Arc Forum | (myeach a b c): set '_' to each value of 'a, then do 'b and 'c in the loop. Or i...

Arc Forum

1 point by palsecam 5625 days ago | link | parent

(myeach a b c): set '_' to each value of 'a, then do 'b and 'c in the loop. Or if 'a is undefined, set 'a to each value in 'b, and do 'c in the loop.

But if 'a is lexically bounded but is not the list/table to traverse, my 'myeach will not work as expected.

   (let v 2 (myeach v '(1 2 3) (prn v)))

will call (each _ v '(1 2 3) c) because 'v is defined.

And this is a problem. What I try to do, call it "optional first arg" is dirty. I mean, terribly unhygienic ;-), imperfect. Using the kind of tricks I'm trying is subject to discussion, and have a lot of 'cons.

But this is not really the point here. The point is, 'if in Arc doesn't work like in Common Lisp. If this is a bug or just a difference is also subject to discussion, but at least know the fact. I mean it is not that natural that Arc acts like that here.

To come back to your questions. I want to use "_" if the first argument is defined. If it is (defined), it supposes it is the list/table to traverse, else it supposes it is the variable to be bind (call it "context intelligence").

Yes, the two arguments solution is easy to do, like is defeating destructuring, and both solutions could be used (I'll certainly adopt one of them). But 1. adds extra ()s and 2. removes a good feature.

These times I feel adventurous, so I tried another thing :-)

Anyway, great thanks for your help & suggestions.

1 point by CatDancer 5625 days ago | link

How do you tell if 'a is defined? Are you using pg's Arc, or Anarki, or something else?

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1 point by palsecam 5625 days ago | link

I'm using this:

   (mac defined args   ; typically, args is one or more symbols
     `(errsafe (do ,@args t)))  ; typically, a pedant functional programmer will have an heart attack seeing this

which is, it's not that I love spamming this term but it must not be forgotten, terribly dirty. I mean using it indicates there is certainly a problem with your (functional) code. Plus here the implementation is tricky.

   arc> (defined a)
   nil
   arc> (let a 2 (defined a))
   t
   arc> (= b 3)
   3
   arc> (defined b)
   t
   arc> (defined (c d) e)
   nil
   arc> (defined 'e)
   t

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1 point by CatDancer 5625 days ago | link

At macro expansion time in Arc, there's no lexical scope yet, as Arc macros operate on the input program as lists. So if you say

  (let a 2 (myeach ...))

'myeach can check if 'a is defined using your function, but it will find out that 'a isn't defined, because the lexical scope for 'a isn't created until after the macro expansion is done.

I don't know about Common Lisp in particular, but there are other more powerful macro expansion languages that iirc can give you information such as whether a variable is in scope of not. 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.

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1 point by palsecam 5625 days ago | link

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.

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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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