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# Name of a particular property

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13 replies to this topic

### #1Bacterius  Members

Posted 22 June 2013 - 04:01 AM

Hello,

in my library I'm working on, I have a few functions which take (among other parameters) a data buffer, with the property that calling such a function twice in succession, with two data buffers, is equivalent to calling it once with the ordered concatenation of both data buffers. In other words:

F("hello")
F(" ")
F("world")

Is strictly equivalent to:

F("hello world")

Is there a name for such a property? For instance a function such that F(F(x)) = F(x) is called idempotent. I'm asking because I'm tired of describing this property in such a long-winded manner in my documentation, and it would be nice if there was a word I could use to sum it all up efficiently.

Thanks!

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

Posted 22 June 2013 - 04:12 AM

Linear?

f(x + y) = f(x) + f(y)

(although linearity also means f(a*x) = a * f(x)).

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### #3JD557  Members

Posted 22 June 2013 - 04:34 AM

I'm not an expert on the topic, but this ressembles a lot the way IO is handled in haskell via monads. You might want to look into that.

Here's a really nice explanation and some more monad examples.

### #4Álvaro  Members

Posted 22 June 2013 - 04:43 AM

I don't know of a name, but I can make one up: "concatenation invariant".

### #5JD557  Members

Posted 22 June 2013 - 05:11 AM

Linear?

f(x + y) = f(x) + f(y)

(although linearity also means f(a*x) = a * f(x)).

This seems ok to me

print("A"+"B")=print("A")+print("B")

print(5*"A")=print("A"+"A"+"A"+"A"+"A")+print("AAAAA")

### #6Bacterius  Members

Posted 22 June 2013 - 05:18 AM

Linear?

f(x + y) = f(x) + f(y)

(although linearity also means f(a*x) = a * f(x)).

That would be fine for a couple of them but many of them do not actually return an output of the same size as the input (or at all) so in those instances "linear" would be meaningless, unfortunately.

I don't know of a name, but I can make one up: "concatenation invariant".

I like it

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

### #7Felix Ungman  Members

Posted 22 June 2013 - 05:23 AM

As you're dealing with data buffer, it seems that the function is some kind of stream writing operator.

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### #8Khatharr  Members

Posted 25 June 2013 - 03:36 AM

I don't know of a name, but I can make one up: "concatenation invariant".

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### #9frob  Moderators

Posted 25 June 2013 - 10:23 AM

I have a few functions which take (among other parameters) a data buffer, with the property that calling such a function twice in succession, with two data buffers, is equivalent to calling it once with the ordered concatenation of both data buffers.

I don't think that is a very good idea.

When it comes to building a language, make it apply in ALL cases.  Something that applies in only SOME cases is usually a bug.

Print("Hello"); Print(" "); Print("World!);

might just happen to be functionally equivalent to Print("Hello World!");

But it is not universally true of all string functions.  It is very different from:

PrintLine("Hello"); PrintLine(" "); PrintLine("World!");

The result should be three distinct entries.  It would be functionally equivalent to Print("Hello\n \nWorld!\n");

Or consider:

ShowDialog("Hello"); ShowDialog(" "); ShowDialog("World!");

I expect three distinct dialog boxes, not one single dialog box with the strings concatenated.

It is one thing to allow adjacent string literals to be concatenated.  It is quite another thing to not call adjacent functions because their parameters happen to be the same type.

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### #10Álvaro  Members

Posted 25 June 2013 - 11:17 AM

I have a few functions which take (among other parameters) a data buffer, with the property that calling such a function twice in succession, with two data buffers, is equivalent to calling it once with the ordered concatenation of both data buffers.

I don't think that is a very good idea.

[...]

I don't understand your objection. Perhaps you misread something? He is not saying that this property is universal, only that he has several functions with that property and he wants to know of a short way to describe it. It sounds very reasonable to me...

### #11freeworld  Members

Posted 25 June 2013 - 04:35 PM

How does the documentation for std::cin describe it?
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I'm not mean, I just like to get to the point.

### #12Bacterius  Members

Posted 25 June 2013 - 08:59 PM

Yes, I am not building a language. But some of my library's functions simply behave that way to allow for arbitrary amounts of data to be provided and at any rate, not limited by the amount of data that can be physically present on one invocation of the function (think about a network socket, for instance, you don't know how much data you are going to receive next, you could receive it all in one shot, or one byte each time you check, so you need a way to make it work the same regardless of how much you've got every time you poll the socket - there are other examples of course). Actually everything is quite consistent overall.

How does the documentation for std::cin describe it?

It doesn't really say anything, actually. The std::cout version says it "inserts" into the stream. I think the notion of stream might help me describe it, though.

Edited by Bacterius, 25 June 2013 - 09:08 PM.

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

### #13Alpheus  GDNet+

Posted 25 June 2013 - 11:10 PM

Hello,

in my library I'm working on, I have a few functions which take (among other parameters) a data buffer, with the property that calling such a function twice in succession, with two data buffers, is equivalent to calling it once with the ordered concatenation of both data buffers. In other words:

F("hello")
F(" ")
F("world")

Is strictly equivalent to:

F("hello world")

Is there a name for such a property? For instance a function such that F(F(x)) = F(x) is called idempotent. I'm asking because I'm tired of describing this property in such a long-winded manner in my documentation, and it would be nice if there was a word I could use to sum it all up efficiently.

Thanks!

I don't know of a name, but I can make one up: "concatenation invariant".

Buffered concatenation........?

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### #14TheChubu  Members

Posted 26 June 2013 - 08:35 PM

Buffered invariant concatenation!

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