Question

data Test = [Int] | Int

foobar :: Test -> Int

What if I wanted something like foobar [1,2,3] = 1 and foobar 1 = 1. In erlang it would be

foobar(X) when is_list(X) -> hd(X);
foobar(X) -> X.

Answer 1

First of all, your data-declaration is invalid. In Haskell, you have to start a data-declaration with a data-constructor, that is later matched upon. For instance, your type Test would be written

data Test = TLst [Int] | TInt Int

Now you can simply match on the type-constructor; it’s field is a list or an int, depending on which constructor you match:

foobar :: Test -> Int
foobar (TLst (x:xs)) = x
foobar (TLst [])     = error "empty list passed to foobar"
foobar (TInt x)      = x