unfoldr in Library Report

Ross Paterson ross@soi.city.ac.uk
Fri, 1 Jun 2001 11:04:34 +0100

While looking through the List module, I came across

   The unfoldr function undoes a foldr operation. Note that, in general,
   only invertible functions can be unfolded.
   [followed by an equation about undoing foldr]

This is quite inadequate: unfoldr is very useful with non-invertible
functions.  I suggest something like:

   The unfoldr function is a `dual' to foldr: while foldr reduces a list
   to a summary value, unfoldr builds a list from a seed value, e.g.,

	iterate f == unfoldr (\x -> Just (x, f x))

   In some cases unfoldr can undo a foldr operation:
   [followed by the existing equation]