Question

What means the code below?

singleton 1 'a' == fromList [(1, 'a')]

Answer 1

singleton and fromList appear in several container modules.

Typically, singleton returns a container with a single element, and fromList returns a container with all of the elements from the list (modulo key duplication in mappings).

The example you give shows singleton taking 2 arguments, which implies that the container is a mapping (Set, IntSet, HashMap, etc…), and expects the key and value of the single element. For mappings, fromList expects a list of key, value pairs.

The equality you show states that the container with a single element that maps the number 1 to the character 'a' is the same whether it is constructed using singleton, or fromList with a single-element list.