What is Expressiveness in a Computer Language

[Ole Nielsby]

David Hopwood <...nospam....uk> wrote:

> A good debugger is invaluable regardless of your attitude
> to type systems.

I found that certain language features help greatly in pinning
the errors, when programming in my own impure fp language
(PILS).

Originally, I implemented a single-stepping debugger for the
language, but I trashed it for three reasons, of which two are
rather uninteresting here: it messed up the GUI system, and the
evaluation order of PILS made single-stepping a confusing
experience.

The interesting reason is: I didn't need single-stepping, partly
because the programming system is unit-test friendly, partly
because of language features that seem to mimick the concept
of taking responsibility.

Basically, the PILS execution mechanism is pessimistic. When
a function is called, the assumption is it won't work, and the
only way the function can return a result is by throwing an
exception. So, if a function call doesn't throw an exception,
it has failed and execution will stop unless the function call
is explicitly marked as fault tolerant, as in

    f (x) else "undefined"

This has been merged with tail call flattening - rather than
throwing the result, an (expression, binding) thing is thrown
(I guess .NET'ers would call it an anonymous parameterless
delegate), and the expression is then evaluated after the throw.

Mimickally speaking, when a function body performs this
throw, it takes responsibility for getting the job done, and
if it fails, it will suffer the peril of having its guts exposed
in public by the error message window - much like certain
medieval penal systems. I will spare you for the gory details,
just let me add that to honor the demands of modern
bureaucracy, I added a "try the other office" feature so that
there are ways of getting things done without ever taking
responsibility.

Strangely, this all started 20 years ago as I struggled
implementing an early PILS version on an 8-bit Z80
processor, at a blazing 4 MHz. I needed a fast way of
escaping from a failing pattern match, and it turned
out the conditional return operation of this particular
processor was the fastest I could get. Somehow, the
concept of "if it returns, it's bad" crept into the language
design. I was puzzled by its expressiveness and the ease
of using it, only years later did I realize that its ease
of use came from the mimicking of responsibility.

I'm still puzzled that the notion of mimicking a cultural/
social concept came from a microprocessor instruction.
It seems like I - and perhaps computer language designers
in general - have a blind spot. We dig physical objects and
mathematical theories, and don't take hints from the
"soft sciences"...

Perhaps I'm over-generalizing, it just strikes me that the
dispute always seems to stand between math versus object
orientation, with seemingly no systematic attempts at
utilizing insigts from the study of languages and cultural
concepts. It's like as if COBOL and VB scared us from
ever considering the prospect of making programming
languages readable to the uninitiated.