Explain concatenative languages to me like I'm an 8-year-old

Question

I\'ve read the Wikipedia article on concatenative languages, and I am now more confused than I was when I started. :-)

What is a concatenative language in stupid people

Answer 1

First I'm going to make a rebuttal to Norman Ramsey's assertion that there is no theory.

Theory of Concatenative Languages

A concatenative language is a functional programming language, where the default operation (what happens when two terms are side by side) is function composition instead of function application. It is as simple as that.

So for example in the SKI Combinator Calculus (one of the simplest functional languages) two terms side by side are equivalent to applying the first term to the second term. For example: S K K is equivalent to S(K)(K).

In a concatenative language S K K would be equivalent to S . K . K in Haskell.

So what's the big deal

A pure concatenative language has the interesting property that the order of evaluation of terms does not matter. In a concatenative language (S K) K is the same as S (K K). This does not apply to the SKI Calculus or any other functional programming language based on function application.

One reason this observation is interesting because it reveals opportunities for parallelization in the evaluation of code expressed in terms of function composition instead of application.

Now for the real world

The semantics of stack-based languages which support higher-order functions can be explained using a concatenative calculus. You simply map each term (command/expression/sub-program) to be a function that takes a function as input and returns a function as output. The entire program is effectively a single stack transformation function.

The reality is that things are always distorted in the real world (e.g. FORTH has a global dictionary, PostScript does weird things where the evaluation order matters). Most practical programming languages don't adhere perfectly to a theoretical model.

Final Words

I don't think a typical programmer or 8 year old should ever worry about what a concatenative language is. I also don't find it particularly useful to pigeon-hole programming languages as being type X or type Y.