Friday, September 09, 2011

syntax-parse and literals

In my last post, I talked about macros and referential auxiliary identifiers—what we usually call a macro’s “literals.” Scheme macro systems only get it half right, though, because while they compare identifiers using referential equality (i.e., using the free-identifier=? predicate), they allow literals to refer to nonexistent bindings. While the comparison is well-defined via the definition of free-identifier=?, at a higher level the idea is nonsensical.

In contrast, syntax-parse requires that every literal refer to some binding. (I’ll sometimes refer to this requirement as the is-bound property for short.) This requirement is problematic in a different way. Specifically, this property cannot be checked statically (that is, when the syntax-parse expression containing the literal is compiled).

That might strike you as bizarre or unlikely. After all, you can easily imagine checking that a syntax-rules macro, say, satisfies the is-bound property. But in Racket, not every macro uses syntax-rules, and—more importantly—not every bit of syntax-analyzing code is a macro. And both of these facts have to do with phases.

In Racket, an identifier might mean one thing in one phase and another thing in a different phase. Actually, this is true for every identifier: it is bound in finitely many phases and unbound in infinitely many more. When an identifier is bound in multiple phases, it is usually bound to the same thing in each phase, but that is a consequence of the way we write (tasteful) code, not a rule imposed by the macro system. Consequently, identifier comparisons must take phase levels into account, and Racket’s version of free-identifier=? takes an optional phase level argument, which defaults to the value of (syntax-local-phase-level), which usually turns out to be just what you want.

There are two modes of dealing with syntax in Racket, which I will label “eval” and “macro.” In “eval” mode, your code is just an ordinary program that manipulates syntax. Perhaps it takes some code, expands it, analyzes the result, and instruments it (e.g., errortrace). Perhaps it constructs code to be executed (e.g., parts of DrRacket’s “module language” implementation). In any case, your program is running at phase 0 and it deals with syntax which represents another program at phase 0.

In “macro” mode, on the other hand, your code is either a macro definition or a helper to a macro definition. Your code is runs at phase 1 and deals with syntax representing code at phase 0. Actually, since your macro might be used in the implementation of some other macro (possibly itself being used in the implementation a macro, and so on), your code runs at phase N+1 and deals with syntax representing code at phase N. You’d better not depend on the particular value of N, either.

So in “eval” mode, you generally want to compare identifiers at phase 0, but in “macro” mode, you want to compare identifiers at phase N-1, where N is the phase of the code currently executing. No problem! Since “macro” code is always at phase 1 or higher, these situations are distinguishable, and we can choose the right default for each of them:

(define (syntax-local-phase-level)
(cond [(= (phase-of-executing-code) 0) 0]
[else (sub1 (phase-of-executing-code))]))

So what’s the problem?

Consider the following bit of code:


#lang racket
(require ???)
; is-lambda? : syntax -> boolean
; Determines if this is a lambda form.
(define (is-lambda? stx)
(syntax-parse stx #:literals (lambda)
[(lambda formals . body) #t]
[_ #f]))
(provide is-lambda?)

What phase will be used for the lambda literal comparison?

Well, if this code is used as a macro helper—that is, this module is required for-syntaxthen the phase will be the phase of the module minus 1, so the module had better have a (require (for-template racket)). On the other hand, if this code is used in “eval” mode, the comparison will be done with respect to phase 0, so we need just (require racket)of course, we have that from the module’s language anyway.

(Actually, it’s even worse than that. Racket’s syntax-local-phase-level isn’t as simple as my definition above; for example, it seems to always be 0 for the initialization of a module, even if the module is instantiated at a phase greater than 1.)

So, should what phase(s) should we check that lambda is bound in? Checking only one is not adequate to enforce the is-bound property, but demanding both is unreasonable—adding a require line is costly at best, semantics-altering at worst. Thus the check must be done dynamically.

Well, not entirely. It turns out that checking if an identifier is bound is relatively expensive. And while we don’t know what phase(s) the comparison will actually use at run time, we know it’s highly likely to be either N or N-1, where N is the phase of the module. If we pre-compute those answers at compile time, we can use them as a fast-path check that only requires numeric comparison (cheap)... and if we get a really weird phase level, we can revert to the slow check.

And that, I think, is the best you can do with literals and standard free-identifier=?. On the other hand, what if you had a cross-phase identifier comparison, one that determines if identifierA refers to the same binding at phaseA that identifierB refers to at phaseB? Then you could fix the phase for the literal identifier and check it at compile time. That’s how syntax-parse’s literal-sets work.

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