[clean-list] Clean's answer to the Functional Achilles-heel

[Marco Kesseler]

----- Original Message -----
From: "Mark Phillips" <mark@austrics.com.au>
> Am I right in thinking that whereas older functional languages
> had the "code reuse" problems described above, newer functional
> languages have mechanisms which allow these problems to be
> overcome?

As far as I can see this is simply a matter of:

- If you expect to make frequent additions to a type, do not use
unions/algebraic data types because with each type change you have to adapt
all functions that access this union. Using a union type (without resorting
to the ultimate union - Dynamic) is even impossible for modules that do not
know the complete type at compile-time. Instead define some interface on the
type, and implement that for each type addition (and typically, IO systems
deal with lots of evolving/unknown types). In Clean, interfaces more or less
come in the form of classes/overloading and Existential types.

- If you expect to add all kinds of functionality to a rather fixed type, an
interface might be a bad idea if adding new functionality means extending
all interface instances. Instead you could use a union/algebraic data type,
so that each functional addition can be put in a single function.

I do not know why this has anything to do with code reuse. To me it seems
more a matter of trying to keep (expected) changes localised to a single
module. Anyway, there is no inherent code reuse problem associated with
functional programming. Good old higher-order functions offer a great way
for code reuse.

regards,
Marco

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