ConstMap after an operation has been applied to each key and
value in the current
public function mapWithKey<Tu>(
(function(Tk, Tv): Tu) $fn,
): ConstMap<Tk, Tu>;
Every key and value in the current
ConstMap is affected by a call to
filterWithKey() where only values that meet a
certain criteria are affected.
The keys will remain unchanged from this
ConstMap to the returned
ConstMap. The keys are only used to help in the mapping operation.
(function(Tk, Tv): Tu) $fn - The callback containing the operation to apply to the current
ConstMap keys and values.