On Tuesday, January 10, 2017 at 4:34:41 PM UTC-5, Alexis King wrote: > Basically, the difference is as follows: #:literals compares > identifiers using free-identifier=?, but #:datum-literals compares > them by using syntax-e and eq?. > > You can observe the difference using two extremely simple macros > that only differ in their use of #:literals or #:datum-literals: > > (define-syntax lit > (syntax-parser > #:literals [add1] > [(_ add1) #'"add1"] > [(_ _) #'"something else"])) > > (define-syntax dat > (syntax-parser > #:datum-literals [add1] > [(_ add1) #'"add1"] > [(_ _) #'"something else"])) > > Observe the behavior of the `lit` macro: > > > (lit add1) > "add1" > > (let ([add1 #f]) > (lit add1)) > "something else" > > (require (rename-in racket/base [add1 my-add1])) > > (lit my-add1) > "add1" > > Contrast the results with the same examples using the `dat` macro > instead: > > > (dat add1) > "add1" > > (let ([add1 #f]) > (dat add1)) > "add1" > > (require (rename-in racket/base [add1 my-add1])) > > (dat my-add1) > "something else" > > By “recognize symbolically”, it means that #:datum-literals looks > at the name of the symbol and nothing else. In contrast, #:literals > is more advanced, since it looks for an identifier with the same > binding as the one specified. > > In my opinion, when in doubt, prefer #:literals. > > > On Jan 10, 2017, at 11:58 AM, Deren Dohoda > > wrote: > > > > I am still making most macros using syntax-rules and syntax-case > > because when I happened to learn macros these were the paths of > > least resistance. Every once in a while I try to learn a little > > more of syntax-parse since the few times I've tried it I really > > liked it. > > > > It appears that, in general, syntax-rules and syntax-case use what > > syntax-parse considers "datum-literals", which the docs say are > > recognized "symbolically" versus actual literals which are recognized > > "by binding." The example in the documents for some reason clarifies > > nothing since both expressions are the same and give the same output, > > making this a distinction without an obvious difference. > > > > Can someone explain the intention behind #:literals as opposed to > > #:datum-literals? In what cases should I consider #:literals? Why > > would I want to avoid #:datum-literals, or vice versa? > > > > Thanks, > > Deren
I have a related question: How does #:literals differ from ~literal in the left-hand side of syntax-parse? I used to think they were the same but it seems that changing from one to another altered my program's behavior... -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
