On 2/12/11 11:41 AM, Tim Chevalier wrote:
What's important is not just that Haskell has static typing, but that algebraic data types are a rich enough language to let you express your intent in data and not just in code. That helps you help the compiler help you.
ADTs are an amazing thing to have. They directly express the types we usually think in, which liberates us to think about those types.
Conversely, in C we have to use structs, untagged unions, and pointers, which makes us spend far too much time worrying about low-level representation issues instead of being able to think about the types we really care about (the lists, the nodes, graphs, trees,...) and worrying about their high-level representation issues (does a list really capture the shape of my data, or would it be better to use a tuple, a priority queue,...?)
Similarly in most OO languages there's no way to define a class which has a fixed non-zero number of subclasses, so it's hard to match the clarity of ADTs' "no junk, no confusion". Instead, we waste time defensively programming against the subclasses our evil users will come up with. This is especially problematic when designing datastructures that have to maintain invariants. And this often causes folks to move program logic from the type realm into the executable realm; how often have you seen methods which simulate dynamic dispatch by case analysis on a state or flag field?
Static typing, type inference, and lambdas are all excellent things, but I think the importance of ADTs is vastly underrated when comparing functional languages (of the ML or Haskell tradition) to procedural languages. Not only do they make life easier, but they also help with proving correctness.
-- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
