Example of Call by name

Question

In my principles of programming class we are talking about different calling methods. Some we discussed were:

• call by value
• call by reference
• call by value/result
• and call by name

I can't find an example of how call by name works. Anyone care to give me an example? I think that when you take an xml file as input this is similar to call by name. Could someone give me a more traditional example?

Pollen

I don't know of any modern examples of call by name. It was a very bad idea back in ALGOL 60, and I doubt that has changed. – Caseinogen

What kind of example are you looking for? An example of a language that uses call-by-name or an example of what evaluating an expression using call-by-name would look like? – Internship

@sepp2k, both could be useful. I am not sure, but from what I have figured out so far it seems like a pretty dead idea (correct me if I am wrong) but it is included in the course material (maybe just for historical reasons?) – Pollen

Answer 1

I'll work in a hypothetical programming language. Let's assume we have a function p(x) that prints out x and then returns it. Now let's define a function:

function foo(x, y) { return y+1; }

Now let's call it with some arguments:

foo(p(123),p(456))

x and y are going to be substituted for the parameters, so the call to foo above is going to result in:

return p(456)+1;

So we're going to print 456 to the screen and return 457. In another evaluation strategy, we would evaluate the function parameters first (printing 123 and 456 to the screen in the process) and then substitute 456 for y in the function body, eventually returning 457.

It's contrived, but I hope you get the idea. It's all about substitution.

Firing

so would call by name also call call by reference? value? Anything specific? – Pollen

@sixtyfootersdude, that sentence made my brain hurt. Care to re-state it? – Firing

Sure. Lets say you have this (Call by name): int x = 0; foo(t(x)); print(x); if it was also "call by value" this would print 0. If it was also "call by reference" it could print something else. – Pollen

Gian! Come back and answer him! – Tartarean

I'm still fairly certain I don't understand the question. In a call-by-reference scenario, it is always possible that the value being referenced could be modified, so the expected semantics of name-passing don't quite hold up to scrutiny. Other than that, I'm still not sure what is being asked. – Firing

Could you explain what exactly the difference to lazy evaluation is, or whether they are the same? (Since it looks very much the same to me and lazy evaluation is not a "dead idea".) – Culinary

Answer 2

http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_name

In call by value, you evaluate arguments, retrieving values that are then passed to the function. foo(bar()) is evaluated as arg = bar();, then foo(arg) is called, and in the body of the function, this newly allocated arg variable is accessible, modifiable, ...

In call by name, you substitues in the body of the function any references to arguments by their code used during the call. Then, evaluating the body, you will evaluate the arguments. foo(bar()) with foo(arg) { return arg; } will be evaluated as foo(arg) { return bar(); }

Flower

Answer 3

Call by name work as call by reference when actual parameter be scaler, but be different when actual parameter is expression or array then actual parameter is re-evaluated on each access.

here is simple example

begin
    integer n;
    procedure p(k: integer);
        begin
        print(k);
        n := n+1;
        print(k);
        end;
    n := 0;
    p(n+10);
    end;

• call by value output => 10 10
• call by name output => 10 11

Hopi

i saw this example elsewhere, you should cite – Nadeau