Adding types to SWI-Prolog

Docs: https://lirias.kuleuven.be/bitstream/123456789/197561/1/shorticlp08.pdf

This is an old playground project for adding typing to Prolog. I have copied it to Github to make sure the code is not lost. The core ideas of this projects were (and are)

Aim at describing as much as possible of normal Prolog practices.
Do not try to be exhaustive
Do not proof type correctness, but try to proof type errors

To achieve these, we envision the following steps

Design a type annotation that is concise, natural and captures most from normal Prolog programming practice.
Implement a constraint based type annotation, i.e., by adding a type to a variable this variable is associated with constraints that prevent it to get a value that does not satisfy the type.
When two type constraint variables are unified, compute the intersection of the types. If this is empty we have a type error.
When two disjunctive paths provide type constraints for a variable the joined type is the disjunction of the types in these paths.
Implement a type checker based on abstract execution using the above type constraints.

What we want from a type declarations

Allow exporting types and provide visibility of types silimar to predicates and operators.
Access predicates:
- (X) is semidet.
- type_constraint(:Type, X)
A type hierarchy as a multiple-inheritance hierarchy.
Interesting challenges:

type_constraint(atom, X), type_constraint(list, X) --> X = []

type_constraint(compound, X), type_constraint(list, X) --> X = [_|T], type_constraint(list, T).
- Atomic types: see whether type decl has actual values that match. Then, replace by eq-constraint?
- Compound:

Hindley-Milner types

  :- type list --->
	( []
	; [_|list]
	).
  :- type list(X) --->
	( []
	; [X|list]
	).
  :- type boolean --->
	( true
	; false
	).
  :- type write_option --->
	( quoted(boolean)
	; ...
	).

Type inheritance

  :- type boolean < [atom] --->
	( true
	; false
	).
  :- type integer < [number].
  :- type atom < [atomic].
  :- type number < [atomic].
  :- type stream.

We also need modes

Input argument. Satisfies the given type at entry
Output argument. Satisfies the given type at exit
? Input/output argument. Satisfies the given type at exit
@ : Input argument. Left unmodified.

Attributed approach

Attributes:

name
type
instantiated

License

This code is available under the BSD-2 clause license.

JanWielemaker / prolog-types