libghc-free-dev 4.12.4-3build4 (armhf binary) in ubuntu bionic

 Free monads are useful for many tree-like structures and domain specific
 languages.
 .
 A Monad n is a free Monad for f if every Monad homomorphism from n to another
 monad m is equivalent to a natural transformation from f to m.
 .
 Cofree comonads provide convenient ways to talk about branching streams and
 rose-trees, and can be used to annotate syntax trees.
 .
 A Comonad v is a cofree Comonad for f if every Comonad homomorphism another
 comonad w to v is equivalent to a natural transformation from w to f.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.