On 8/30/07, Andrew Coppin
Yeah, but... programs aren't like mathematics. I know people claim that they are, but they aren't.
But the raison d'etre of Haskell is to make programming more like mathematics. That motivates everything from the fact that it's a declarative language, and the support for equational reasoning, to the fact that IO happens in a monad, and the option to use primes on variables names.
In mathematics, if you write "x = f y" you mean that these two expressions are equal. In Haskell, if you say "x = f y" you mean *make* then equal!
Haskell is a declarative language, not an imperative language. When you write "x = f x" in Haskell, you're declaring to the compiler that x equals f x. In an imperative language like Java, the line x = f(x) gives the compiler the imperative to emit instructions to store the value of f(x) in a 'box' called x. In Haskell, there is no box. (When you get down to the nuts and bolts, a Haskell compiler and a Java compiler may ultimately actually do the same thing here, but the way you think about a language is as important as what instructions the code generator emits.) -- Dan