpQ } now we just need to get the compiler to run an arbitrary TH splice and check that the types match after `runThMeta` is called. I think this is already possible via the GHC api. It would have the undesirable property that some expressions could be ill-typed, and this wouldn't be known until run-time, but it's a start. That's about as far as I got before I discovered a much more worked-out version on Oleg's site (with Chung-chieh Shan and Yukiyoshi Kameyama). Of course they've tackled a lot of the awkward typing issues that my simple construct rudely pushes onto the user. I'm probably showing my naivety here, and I haven't fully digested their papers yet, but I wouldn't be surprised to see applications using Haskell metaprogramming within a relatively short time. John L.