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1 point by palsecam 5625 days ago | link | parent

CatDancer, thanks once again, you're full of advice!

> arc macros operate on the input program as lists

I agree.

> (let a 2 (myeach ...)) will see 'a undefined. [...] Lexical scope explanation [...]

I believe you know Arc internals way better than me, and I certainly miss some points in your explanation but it seems to work for me:

   arc> (mac myeach (first expr . body)  
          `(if (defined ,first)  
             (each _ ,first ,expr ,@body)
             (prn "undefined")))
   #3(tagged mac...)
   arc> (let a '(1 2 3) (myeach a (prn _)))
   1
   2
   3
   nil
   arc> (myeach a (prn _))
   undefined
   "undefined"
> More powerful macro systems [...] can give you information such as whether a variable is in scope of not.

Certainly yes. I don't know either.

> So I think the issue that you're running into isn't in the behavior of macros and 'if, but that Arc isn't able to tell you at macro expansion time whether a variable is defined or not.

Being (or not) able to tell me at macexpansion time if a variable is defined is part of what I'd call the behaviour of macros ;-)

But actually, 'defined was just a way I tried to deal with Arc macro expansion stuff, but it was just a consequence of my problems with 'if, not the beginning.

However yes, for the precise case of 'each (but again, this topic was not limited to it), maybe the 'if behaviour would not be the main issue anyway.



2 points by CatDancer 5625 days ago | link

> it seems to work for me

  (mac myeach (first expr . body)  
          `(if (defined ,first)  
             (each _ ,first ,expr ,@body)
             (prn "undefined")))
ah, but now you're doing the 'defined test at run time. What you want is to be able to do the 'defined test at macro expansion time in order to affect how your macro is expanded, and that, as far as I know, Arc isn't able to do for you.

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1 point by palsecam 5625 days ago | link

> now you're doing the 'defined test at run time.

Yes, but check the previous messages, I've always wanted to do so. The definition of 'myeach basically never changed.

> What you want is to be able to do the 'defined test at macro expansion time in order to affect how your macro is expanded

Not necessary (maybe yes in the current Arc because of the 'if behaviour, but between making the language works for me [change its behaviour] or works for the language [change my behaviour], you'll guess what I prefer), and no, this is too precise. What I want is 'each to be smart and have an "anonymous form". Which tricks to use to get that, I don't care. I don't care of 'defined, this thing should certainly not exist anyway.

> and that, as far as I know, Arc isn't able to do for you.

You're certainly right here.

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