What does "the composition of UNIX byte streams" mean?

Question

In the opening page of the book of "Lisp In Small Pieces", there is a paragraph goes like this:

Based on the idea of "function", an idea that has matured over several centuries of mathematical research, applicative languages are omnipresent in computing; they appear in various forms, such as the composition of Un*x byte streams, the extension language for the Emacs editor, as well as other scripting languages.

Can anyone elaborate a bit on "the composition of unix byte streams"? What does it mean? and how it is related to applicative/functional programming?

Thanks,

/bruin

Answer 1

My guess is that this is a reference to something like a pipe under linux.

cal | wc

the symbol | it's what invokes a pipe between 2 applications, a pipe is a feature provided by the kernel so you can use pipes where the applications are written using this kind of kernel APIs.

In this example cal is just the utility that prints a calendar, wc is an utility that counts words, rows and columns in the input that you pass to it, in this case the input is the result of piping cal to wc which makes things easier for you because it's more functional, you only care about what each applications does, you don't care, for example, about what is the name of the argument or where to allocate a temporary file to store the input/output in between.

Without the pipes you should do something like

cal > temp.txt
wc temp.txt
rm temp.xt

to obtain pretty much the same information. Also this second solution could possibly generate problems, for example what if temp.txt already exists ? Following what kind of rationale you will tell to your script to pick a name for your temporary file ? What if another process modifies your file in between the 2 calls to cal and wc ?