Procedure types

So far, we have regarded procedures exclusively as program parts, i.e. as texts that specify actions to be performed on variables whose values are numbers, logical values, characters, etc. However, we may take the view that procedures themselves are objects that can be assigned to variables. In this light, a procedure declaration appears as a special kind of constant declaration, the value of this constant being a procedure. If we allow variables in addition to constants, it must be possible to declare types whose values are procedures. These are called procedure types.