This is actually a pretty common technique in Haskell. If you look at the Alex lexical analyzer generator's source code, it represents CharSets as functions from Char -> Bool: they return true if the character is in the charset or false if it is not. Internally, they work just like the article describes: the union of two CharSets doesn't actually enumerate every possible character in the set, it just calls the two CharSets and performs an OR.

There're other languages that do this two, eg. JavaScript written by functional programmers often ends up looking like this. In Arc, the distinction is completely meaningless, because you "call" a hashtable to index an object anyway.

The problem with it, I've found, is the lack of debugability. You can enumerate a hashtable, and you can inspect it in the debugger. You generally can't enumerate a function. I've found that with complex systems, the biggest correlation to programming productivity is the amount of debug information available, not the size of the code. Trading off debugability to get short code is usually a poor trade-off, once you get past a certain code size.