balanced fold for Data.List

Brent Yorgey byorgey at
Fri Jun 28 23:44:49 CEST 2013

How would people feel about adding a function

  foldb :: (a -> a -> a) -> a -> [a] -> a

to Data.List which performs a *balanced* fold of a list?  Right now I
have this function in the Diagrams.Util module of diagrams-lib but it
seems generally useful.  Here's a (strawman) implementation:

  foldb :: (a -> a -> a) -> a -> [a] -> a
  foldb _ z [] = z
  foldb f _ as = foldb' as
    where foldb' [x] = x
	  foldb' xs  = foldb' (go xs)
	  go []         = []
	  go [x]        = [x]
	  go (x1:x2:xs) = f x1 x2 : go xs

I could also stick it in a separate package but it seems silly to have
a whole package for such a small function.

Also, does this already exist anywhere?  Can it be implemented more
simply in terms of existing machinery? etc.?


More information about the Libraries mailing list