A reference to a function. The type arguments encode the function return type and parameter types. The parameter types are typically represented as a tuple type. For example, the type of the function reference plus<Float> is:

Callable<Float,[Float,Float]>

which we usually abbreviate Float(Float,Float). Any instance of Callable may be invoked by supplying a positional argument list:

Float(Float,Float) add = plus<Float>;
value four = add(2.0, 2.0);

or by supplying a tuple containing the arguments:

Float(Float,Float) add = plus<Float>;
[Float,Float] twoAndTwo = [2.0, 2.0];
value four = add(*twoAndTwo);

This interface may not be implemented by user code.