Cristian Baboi wrote:
http://en.wikipedia.org/wiki/First-class_object I'll guess that 5,9,12 does not apply to Haskell functions.
I think there is a basic semantic difference between what the author of that article meant by the word "function" and what we mean by that word when we are talking about Haskell. In the article, "function" means a concrete data object that specifies how to compute something. In Haskell, a function is closer to the mathematical idea of a function. Functions specify relationships between elements of certain types, and then the compiler uses them to create code to do your computation. But there is no obligation for a compiler to create any concrete data structure that corresponds to a function. Often it does in practice, but not always. On the other hand, functions are members of types that are just like any other Haskell type. They are first-class in that sense. Like any type, only certain operations make sense on functions. Strings can be compared to each other for equality and written to a disk, and you can take the logarithm of a float, but none of those operations make sense for functions. In particular, two functions are equal only if they produce the same value for every input, and in general it is impossible for a computer to check that. -Yitz