What is Expressiveness in a Computer Language

[Matthias Blume]

Pascal Bourguignon <pjb at informatimago.com> writes:

> Moreover, a good proportion of the program and a good number of
> algorithms don't even need to know the type of the objects they
> manipulate.
>
> For example, sort doesn't need to know what type the objects it sorts
> are.  It only needs to be given a function that is able to compare the
> objects.

Of course, some statically typed languages handle this sort of thing
routinely.

> Only a few "primitive" functions need specific types.

Your sort function from above also has a specific type -- a type which
represents the fact that the objects to be sorted must be acceptable
input to the comparison function.

> So basically, you've got a big black box of applicaition code in the
> middle that doesn't care what type of value they get, and you've got a
> few input values of a specific type, a few processing functions
> needing a specific type and returning a specific type, and a few
> output values that are expected to be of a specific type.  At anytime,
> you may change the type of the input values, and ensure that the
> needed processing functions will be able to handle this new input
> type, and the output gets mapped to the expected type.

...or you type-check your "black box" and make sure that no matter how
you will ever change the type of the inputs (in accordance with the
interface type of the box) you get a valid program.