I am talking about simple string concatenation. Take two python programs "a.py" and "b.py", exclude command line arguments and stdio file descriptors - and, yes

I am talking about simple string concatenation. Take two python programs "a.py" and "b.py", exclude command line arguments and stdio file descriptors - and, yes, the string concatenation of "a.py" and "b.py" will generally execute the same computation / "function" that executing "a.py", followed by executing "b.py" will compute. That's true of Forth as well (and you don't even need to exclude the stack state), but not of C. And indeed, it's not worth arguing about a non-rigorous definition, except that at the surface level there's a very inherent difference between the definition you presented and the one I know - which goes to the (rather unusual) definition of "concatenation" which appears to require function application in your definition, unlike the common use of the term in software in general. A quick look at Wikipedia[0] indicates we are both right; The overall definition does refer to programs as functions, but the "Properties" part says "Any subexpression can be replaced with a name that represents the same subexpression. This is referred to in the concatenative community as factoring and is used extensively to simplify programs into smaller parts." It is likely an equivalent requirement (from what I remember, it is, but I don't remember all the details), but relies on the standardized, general meaning of "concatenate" rather than a narrow one specifically for this definition. [0] https://en.wikipedia.org/wiki/Concatenative_programming_language https://en.wikipedia.org/wiki/Concatenative_programming_lang...