Friday, November 30, 2007

Haskell design patterns are (probably) needed

I was reading that post, and thinking: well when I start a Haskell project (for fun, yes, I don't use Haskell in work) I do wonder how best to design it. Yes I know about higher order functions, generics and such, but I don't think these low level constructs are enough to express, on their own, the design of the system as the design patterns from the GoF do. I would love to see patterns on how to allow extensibility, separation of concerns and such in Haskell. Years of experience in Java mean that I see a lot of things in terms of objects, inheritance vs composition, interfaces, etc, and not all of that apply in Haskell. I see a lot of low level algorithms in Haskell, but few software design papers (papers like this one are useful, we need more of them).

Monday, November 26, 2007

Structure of a functional Java, er, method

A method in FLOJ (remember, the functional Java based language I'm toying with) has a simple structure:
- a method definition 100% similar to a normal Java method: visibility generic return-type type parameters.
There are no exceptions, of course, since exceptions are things Best Done in Monads, once I've figured out
out I'll implement monads.
- a series of assignments of the form x=e where x is a variable name, and e an expression
- a return expression

For example:

public static String[] testMap(){

Since the return expression is always the last expression, there is no need for a return keyword, but it's accepted by the parser. In the exemple above, the method returns the list of String TOTO and TITI.
Note also that the assignments do not allow nor require the type definition of the left hand side of the assignment. The type is inferred from the
return type of the right hand side expression (in the exemple above, a is a list of String). It's not as powerful as the full blown Haskell type inference but at least it saves some typing.

Of course, the generated Java code will add a sneaky final keyword to all the assignments, to enforce the fact that there's no variables in a functional language.

All in all, that structure makes the code easy to read and to understand: the sequence of calculations and the final expression yielding the method result.

I don't think my FLOJ language will ever be a full blown industrial-strength language, but I know that by implementing my own functional language I will learn a lot
more about functional languages than by just using one. Just implementing method pointers, type inference, combinator parsers and such, I understand more the why and how of Haskell.