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1 point by Pauan 2808 days ago | link | parent

Exactly! nil too. :P


2 points by shader 2808 days ago | link

Hmm... the issue with that is that you might start having to quote nil or t whenever you want to actually mean nil or t, instead of just typing them in normally.

I do wish there was an easier way to tell whether or not a value was provided as nil, or was left empty and defaults to nil. Maybe doing destructuring on rest args would help solve that problem in most cases?

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1 point by Pauan 2807 days ago | link

"I do wish there was an easier way to tell whether or not a value was provided as nil, or was left empty and defaults to nil."

I too have sometimes wished for that in JavaScript, but let me tell you a little story. I was writing a syntax highlighter, and got it working fine in Chrome and Firefox 3.5, but there was a bug in Firefox 3.0.

You see, I was using this bit of code here:

  output.push(text.slice(curr.index[1], next && next.index[0]));
If `next` doesn't exist, it will pass the value `undefined` to the `slice` method. In JS, if you don't pass an argument, it defaults to `undefined`, so this is supposed to behave like as if I hadn't passed in the argument at all.

But in Firefox 3.0, the slice method behaves differently depending on whether you pass it `undefined`, or don't pass it any arguments. So, I had to use this instead:

  if (!next) {
      output.push(text.slice(curr.index[1]));
  } else {
      output.push(text.slice(curr.index[1], next.index[0]));
  }
This was (thankfully) fixed in 3.5. The moral of the story: most of the time it doesn't matter whether the caller passed nil, or didn't pass anything. You can treat the two situations as the same.

Consider this hypothetical example in Arc:

  (your-func 5 (and x y z))
If x, y, or z are non-nil, it will be passed in as usual. On the other hand, if any of them are nil, it will be like as if you had used (your-func 5 nil).

By behaving differently when nil is passed in vs. not passing in an argument, you might cause the above example to break. Or perhaps it would work, but the behavior would be subtly different... introducing bugs.

By having different behavior depending on whether an argument is passed or not, you force callers to do this, instead:

  (iflet val (and x y z)
    (your-func 5 val)
    (your-func 5))
Note the redundancy. In fact, this is even more important in Arc (compared to JavaScript) because you can use any expression, such as (if), a macro call, etc.

So... let me ask: what situations do you really need to know whether the caller actually passed in nil, or didn't pass anything at all?

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1 point by rocketnia 2807 days ago | link

Great point. In fact, I don't check whether an optional argument was passed very often, and the times I do, I usually expect to regret it at some point, for exactly that reason. ^_^

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1 point by rocketnia 2808 days ago | link

"I do wish there was an easier way to tell whether or not a value was provided as nil"

I share this sentiment. One thing we could do is have a space of hidden-from-the-programmer variables which tell you whether other variables have been bound. They can be accessed using a macro:

  (= given-prefix* (string (uniq) "-given-"))
  (mac given (var)
    ; NOTE: I don't think this will work properly for nil, but nil is
    ; never a local variable name anyway.
    (sym:+ given-prefix* var))
The implementation of argument lists would need to be aware of 'given-prefix* and bind the prefixed variables at the same time as the regular ones.

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"Maybe doing destructuring on rest args would help solve that problem in most cases?"

What do you mean by that?

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2 points by shader 2808 days ago | link

Well, if you use a rest arg for all optional values, and then use some form of destructuring bind on that list to extract your optional arguments, then you can tell whether or not they were passed in or merely defaulted to nil by just searching the arg list.

  (def test args
    (if (assoc 'c args)
          (pr "c was passed")
        (pr "c was not passed")))

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1 point by rocketnia 2808 days ago | link

I still don't follow. We can already manage the argument list manually, but in most of the suggestions here, we can only do it if we don't destructure it in the signature (unless we use more complicated kinds of destructuring).

  ; Current options:
  
  (def test args
    (let ((o c)) args
      (pr:if (len> args 0)
        "c was passed"
        "c was not passed")))
  
  (let missing list.nil  ; This is just something unique.
    (def test ((o c missing))
      (pr:if (is c missing)
        "c was not passed"
        "c was passed")))
  
  
  ; Some hypothetical options and non-options:
  
  (def test (& (c))
    (pr "no way to tell if c was passed"))
  
  (let missing list.nil
    (def test (& (c))
      (pr "still no way to tell if c was passed")))
  
  (def test (& args)
    (let ((o c)) args
      (pr:if (len> args 0)
        "c was passed"
        "c was not passed")))
  
  (def test (& (&both args (c)))  ; Destructure twice.
    (pr:if (len> args 0)
      "c was passed"
      "c was not passed"))
  
  (def test ((o c nil c-passed))
    (pr:if c-passed
      "c was passed"
      "c was not passed"))
  
  (def test ((o c))
    (pr:if given.c
      "c was passed"
      "c was not passed"))
  
  (def test (c)  ; Parameter lists are just destructuring.
    (pr:if given.c
      "c was passed"
      "c was not passed"))
  
  (def test (&both args (c))
    (pr:if (len> args 0)
      "c was passed"
      "c was not passed"))

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1 point by Pauan 2807 days ago | link

Brilliant! In fact, you could write a macro that would do that for you:

  (mac defreq (name args . body)
    `(w/uniq gen
       (def ,name ,(map (fn (x) `(o ,x gen)) args)
         ,@(map (fn (x) `(if (is ,x gen) (err:string "parameter " ',x " is required"))) args)
         ,@body)))

  (defreq foo (x y) (+ x y))
  (foo)     -> x is required
  (foo 1)   -> y is required
  (foo 1 2) -> 3
It probably breaks with rest arguments, but I think you could get those working too.

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1 point by Pauan 2807 days ago | link

Or this version, which is even better:

  (mac defreq (name vars . body)
    (if (isa vars 'cons)
          (let exp (len vars)
            `(def ,name args
               (let giv (len args)
                 (if (< giv ,exp)
                       (err:string "expected " ,exp " arguments (" giv " given)")
                     (apply (fn ,vars ,@body) args)))))
        `(def ,name ,vars ,@body)))


  (defreq foo (x y) (+ x y))
  (foo)     -> error: expected 2 arguments (0 given)
  (foo 1)   -> error: expected 2 arguments (1 given)
  (foo 1 2) -> 3
  
  (defreq foo args args)
  (foo)     -> ()
  (foo 1)   -> (1)
  (foo 1 2) -> (1 2)
  
It fails on functions that take required and rest args, though:

  (defreq foo (x y . args) (list x y args)) -> error
Err... right, you were talking about detecting if an argument was nil or not given... but I realized that the same technique could be used to write a version of def that implements required arguments even in a language where every argument is optional.

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1 point by Pauan 2808 days ago | link

Only if you actually rebind them. It's like using `quote` as a variable name: you can do it, but most people won't because that's silly. I just think it's nice to allow it, on the off chance it's actually useful. It just feels weird to arbitrarily say "you can't rebind nil and t" but allow rebinding of everything else.

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