No defmacro support

[BracketMaster]

Sometimes I like to write quick and dirty macros, but it seems cyclone doesn't support this? It seems SCM will allow you to make macros hygenic with gentemp for example. Is this available in cyclone?

[justinethier]

There is no defmacro but you can use explicit renaming macros if you want something quick and dirty, EG:

    (define-syntax when
      (er-macro-transformer
        (lambda (exp rename compare)
          (if (null? (cdr exp)) (error/loc "empty when" exp))
          (if (null? (cddr exp)) (error/loc "no when body" exp))
          `(if ,(cadr exp)
               ((lambda () ,@(cddr exp)))))))

[BracketMaster]

Thanks. I also wonder if there's a way to "preview" the results of a macro expansion before things get evaluated. This would help with debugging.

[justinethier]

There are a few ways to do that:

• You can compile a file with the -t option which will write all of the intermediate transformations - including macro expansions - out to the .c file.

• From the interpreter you can use expand, EG:

  cyclone> (expand '(when #t (+ 1 2 3)) global-environment '()) (if #t ((lambda () (+ 1 2 3))) )

• Alternatively when developing an ER macro, since its just a Scheme function, you could define it as a lambda and pass it your own quoted expression to debug.

[BracketMaster]

it seems expand has three arguments, what is the third for? https://justinethier.github.io/cyclone/docs/api/scheme/cyclone/macros#macroexpand

[BracketMaster]

It also seems er-macro-transformer is implicitly defined within define-syntax. https://github.com/justinethier/cyclone/blob/7f4f67f612bcccd2d13a75cbe1e3f282e649a26b/scheme/eval.sld#L533

[justinethier]

it seems expand has three arguments, what is the third for? https://justinethier.github.io/cyclone/docs/api/scheme/cyclone/macros#macroexpand

Good question. The third argument is an environment of variables renamed directly by macros. For simple testing you can just pass '() to indicate an empty environment.

Let me see about putting together some documentation to better explain how to use this function.

[justinethier]

It also seems er-macro-transformer is implicitly defined within define-syntax. https://github.com/justinethier/cyclone/blob/7f4f67f612bcccd2d13a75cbe1e3f282e649a26b/scheme/eval.sld#L533

That is true. Do you have concerns?

[BracketMaster]

Not a problem, just something I observed.

Also, your macro docs are down: https://justinethier.github.io/cyclone/docs/api/scheme/cyclone/macros

Lastly, the following seems to not work:

; myfile.scm
(define-syntax while
  (syntax-rules ()
    ((_ condition . body)
     (let loop ()
       (cond (condition
          (begin . body)
          (loop)))))))

(define x 0)
(expand '(while (< x 3) (set! x (+ x 1)) (display x)) *global-environment* `())

(exit 0)

icyc -s myfile.scm complains Error: Invalid type: expected pair, found : ()

[justinethier]

Thanks for the feedback on this :)

The (scheme cyclone macros) module is no longer used. expand is documented here: http://justinethier.github.io/cyclone/docs/api/scheme/eval#expand

In your example, the macro expansion itself seems fine. The issue is with calling expand. You know, when debugging I generally use the other methods and do not call expand directly. But it would be helpful if we had a way to reliably debug macro expansions from the REPL. Let me take a closer look and see what can be done.

[justinethier]

Apologies, the appropriate call is:

(expand '(while (< x 3) (set! x (+ x 1)) (display x)) *global-environment* (env:extend-environment '() '() '())

That works but is a lot to type and remember... So expand has been updated on master to only require a single expression argument:

(expand '(while (< x 3) (set! x (+ x 1)) (display x)))

This is available now if you are building from source and will be included in the next release.