[TheZenPsycho]

What most C derivative programming languages call a "function" is not actually a "function" in the sense meant in "functional programming". It is more accurately called a "procedure". While you can write a "functional function" in php, there's nothing inherent about php's "functions" that makes them "functional". it's up to the programmer to make them so.

On the other hand, a "formula" in excel is inherently a "functional" function or "first order function". Why? two things: No side effects, and statelessness- that is, the order of the computation is theoretically implicit: a value can be computed in a number of different arbitrary orders without effecting the final result.-- not explicit- like in php, where you are specifying an exact order of operations by the order of statements in a procedure ("function").

Another thing about statelessness is that given the same input, a "first order function" must always return the same output.

The fact that order is not explicit in excel, that there is no user controlled "state" means that there's certain associative, commutative and compositional properties available to "first order" functions that are not possible with a procedure that is not guaranteed to be a first order function.

it's the difference between deciding to do functional programming in an imperative language, and having functional programming structurally enforced by the language.

[ww520]

The "pure" functions are just a subset of "procedure" functions. One can always restrict to use the "pure" function in PHP and yes, that's "first-order functional programming" in PHP. The question is: do we call PHP a functional language?

Likewise, Excel can do stateful update with side effect. The cell formula function happens to be a subset of all it can do. Sure, we can claim it's "first-order functional programming." But do we want to call it a functional language?

My point is that "first-order functional programming" is a really weak qualifier to call a language functional.

[TheZenPsycho]

You can do "Object Oriented programming" in C++. you can also do "OOP" in plain C, but why do we call C++ an "OO Language" but not C? because the design of the language itself encourages and enforces a particular style of "OOP".

In this case, I believe the "Excel is a functional language" meme is using "Excel" not to include ALL the things that excel does, which includes things like VB and Javascript scripting, but as shorthand for the spreadsheet model of application, and more specifically aimed at Excel's formula language, which enforces the creation of first order functions by excluding the possibility of state-fulness and side-effects from the language's design. It is the fact that this is inherent in its design that merits calling it a functional language, while in php there are no such design choices which enforce or even encourage a functional way of writing code.

[TheZenPsycho]

Another point to make is that the "Excel is functional" meme is so useful is because, as soon as you tell most programmers that "functional programming" has no variables, no state, and no side-effects, they can't imagine how it's possible to actually do anything useful in a functional language. On the other hand, most programmers do know how to do useful things in a spreadsheet program- which happens to be effectively equivalent to a stateless side-effect free "pure" functional programming language.

[ww520]

That's kind of false advertisement. Excel (or spreadsheet in general) is great and easy to use because of declarative programming, not because of functional programming. The functional programming aspect of it is minimal. No variables, no state, and no side-effects are mainly attribute of declarative programming.

The most important aspect of being functional, high order function, is completely missing.

[gruseom]

Even "no variables" and "no state" are arguably untrue. The state of a spreadsheet consists of the values sitting in its cells. You mutate them by editing them.

One FP attribute that spreadsheets do have is referential transparency: the same inputs given to the same formula will always produce the same result.

[qu4z-2]

But it only recomputes them when you edit them. It's like changing the source code of the program and rerunning it.

  let x = 3, y = 5 in
    x + y

No (mutable) variables there, but if I change the y = 3 bit to y = 4, the value of the expression changes. That's what it's like in excel, as well.

[erichocean]

From Wikipedia[0]:

Common declarative languages include those of database query languages (e.g., SQL, XQuery), regular expressions, logic programming, and functional programming.

I guess you're both right then. :)

[0] http://en.wikipedia.org/wiki/Declarative_programming