Monadic Tree Print

Directed acyclic graphs and trees in particular belong to the most extensively used data structures. Visualizing them properly is a key to a success when developing complex algorithms that make use of them. Textual visualizations a la UNIX tree command is essential when the urge is to deal with large trees. Our aim was to design a library that would exploit this approach and to make an implementation of it for a purely functional programming language. The library uses monads to print directly into an output stream or to generate immutable Strings. This paper gives a detailed overview of the solution.