The set type models the mathematical notion of a set. The set's basetype can
only be an ordinal type of a certain size, namely:
* ``int8``-``int16``
* ``uint8``/``byte``-``uint16``
* ``char``
* ``enum``
or equivalent. The reason is that sets are implemented as high
performance bit vectors. Attempting to declare a set with a larger type will
result in an error:
.. code-block:: nim
var s: set[int64] # Error: set is too large
Sets can be constructed via the set constructor: ``{}`` is the empty set. The
empty set is type compatible with any concrete set type. The constructor
can also be used to include elements (and ranges of elements):
.. code-block:: nim
type
CharSet = set[char]
var
x: CharSet
x = {'a'..'z', '0'..'9'} # This constructs a set that contains the
# letters from 'a' to 'z' and the digits
# from '0' to '9'
These operations are supported by sets:
================== ========================================================
operation meaning
================== ========================================================
``A + B`` union of two sets
``A * B`` intersection of two sets
``A - B`` difference of two sets (A without B's elements)
``A == B`` set equality
``A <= B`` subset relation (A is subset of B or equal to B)
``A < B`` strong subset relation (A is a real subset of B)
``e in A`` set membership (A contains element e)
``e notin A`` A does not contain element e
``contains(A, e)`` A contains element e
``card(A)`` the cardinality of A (number of elements in A)
``incl(A, elem)`` same as ``A = A + {elem}``
``excl(A, elem)`` same as ``A = A - {elem}``
================== ========================================================
Sets are often used to define a type for the *flags* of a procedure. This is
a much cleaner (and type safe) solution than just defining integer
constants that should be ``or``'ed together.