2016-07-07

Weirdness of self evaluating vector

Scheme's vector has a history of being non-self evaluating datum and self evaluating datum. The first one is on R6RS, and the latter one is R7RS (not sure about R5RS). Most of the time, you don't really care about the difference other than it requires ' (quote) or not. However, you may need to think about the difference and maybe also think self evaluating causes more trouble. One of the particular case (and this is the only case I think self evaluating vector is evil) is when vector is used in macro.

Have a look at this case:
(import (rnrs))

(define-syntax foo
  (syntax-case ()
    ((_ e) e)))

(foo #(a b c))
What do you think how it behaves? The answer is depending on the standard. On R6RS, vectors are not self evaluating data so this should be an error. So you can't complain if you'd get a daemon from your nose. On R7RS (of course you should change the importing library name to (scheme base)), on the other hand, vectors are self evaluating data so this should return the input vector.

Now, how about this case?
(import (scheme base) (scheme write))

(define-syntax foo
  (syntax-rules ()
    ((_ "go" (v ...) ())         #(v ...))
    ((_ "go" (v ...) (e e* ...)) (foo "go" (v ... t) (e* ...)))
    ((_ e ...)                   (foo "go" () (e ...)))))

(foo a b c d e)
What would be the expansion result of macro foo? I think this is totally up to implementations (if it's not please let me know). For example, Chibi returns vector of something like {Sc #22 #<Environment 4365836288> () t} (syntactic closure, I think), Sagittarius returns vector of identifier, and Larceny returns vector of symbol t. If you put ' (quote) to the result template then the expansion result should be the same as Larceny returns (though, Chibi still returned a vector of syntactic closure, so this might not be defined, either).

Back to the first case. The first case sometimes bites me when I write/use R6RS macro in R7RS context. For example, SRFI-64 is implemented in R6RS macro and using it like this:
(import (scheme base) (srfi 64))

(test-begin "foo")

(test-equal "boom!" #(a b) (vector 'a 'b))
;; FAIL!!

(test-end)
On Sagittarius, R6RS macro transformer first converts all symbols into identifiers, then syntax information will be stripped only if expressions have quote. Now, SRFI-64 is implemented on the R6RS macro transformer and the vector doesn't have quote. Thus, symbols inside of the vector are converted to identifiers. If it's R6RS, then it's an error. But if it's R7RS, it should be a valid script.

I have sort of solution (not sure if I do it or not): Internally, symbol and the identifiers converted from symbols without any context (c.f. not using datum->syntax) are theoretically the same. So if compiler sees such an identifier, then it should be able to unwrap it safely.

I haven't decided how it should be. So for now, just a memo and let it sleep.

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