unfold

[Stephen Weeks]

Henry, are these reasonable definitions of unfold and unfoldr?  If so, I will
add them to my library (as well as the versions for vectors and arrays).  I
agree that this is a nice abstraction.

val ('a, 'b) unfoldr: int * 'a * ('a -> 'b * 'a) -> 'b list =
   fn (n, a, f) =>
   if n < 0
      then raise Size
   else
      let
	 fun loop (i, a, bs) =
	    if i >= n
	       then bs
	    else
	       let
		  val (b, a) = f a
	       in
		  loop (i + 1, a, b :: bs)
	       end
      in
	 loop (0, a, [])
      end

val ('a, 'b) unfold: int * 'a * ('a -> 'b * 'a) -> 'b list =
   fn z => rev (unfoldr z)