arc> (= (s 0) #\m)
  Error: "string-set!: expects type <mutable string> as 1st argument, given: \"foo\"; other arguments were: 0 #\\m"

http://comments.gmane.org/gmane.lisp.scheme.plt/14787

In ac.scm, in the arc compiler ac, arc strings are translated using ac-string. I've tried instrumenting the output of ac-string, and it seems mutable when it's encountered. And yet it's immutable by the time I return to the prompt:

  arc> ($.immutable? "abc")
  #f  # printed inside *ac-string*
  #t  # return value

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I tried bisecting the git history, and the stable branch, which still requires mzscheme, has mutable strings. But sometime in 2011 when the master branch switched to using racket and stopped working with mzscheme, strings went immutable again. Still investigating..

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Back in Feb 2011, waterhouse provided a bugfix for arc's mutable pairs. http://arclanguage.org/item?id=13616. https://github.com/arclanguage/anarki/commit/cea8a4c5e9. This change requires a racket-only lib, and so arc stopped working with the old mzscheme versions. Prior to it, modifying strings worked in mzscheme but not in racket. So it seems to be something about the move from mzscheme 372 to racket.

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Update 34 minutes later: It seems eval in racket emits immutable strings even when given mutable strings. Since everything passes through racket's eval it doesn't matter how much arc twists and turns to avoid immutability.

  $ racket
  > (immutable? "abc")
  #t
  > (immutable? (string-copy "abc"))
  #f
  > (immutable? (eval (string-copy "abc")))
  #t

Here is a quick and dirty patch for arc3.1, it postpones string-copy and let it run by eval.

Not sure if it will cause some other problem.

  50,51c50,51
  <           (unescape-ats s))
  <       (string-copy s)))          ; avoid immutable strings
  ---
  >           `(string-copy ,(unescape-ats s)))
  >       `(string-copy ,s)))          ; avoid immutable strings

https://github.com/Pauan/ar/commit/a2c1939936846b665eccf06d0...

BTW, I think `(unescape-ats s)` also needs to be treated as the same.

I'd like to run some tests to make sure it does not cause much problem.

I found this one:

https://github.com/conanite/rainbow/tree/master/src/arc/lib/...

But it failed with following error when using arc3.1.

  arc> (load "unit-test.arc")
  nil
  arc> (rat)
  Error: "_dfn: undefined;\n cannot reference undefined identifier"

I've added a few piecemeal tests over the years (all the .t files in the repo), and there's also some tests in my curated repo (http://github.com/akkartik/arc). All those seem ok..

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Looks like dfn is a rainbow innovation: http://arclanguage.org/item?id=10539

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I've also added/plagiarized rainbow's tests to the repo: https://github.com/arclanguage/anarki/commit/68be3c0f4d. Joys of the perl artistic license.

There's still a couple of syntax tests that are failing, likely because of test harness issues. And I'd like to unify all the different tests into a single framework. For future work..

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