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by catnaroek
3496 days ago
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> Please tell me how Haskell represents HOAS and how this is not possible in Lisp. I'm not talking about representing some object language's syntax in Haskell or Lisp. I'm talking about representing Haskell or Lisp's syntax in some other metalanguage (possibly Haskell or Lisp itself). How would you implement a macro expander that operates on HOAS? > Barzilay's paper (http://scheme2006.cs.uchicago.edu/15-barzilay.pdf) The object language implemented in that paper isn't the whole of Scheme, and in particular, it doesn't have macros. > Ergo Project (1988! http://repository.cmu.edu/cgi/viewcontent.cgi?article=2763&c...) Sorry, I can't find an explicit description of any object language's syntax in this paper. |
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Hmm, you would expect that HOAS actually be helpful in this area in some ways. Since variables are no longer resolved by matching symbols to binding information in a scoped environment (all we have is direct names references to the binding inserted in the code, or something like that), we don't have hygiene issues when we move code around without having to do any alpha-renaming, or gensyms. It sounds rather convenient, unless we expressly want to do something non-hygienic.
So how would we implement a macro expander? Like we implement anything else: in whatever way that implementation satisfies the specification of macros under HOAS. What is the specification: that is the question. What are HOAS macros, or macros under HOAS?
Not having a formal specification, we might want at least some examples: OK, I have this HOAS artifact, and would like to be able to write it in a more condensed way using this other HOAS: now how to make one into the other? That gives us a function, where we can identify the inputs, which have to be arguments to the macro somehow and so it goes.
Some kind of dual macro system could be useful: a raw source code macro system for doing un-hygienic things, and then a parallel one which kicks in when the code is converted to HOAS.