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Problems while trying to run news in arc/nu
4 points by Oscar-Belletti 863 days ago | 14 comments
I've installed arc/nu from github and it runs ok. I made a link of the "arc" file and put it in my ~/bin so I can get to the repl by typing "arc" at the terminal from any directory. Then I decided to try to run the news from a directory(not the installation one). So i put all the library files in that directory and started arc's repl, but, when I tried to run the news, after entering (load "news.arc") I got an error message: "error: open-input-file: contract violation expected: path-string? given: 't ". Then I tried to reload it and got the "* redefining " messages. They interrupt at "* redefining ignore" and since I didn't get "* redefining log-ignore" the error happened at (diskvar ignore-log* (+ newsdir* "ignore-log")). I tried from the installation folder, and it works. The error appears only when I try to call it from a different folder.

4 points by rocketnia 863 days ago | link

"I tried from the installation folder, and it works. The error appears only when I try to call it from a different folder."

That's good to hear. Thanks for sharing your process.

It would be useful to find out what's causing that error in particular, so I'll check on that in a moment.

(This is something that the official releases of Arc were pretty picky about; the language would load paths relative to the current directory, so you pretty much had to start Arc from the same directory as strings.arc, etc. I think Anarki has occasionally made changes to this behavior, but maybe Arc/Nu is still picky.)


4 points by rocketnia 862 days ago | link

Hmm... It looks like it's my turn to have trouble! What version of Racket are you using? I might need to revert to that version to get Arc/Nu working on my system. XD


"Then I decided to try to run the news from a directory(not the installation one). So i put all the library files in that directory [...]"

Now that I've read this again, that sounds like something that would break the libraries in some way. The files of Arc/Nu contain relative paths to each other. If you move some files, you might need to edit the paths that occur in the other files. Which files were you trying to move where?


For anyone here who can help, here's the problem I'm encountering.

When I try to start Arc/Nu on Windows 10 with Racket 6.4 or 6.5 installed, I get this error:

  hash-ref: contract violation
    expected: hash?
    given: #f
    argument position: 1st
    other arguments...:
I've tracked it down to a specific place: The execution of the Racket code (mac % args (% args)) in lang/arc/3.1/main.

That line is trying to set a hash table entry in Arc/Nu's (globals) table. In the file "compiler", when (namespace-require path) is called to load lang/arc/3.1/main dynamically, the (globals) parameter has been parameterized to contain an appropriate table to store that definition.

However, when lang/arc/3.1/main requires the "compiler" module, it loads a second instance of the module. So when it tries to access (globals), it gets the original value: (define globals (make-parameter #f)). Since #f isn't a hash table, hash-ref raises an exception.

Does anyone know if there's a way to make lang/arc/3.1/main reuse the same "compiler" module instance that's already been loaded?


3 points by Pauan 859 days ago | link

That sounds super strange. From what I understand, Racket automatically caches modules, so they are only loaded once.

But then again, Racket does have multiple phases, so it's possible that it's loading the "compiler" file in two different phases, causing duplicate variable definitions.

In addition, Racket doesn't allow for mutually recursive modules, but Arc/Nu is using "namespace-require", which apparently bypasses that restriction. It's possible that there is a race condition that is causing the "arc/3.1/main" file to be loaded before the "compiler" file is finished loading, which then triggers a second load of the "compiler" file.

Just to clarify: have you tried inserting a "displayln" in the "compiler" file to verify that it is being loaded twice?


4 points by rocketnia 858 days ago | link

That is exactly how I determined it was being loaded twice.

I was meaning to try on other versions of Racket but I've gotten distracted.


3 points by Pauan 851 days ago | link

Okay, I just tried upgrading to Racket 6.5, but it still works correctly for me.

I wonder if it's a bug with the Windows version of Racket?


2 points by Oscar-Belletti 861 days ago | link

Actually I was wrong in saying that in the installation folder it worked. In the installation folder it succeeded in loading the "news.arc" file, and failed at runtime. While in the other folder that failed, because in that folder I already tried with anarki and created some news/bans and this is why it failed to load: the diskvar didn't work. However the problem wasn't in diskvar: it was in "file-exists" which should return the name of the file in case of true: because of this when I just modified "diskvar" it still gave me the contract violation error, I think because the arc code relies on the fact that file-exists returns true. I changed in "lang/arc/3.1/main" the line "(def file-exists (x) (tnil (file-exists? x)))" in "(def file-exists (x) (if (file-exists? x) x nil))". Now I successfully run the news!


2 points by akkartik 860 days ago | link

Awesome! If you submit a pull request on github we'll give you (more) credit for fixing this.

I see some more tnil in lang/arc/3.1/main. It would also be good to check if any other functions are incompatible with Arc 3.1.


4 points by Oscar-Belletti 862 days ago | link

I think I found the problem. Diskvar is broken:

  (mac fromdisk (var file init load save)
    (w/uniq (gf gv)
      `(unless (bound ',var)
         (do1 (= ,var (iflet ,gf (file-exists ,file)
                                 (,load ,gf)
              (= (savers* ',var) (fn (,gv) (,save ,gv ,file)))))))
Notice the "(iflet ,gf (file-exists ,file)". It expexts file-exists to return the name of the file if it exists. file-exists doesn't, it returns t if files exists, so here is the cause of "error: open-input-file: contract violation expected: path-string? given: 't ".


4 points by Pauan 859 days ago | link

Hey, I'm the author of Arc/Nu.

Thanks for letting me know about this.

In my opinion, returning t/nil makes more sense than returning the file path, but Arc/Nu is supposed to be backwards compatible with Arc 3.1, so I fixed this. I also fixed "dir-exists", which has the same issue.

I just pushed out the fix to the Arc/Nu GitHub repository, please try it out and let me know if it solves the problem for you.


4 points by Oscar-Belletti 857 days ago | link

I've just tried running the news site with the new version of Arc/Nu and it runs fine.


3 points by Pauan 856 days ago | link

Thanks for letting me know, I'm glad it's working now.


2 points by akkartik 862 days ago | link

You're right. It looks like file-exists has changed its behavior in Arc-Nu to return a boolean even though its being loaded from lang/arc/3.1/main, which might lead one to think it's trying to be compatible with Arc 3.1 [1]. Let me try to reach out to the original author.

[1] Arc has always returned the name on success, as far back as the Arc 0 release.


1 point by akkartik 862 days ago | link

What platform are you on? I'm reminded of this other bug with diskvar on Windows (since fixed on Anarki):

I haven't tried Arc/Nu, but Arc 3.1 and Anarki both return a string path name on success.


3 points by Oscar-Belletti 862 days ago | link

I am using racket v5.3.6 on Xubuntu.