Is there a relationship between untyped/typed code quotations in F# and macro hygiene?

Question

I wonder if there is a relationship between untyped/typed code quotations in F# and the hygiene of macro systems. Do they solve the same issues in their respective languages or are they separate concerns?

Answer 1

The meta-programming aspect is the only similarity, and even in that regard, there is a big difference. You can think of the macro's transformer as a function from syntax to syntax like you can manipulate quotations, but the transformers are globally coordinated so that names used as binders follow a specific protocol:

1) Binders may not be the same as any free name in input to the macro (unless you use an unhygienic escape hatch)

2) Names bound in a macro definition's context that are free in the macro's expansion must point to the same thing at macro use time. (this needs global coordination)

Choices for names are made so that expansion does not fail if you used the wrong name (unless it turns out that name is unbound).

Transformers of typed quotations do not have this definition time context idea. You manipulate quotations to form a program that does not refer to any names in your program. They are not meant to provide a syntactic abstraction mechanism. Arbitrary shapes of syntax? Nope. It all has to be core AST shapes.

Open code in typed quotation systems can be closed with anything that fits the type structure of the expected context - there is no coordinated composition of several open components into a coherent structure.

Answer 2

Quotations are a form of meta-programming. They allow you to manipulate abstract syntax trees programmatically, which can be in turned spliced into code, and evaluated.

Typed quotations embed the reified type of the AST in the host language's type system, so they ensure you cannot generate ill-typed fragments of code. Untyped quotations do not offer that guarantee (it may fail with a runtime error).

As an aside, typed quotations are strongly similar to Template Haskell quasiquotations.

Hygenic macros in Lisp-like languages are related, in that they exist to support meta-programming. The hygiene however is for simple name capture confusion, something that typed quasi quotations already avoid (and more).

So yes, they are similar, in that they are mechanisms for meta-programming in typed and untyped languages, respectively. Both typed quasi quotes and hygenic macros add additional safety to fully untyped, unsound meta programming. The level of guarantee they offer the programmer though is different. The typed quotes are strictly stronger.