A question on modification of a list via a function invocation

[Chris Angelico]

On Mon, Sep 4, 2017 at 6:16 PM, Gregory Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> Chris Angelico wrote:
>>
>> This is another proof that you can't divide everything into "pass by
>> value" vs "pass by reference"
>
>
> True, but that doesn't mean you should deny that something
> is pass-by-value when it actually is.
>
>> In C, a string is not an
>> entity; it's simply an array of characters. Arrays are never passed by
>> value;
>
>
> I think it's more accurate to say that arrays are never
> passed at all in C.

This is technically true; however, it's common to describe a function
as "accepting a string" as a parameter. What it actually accepts is a
pointer to char. Is that "passing an array by reference"? Not quite.
Is it "passing a pointer by value"? Technically correct, but useless.
Is it "passing a string by <anything>"? Technically incorrect, but
probably more useful

> A better name for pass-by-value would be "pass-by-assignment".
> Passing a parameter by value is equivalent to assigning it
> to a local name.

That's true in Python too though - parameter passing is equivalent to
assigning to a local name. And in C++, where you have actual
references, a reference parameter is equivalent to initializing a
local reference with the same result. I expect that "parameter passing
is equivalent to assignment" is true in the bulk of languages.

>> yet everything in C is passed by value. So you pass a
>> pointer... by value.
>
>
> Yes, because that's what happens when you assign an array
> in C.
>
> If it seems screwy, it's because assignment is screwy in
> C, not parameter passing.

Sure, if you define "screwy" as "different from Python". But that's
the exact problem that got us where we are.

>> What would you define LISP's semantics as? Pass by value? Pass by
>> reference? Pass by name? Pass by immutability? Pass the salt?
>
>
> Let's see... the expression being passed gets evaluated
> once at the point of call, and the result gets bound to
> a local name. Looks exactly like pass-by-value to me.

Oh, looks exactly like pass-by-name-binding to me, then. Nothing's
getting copied, so it can't be pass-by-value.

See how non-simple it is?

ChrisA