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Predicate flatten/2
Availability::- use_module(library(lists)).(can be autoloaded)
Source[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
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Jan Wielemaker said (2015-01-08T15:58:51):1 upvotes 0 -1 downvotes
Picture of user Jan Wielemaker.
Indeed. There is no plan to remove this predicate from the library. Removed the @deprecated tag.
Kilian Evang said (2015-01-08T15:47:54):0 upvotes 0 0 downvotes
Picture of user Kilian Evang.
Needing it often often indicates a bad design, but that is no reason to deprecate it, is it??