Hi Jens, Thank you for telling me that Racket does not support capture and extend environments.
So that I can change direction. Hi all, I found that Guile has a function called `local-eval`, which can be used to simulate FEXPRs. see https://www.gnu.org/software/guile/manual/guile.html#index-local_002deval I implemented do-notation by `local-eval`. ``` ;guile 2.2.3 (use-modules (ice-9 local-eval)) (use-modules (ice-9 match)) (use-modules (srfi srfi-9)) (define-record-type Just (make-Just x) Just? (x Just-x)) (define-record-type Nothing (make-Nothing) Nothing?) (define (extends-env env var-name value) (local-eval `(let ((,var-name ,value)) (the-environment)) env)) (define (do env binds return) (match binds [() (begin (local-eval return env))] [((var e) rest-binds ...) (begin (match (local-eval e env) [($ Nothing) (make-Nothing)] [($ Just x) (do (extends-env env var x) rest-binds return)]) )])) (define (bigger-than-two n) (if (> n 2) (make-Just n) (make-Nothing))) (display (do (the-environment) (list '[x (bigger-than-two 4)] '[y (bigger-than-two 5)]) '(make-Just (+ x y)))) ``` I was surprised that it works. PS: The code above can be compiled and run in guile 2.2.3. You can test it at https://rextester.com/l/scheme_online_compiler, if someone interested, Best regards, Siyuan Chen On Mon, Jun 8, 2020 at 12:23 AM Jens Axel Søgaard <[email protected]> wrote: > Den søn. 7. jun. 2020 kl. 14.09 skrev Siyuan Chen <[email protected]>: > > Unfortunately, this code doesn't work, because it lacks two functions, >> `get-current-env` and `extends-env`. >> > > These are not available since static lexical scope makes it possible for > the compiler to determine where > a variable is stored at compile time. Therefore an explicit environment is > not needed. > (See "early binding" vs "late binding"). > > >> Is there any way to work around this issue? >> > > >> I have searched Racket's document. Racket has namespaces that are very >> similar to the environment here. >> >> But it seems that `current-namespace` can not get binding (e.g. Just) and >> `namespace-set-variable-value!` is a mutable function which is not suitable >> as well. >> > > Namespaces are "environment-like" but contain only top-level variables. > > Could someone give me some advice? >> Or we can modify the code above to make it work in Racket. >> > > Since this for educational/fun use and not practical, I suggest using a > Scheme interpreter where the environments > are explicit. Then you can easily add `get-current-env` and `extends-env` > yourself. > > One option is the interpreter page LiSP (Lisp in Small Pieces) where an > interpreter for fexprs is discussed > with full working code. > > /Jens Axel > https://racket-stories.com > > > > > -- 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]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAHWTsYkD7i2ctsc-rAYrjjKBNKBBd8rQvqni2%2B-F%3DHV8PHtWiw%40mail.gmail.com.
