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The type of a map is represented as a three-tuple {Pairs, DefaultKey,
DefaultValue}. DefaultKey and DefaultValue are types. Pairs is a list of
three-tuples {Key, mandatory | optional, Value}, where Key and Value are
types. All types Key must be singleton, or "known at compile time," as
the EEP put it. Examples:
#{integer()=>list()} {[], integer(), list()}
#{a=>char(), b=>atom()} {[{a, optional, char()},
{b, optional, atom()}],
none(), none()}
map() {[], any(), any()}
A more formal description of the representation and its invariants can
be found in erl_types.erl
Special thanks to Daniel S. McCain (@dsmccain) that co-authored a very
early version of this with me back in April 2014, although only the
singleton type logic remains from that version.
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