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.