Hi Richard,
Yeek. Why do you want to do _that_?
Heh. I've got a parser and I want to check what I've parsed (it's an exercise in Write Yourself a Scheme in 48 Hours).
check (Atom _) (Atom _) = True check (Bool _) (Bool _) = True check _ _ = False
Yes I came up with this too, but it seemed ugly to create unnecessary new values just to take them apart again.
is_atom (Atom _) = True is_atom _ = False
This is nicer. It still requires listing out the possible constructors (Bool, Atom ... the real code obviously has more). I don't like that, because I've already listed them out once, in the type declaration itself. Surely, I shouldn't have to list them out again?
There are various meta-programming ("Scrap Your Boilerplate", "Template Haskell") approaches you can use to automate some of these.
You hit the nail on the head. "Why I am doing this" is because of boilerplate. Boilerplate gives me rashes and bulbous spots on the nose. Consider the following Ruby code: def check(zeClass, zeValue) zeValue.is_a? zeClass end This does not require a new function for every class defined in Ruby. (To be fair, though, the class of a Ruby object tells you precious little, compared to a Haskell type constructor). I figured there would be a clever Haskell idiom that would give me a similarly concise route. Does it really require Template Haskell? I can barely parse regular Haskell as it is.. Cheers, - Dan