This page is part of the web mail archives of SRFI 62 from before July 7th, 2015. The new archives for SRFI 62 contain all messages, not just those from before July 7th, 2015.
> Robby Findler writes: >> David Van Horn writes: >> What does the following evaluate to? >> >> (list 'x #;#;'y 'z) > > FWIW, mzscheme and chez scheme do the same thing: > ... > > (list 'x #;#;'y 'z) > (x) > > Taylor Campbell writes: > MzScheme & Chez, as Robby Findler demonstrated, both give the list (X). > This is correct. (SISC & Chicken do, too.) The reason is that #; > ignores the single next following S-expression and allows the reader to > continue on after that. In the string "#;'y 'z", the #; comments out > the 'Y part, leaving the 'Z part. Since a #; precedes that string in > your example, the 'Z part is ignored, too, so the whole thing is read > as (LIST (QUOTE X)), which evaluates to the list (X). > > This is an excellent & possibly initially confusing example (though the > explanation is simple & straightforward), so I'll add it to the SRFI > document. I'll also add another example of nested comments, too: > > (list 'a #;(list 'b #;'c 'd) 'e) ==> (LIST (QUOTE A) (QUOTE E)) > > The inner #; comments out the following 'C, but the list structure that > lies a layer above it is still read as a complete S-expression -- in > particular, (LIST (QUOTE B) (QUOTE D)) --. Then the outer #; comments > that out, leaving only (LIST (QUOTE A) (QUOTE E)), which evaluates to > the list (A E). Personally I believe this is not a good idea, it's neither syntactically consistent with scheme, nor visually expected: more simply and consistently I would expect a #; comment to lexically remove the expression/token it's been lexically prepended to, nothing else. (including white-space). i.e.: #; => #;#; => #;a => #;#;a => #;#; a => a #;(a b) => #; (a b) => (a b) (#;a b) => (b) (#; a b) => (a b) (#;#; a b) => (a b) (#; #; a b) => (a b) (#; #;a b) => (b) (#;a #;b) => () (a #; b #;c) => (a b) And just to be certain, as earlier comments make reference to strings, although suspect not intended, #; should defiantly not affect what's inside of strings, as strings are themselves an literal expression unto themselves: #; "a b c" => "a b c" #;"a b c" => "#;a b c" => "#;a b c" Let's at least try to make such a feature as consistent with similar syntactic abbreviations; As I don't suspect anyone would expect: ''a b c => (quote a) (quote b) c ; would they? They'd expect: ''a b c => (quote (quote a)) b c Where possibly with respect to: > Aubrey Jaffer wrote: > -- Function: comment string1 ... > Appends STRING1 ... to the strings given as arguments to previous > calls `comment'. > > -- Function: comment > Returns the (appended) strings given as arguments to previous calls > `comment' and empties the current string collection. > > -- Load syntax: #;text-till-end-of-line > Behaves as `(comment "TEXT-TILL-END-OF-LINE")'. (comment ...) seems like a nice feature, but personally suspect it's just As convenient and feature preserving to utilize #; to lexically remove expressions it's been prepended to, while preserving the explicit use of (comment ...) within code. (but it's just an opinion).