Beginning of a type system

Last night’s checkin added a new binary operator: has lets you inquire about a container’s attributes. The has operator has the same precedence as the comparison operators. Its left operand must be a container and its right operand may be any value. The result is a boolean, which is true if looking up the value in the container would result in a non-exceptional value, false if the lookup would fail.

Combining this with the new symbol-literal syntax lets you ask whether a given object implements some method:

if foo has :bar:
    foo->bar(42)
end if

Testing for a set of related methods lets you determine whether an object implements some interface:

function stack?(foo):
    result = foo has :push
    result = result and foo has :pop
    result = result and foo has :head
end stack?