flatten/2

Availability::- use_module(library(lists)).(can be autoloaded)

[det]flatten(+NestedList, -FlatList)

Is true if FlatList is a non-nested version of NestedList. Note that empty lists are removed. In standard Prolog, this implies that the atom’[]’is removed too. In SWI7, [] is distinct from’[]’.

Ending up needing flatten/2 often indicates, like append/3 for appending two lists, a bad design. Efficient code that generates lists from generated small lists must use difference lists, often possible through grammar rules for optimal readability.

See also: append/2