3.3.1.4.1 What types are instantiable?
Paul Haahr
``Instantiable type'' : [...] The unifying concept is that all ``instantiable types'' are types which can be instantiated.
The rule specified in the DIRM is that make may be called on ``instantiable types,'' of which there are two defined by the language: classes and limited collections. (The latter may be implemented as classes, according to the design note.)
Note, by the way, that not all classes are instantiable types. Abstract classes are by default not instantiable. And, by defining a make method I can switch the classification for a particular type.
Jeff Dalton
What do they have in common apart from that? Why are they, and they alone, instantiable?
Paul Haahr
The rule is: 
   The first argument to make must be an instantiable
   type.
The definition is: 
   An instantiable type is a type suitable for use as
   an argument to make.
I don't mind living with the circularity, but one of the reasons that it's hard to pin down is that you can change what's instantiable by defining methods on make.
If you want a set of rules, here's a set off the top of my head.
    • Most concrete classes are instantiable, but you can change this by defining a make method which signals an error.  Some builtin concrete classes, notably those with special behavior for the == function  (e.g., numbers), are not instantiable.
    • Abstract class are not instantiable unless there is a make method. It is common among the builtin abstract classes for there to be a   make method which instantiates a subclass of the abstract class.
    • Limited collection types are instantiable, though redefining a make  method might interfere with this.
    • Other types are generally not instantiable, though a make method can probably change this.
The important idea here, to my mind, is that instantiability is purely an operational aspect of make's behavior.  It is not a property of the type system.
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