This is my work repository for playing with the concepts explained in Generalized Algebraic Data Types and Object-Oriented Programming.
The following sentence is a little bit obscure:
The root of the problem is that a virtual method must assume that the type of its receiver (this) is a generic instance of its class: it cannot impose nor make use of any pre-conditions on the class instantiation. Luckily, for functions over parameterized ADTs, such casts can always be avoided: we can re-code Flatten as a static method that uses a cast-free implementation of the Visitor pattern [6] to traverse its list, this time at a more specific instantiation.
The main problem, as far as I'm concerned, is the lack of sample implementation. This is provided here.
It’s not sensible to define equality between unevaluated typed Exp (consider the case for Fst).
TODO: why?
The restriction that constructors all return ‘generic’ instances of the datatype can be removed. Constructors of
Exp[T]do not need to depend onT- eg,Literal, which is anExp[Int].
The regularity restriction can be removed, permitting datatypes to be used at different types within their own definition.
TODO: I do not understand this.
A constructor can be allowed to mention additional type variables that may appear in its argument types but do not appear in its result type. The actual types at which such parameters are instantiated is not revealed by the type of the constructed term.
For example, Fst[Left, Right](pair: Pair[Left, Right]) extends Exp[Left].
Right appears in Fst, but not in Expt[Left].