Question

I wonder if there is a “functional programming” reason for the function heads not to be implemented in List (or more generally in TraversableLike). To me, heads would be the exact opposite of tails but I must miss something.

As Scala is easy to read, here is what I would see (for the List case):

def heads[T](li:List[T]):List[List[T]] = li match {
  case Nil => Nil
  case head::tail => (Nil::heads(tail)).map(head::_)
}

So, here are the few possibilities I have thought of for this function not to be implemented:

1. head represents the first element of a List, so it would not make sense to create a function to iterate over all heads are there is only one. Then why not a function prefixes?
2. It is not possible to make this function tail recursive so we prefer to ignore it.
3. It is useless (is it?)
4. It is not compatible with the philosophy of functional programming (why?)

Thanks in advance for your answer.

Answer 1

I think you’re looking for inits.

head = first element
last = last element
tail = all except head
tails = recurses tail
init = all except last
inits = recurses init

See the descriptions under trait Traversable in the Collections API.