Hacker News Reader

Indeed. Definition 2 is broadly in line with what "reference" means in general usage, while definition 1 is a term of art in the vocabulary of certain languages.

Before languages had reference types or pointers, the distinction between pass-by-reference and pass-by-value (or also pass-by-name in Lisp and Algol) told you important things about the semantics of the language, such as whether a function you called could modify the objects you gave as arguments, or incorporate them by reference into some other object, and how arguments that are expressions are handled. Some of those distinctions become blurred when pointers or references are passed by value; the distinction on how you pass them now becomes a distinction on what you pass. In practice, is there any language that passes pointers or references by reference?

It may be what reference means, but just because a language has references doesn't mean that it is passing-by-reference. There are plenty of languages that allow actual pass-by-ref, like C++. The usual litmus test is writing a function that takes two ints (not int refs!) and they're swapped at the end. In C++, you can tell because of "int&" in the signature of the function you're calling; as opposed to plain int.

In languages with macros, you get to cheat a little, and write something that looks like the above without _really_ having those semantics. However, in languages like Lisps where your macro language is extremely powerful, you might not really miss it so much.

There is a lot of convenience in _not_ having pass-by-ref! Sometimes I feel like the ardent opposition just doesn't want to admit their favorite language is missing a feature ;-) If I call a function f(a, b), I know that a, b are still pointing at the same object in e.g. Python. That is not the case in C++ (pass-by-ref) or Lisp (arbitrary macros). Predictable language semantics make code easier to reason about. That's a good thing, and it's quite plausible that the added complexity and confusion of supporting pass-by-ref isn't worth the expressiveness.

I appreciate that the term is confusing, but it does mean something. I submit that making it mean two things would be even more confusing. If it's confusing, just say "passing (an argument) _as a reference_" which is unambiguously about what you're passing, and not how you're passing it.

"If I call a function f(a, b), I know that a, b are still pointing at the same object in e.g. Python."

  def f(x):
    global a
    a = "No, you don't"

  a = "I know"
  f(a)
  print a

This counterexample may be far-sought, but it is a minimal version of what can easily happen in larger programs that have shared mutable state.

But yes, call by reference can be confusing. It also can be very useful, though. That's why many modern languages have call by reference, but also require callers to specify it at each call site. I think that gives you the best of both worlds.

Whenever a word with a general meaning is adopted, and specialized, as a term of art, a double meaning is created. I get the impression that philosophy prefers to invent new words (unless it is reaching into the darkest corners of the vocabulary.)

What has happened here is that the double meaning of 'reference' has caused some confusion over the term pass-by-reference, which is purely a term of art. So long as a person understands what gets passed and, especially, its implications, I am not particularly bothered by the mistaken usage.

Object Pascal (Delphi, Free Pascal) allows this. You can use a var (variable) parameter to pass any type by reference to a function/procedure/method. This is used quite a bit in the Win32 API interface units in the run-time library because it simplifies the usage of the API by removing the requirement for double indirection when dealing with pointer parameters that need to be updated by the callee, as well as pointers in general for other type/structure parameters that need to be updated by the callee. As the caller, instead of passing a pointer to a structure as a parameter, you simply pass the structure. There are also out parameters that work similarly, but are, obviously, output-only.

In general, you can use pointers in Object Pascal, but the language offers a lot of options for avoiding them, if you want to, and you get type-checked references in the process. Records (structs) are stack-based and pass-by-value, by default, and class references/class instances (instances are always heap-allocated) are pass-by-reference, by default. So, for the majority of applications, you can use parameters in a myriad number of ways in order to improve performance or provide the proper calling semantics, without any of the hassle of address-of or de-referencing operators.

Don't know of any mainstream Lisp with "pass by name". Which dialects have or had that?

Not the author, but they may be referring to NLAMBDA/FEXPR. (That hasn't been in Lisp since the eighties, because, well, Lisp had better tools. NLMABDA/FEXPR basically made AOT compiling Lisp impossible, and all of the use cases were much more neatly handled by a macro system.)

https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_na...

Call by name is actually a closure based mechanism under the hood. It's like lazy evaluation, but with side effects allowed:

  (let ((0 42))
        (a #(1 2 3))
    (foo (aref a (incf i))))

If foo is call by name, then it receives a thunk (basically a lexical closure) for the argument. The aref expression is then evaluated inside foo whenever foo refers to the corresponding argument. (Unfortunately, repeating the incf side effect). We could simulate this with:

   (foo (lambda () (aref a (incf i)))

with foo using (funcall arg) to get the argument's value.

Call by need, described below that, seems exactly like the insertion of delay and force. The call-by-need argument expression is basically wrapped in an implicit delay, and callee refers to the argument value using implicit force. delay and force are just additional wrapping around a closure to add a flag to squash multiple evaluations.

Ah!

I just remembered I implemented some macros to simulate call by name in the TXR solution to the "Man or boy test" task on Rosetta Code:

https://rosettacode.org/wiki/Man_or_boy_test#TXR

With these macros, Knuth's test is expressed exactly in the original form (modulo the Algol being cast into S-exp syntax).

Call by name isn't just lambda with funcall (as might be wrongly inferred from my comments in the other reply), because it supports assignment. If we pass a variable A with call by name, the caller can assign to it. This all works with the given macros, and it probably more or less corresponds to the Algol mechanisms.

(The task description is "Imitate Knuth's example in Algol 60 in another language, as far as possible." so the solutions which don't actually implement a call-by-name syntax and semantics don't go as far as this solution.)

It has been a while since I used any form of Lisp, but nlambda seems to be what I was remembering as pass-by-name (and hand-annotated as deprecated, here:) https://www.cl.cam.ac.uk/teaching/1213/ConceptsPL/l4.pdf

Thank you sir for pointing this out. I recently ran into a similar argument with an Oracle trainer whether Java has pass-by-reference semantics. The entire Java community seems adamant that Java has no pass-by-reference, but only pass-by-value. This blog post reminds me of that argument.

In Java, Python, Javascript and many other languages, objects pointers are passed by value, but the contents of which are shared between the caller and the callee, so the modification of which is visible from both sides. If we take the definition of pass-by-reference or call-by-reference semantics to mean that, then these languages are pass by reference by default. Apparently, that's not the definition, which in my opinion is rather strange that we are still stuck in a definition that predates OOP, when people mostly concerned with simple primitive values as opposed to composites.

It appears that Barbara Liskov has recognised this and coined call-by-sharing in 1974. 43 years later, practitioners are still arguing in the C++ frame of mind and failed to disseminate the more modern and suitable term of call-by-sharing. This is rather disappointing to me.

Aren't reference variables just syntactic sugar on top of pointers? I've seen this debate about pointers vs. references so many times that I'm starting to think I might be missing something.

This has always bugged me too. I don't understand why people make such a big deal about pass-by-value and pass-by-reference when the latter is essentially a small layer of syntactic sugar over the former. Anything you can do with reference variables can be done with pointers, with a few more characters (and arguably easier to read, if you're not used to reference variables), so from a semantic point of view it doesn't add anything new.

PBV and PBR seems to be a tiny distinction and yet, we have blog posts and interview questions and online debates about whether a language falls into the first or second category.

Yes.

Yap, your comment is better than the entire article. Thanks!

And Java just added to the problem.

For PR reasons, they didn't want to have "pointers" (arithmetic, etc. ew). So Java has "references".

But what happens when you call a method on a null reference? Not NullReferenceException, but NullPointerException.

>For PR reasons, they didn't want to have "pointers" (arithmetic, etc. ew). So Java has "references"

Well, it's just a rename for PR reasons. They actually don't have pointers.

>But what happens when you call a method on a null reference? Not NullReferenceException, but NullPointerException.

That's because it's the underlying pointer access that causes the exception. Doesn't mean the reference is itself a pointer -- it just abstracts over one.

> They actually don't have pointers.

They do. Everywhere.

Every Java variable of Object type is a pointer. (Also, they're all nullable....ugh.)

If Java had real references (see original comment by jasode) you'd be able to write something like C#'s int.TryParse ( https://msdn.microsoft.com/en-us/library/f02979c7(v=vs.110).... ).

>Every Java variable of Object type is a pointer.

No, it's a reference. It just has a pointer underneath.

>If Java had real references (see original comment by jasode) you'd be able to write something like C#'s int.TryParse

If Java had one specific type of references. The common use of the term though is still what Java has.

> The common use of the term though is still what Java has.

The way you are defining "reference" makes it immaterially different from "pointer".

Can a pointer be null? Yes, in C.

Can a "reference" be null? Yes, in Java.

Can a pointer be used in arithmetic operations? Yes, in C.

Can a pointer not be used in arithmetic operations? Yes, in Go.

If this is really the common definition, it isn't a useful one.

Whether a thing can be null or not is orthogonal to whether it is a pointer or a reference.

The main difference in this definition is on whether you need to dereference to access the value pointed at (like you do in C) or not.

Not sure I understand you. A concrete PHP example:

   function noop($a, $b) {
     $tmp = $a;
     $a = $b;
     $b = $tmp;
   }

   function swap(&$c, &$d) {
      $tmp = $c;
      $c = $d;
      $d = $tmp;
   }

   $first = [1, 2];
   $second = [3, 4];
   noop($first, $second);
   swap($first, $second);

$a and $b work much differently than $c and $d.

I would call the first two pointers (to [1,2] and [3,4]), and the latter two references (to $first and $second). What would you call them?

(Note: the latter function is impossible to write in Java.)

>I would call the first two pointers (to [1,2] and [3,4]), and the latter two references (to $first and $second). What would you call them?

In the first case, $a and $b pass the arrays by copying their values. The second example uses references.

Java can't do the change visible externally because it passes its references by value -- but that's not a necessity for all languages with references.

As the PHP manual itself says: "References in PHP are a means to access the same variable content by different names. They are not like C pointers; for instance, you cannot perform pointer arithmetic using them, they are not actual memory addresses, and so on".

They also insists Java is pass-by-value whereas technically it's pass-by-value for primitive unboxed types and call-by-sharing for others. The reason of which escapes me.

"Call-by-sharing" is just a different, and IMHO unnecessary and unhelpful, name for pass-by-value when the values happen to be pointers.

To put it another way, the inconsistency in Java isn't that primitives and objects are passed differently, it's that they're stored differently. Given how they're stored, they're passed the same way.

Since primitives are immutable have value-equality, it doesn't matter.

If your cognitive model prefers to think of everything as pass pointer by value, you think that.

I've got a credit in a game by pointing out that the reason the teams tools kept segfaulting was because the authors believed that if they had a reference they didn't need to check it for null. Unfortunately, they'd assign these references from pointers that could totally be null, and didn't error check at that point. References in C++ can totally be null.

That's undefined behavior. References in valid C++ cannot be null.

I can write a method that is valid C++, but it gets called by something that isn't valid C++. If I don't check for null references, then my method will crash. My method is valid C++, and yet it gets a null reference. So clearly, references in valid C++ can be null, and must be checked for.