Static typing (was: Python from Wise Guy's Viewpoint)

[Dirk Thierbach]

Pascal Costanza <costanza at web.de> wrote:
> You need some testing discipline, which is supported well by unit 
> testing frameworks.

IMHO it helps to think about static typing as a special kind of unit
tests. Like unit tests, they verify that for some input values, the
function in question will produce the correct output values. Unlike
unit tests, they do this for a class of values, instead of testing
statistically by example. And unlike unit tests, they are pervasive:
Every execution path will be automatically tested; you don't have
to invest brain power to make sure you don't forget one.

Type inference will automatically write unit tests for you (besides
other uses like hinting that a routine may be more general than you
thought). But since the computer is not very smart, they will test
only more or less trivial things. But that's still good, because then
you don't have to write the trivial unit tests, and only have to care
about the non-trivial ones.

Type annotations are an assertion language that you use to write down
that kind of unit tests. 

> Static type systems are claimed to generally improve your code. I
> don't see that.

They do it for the same reason that unit tests do:

* They are executable documention.

* By writing them down first, you focus on what you want to do.

* They help with refactoring.

etc.

Of course you can replace the benefits of static typing by enough unit
tests. But they are different verification tools: For some kind of
problems, one is better, for other kinds, the other. There's no reason
not to use both.

- Dirk