In the following, the statement "M has type A" (or equivalently, "A is a type of M"), notation |- M : A or simply M : A, stand for "the statement {} |- M : A has a proof in the type system of Curry". Note that the set of assumption must be empty.
(var) ---------------- x : A |- x : A (abs) ------------------ |- \x.x : A -> AWe can also prove that \x.x has type (B -> B) -> (B -> B) for any type variable B. In fact, by replacing A by B -> B in the proof above, we obtain the following:
(var) -------------------------- x : B -> B |- x : B -> B (abs) -------------------------------- |- \x.x : (B -> B) -> (B -> B)In a sense, however, the first type (A -> A) seems a "better answer". In fact, the type (B -> B) -> (B -> B) can be seen as a particular case of A -> A, but not viceversa. In other words, A -> A is more general than (B -> B) -> (B -> B). We can indeed prove that A -> A is the most general (or principal) type of \x.x.
Theorem If a lambda term has a type, then it has a principal type, unique modulo renaming.
We don't give a formal proof of this theorem, but we show how to construct the principal type. Let us start with the example of the term \x.x. Intuitively, any proof of a type statement for \x.x must have the following form:
(var) ---------------- A = B x : A |- x : B (abs) ---------------- C = A -> B |- \x.x : CThe equation C = A -> B indicates the condition under which (abs) is applicable, and the equation A = B indicates the condition under which (var) is applicable. Hence A = B , C = A -> B are the conditions under which the proof is valid. Any solution of these equations (i.e. any substitution which makes A and B identical, and C and A -> B identical), applied to the type in the conclusion (i.e. C), will derive a type for \x.x. Clearly, the most general type is obtained by taking the most general solution of the equations (see section on unification below). One form of the most general solution is the substitution theta = { B |-> A , C |-> A -> A }. theta, applied to C, gives the type A -> A. (Alternatively we could have considered the substitution { A |-> B , C |-> B -> B }, that would have given the type B -> B, which is equivalent to A -> A modulo renaming.)
the principal type of M is A theta.If, on the contray, the generic proof cannot be built, or the equations are not solvable, then M is not typeable.
- val f = fn x => x;(remember that fn x => E is the ML syntax for \x.E), the ML answer is:
val f = fn : 'a -> 'a(where 'a represents a type variable), meaning that f is a function of type 'a -> 'a.
Intuitively, it was to be expected that \x.x x has no type. In fact, x x represents the application of a generic function x to itself. Now, there are functions for which it makes sense to be applied to themselves (for instance the identity function), but this is not the case for all functions.
For the same reason, also the fixpoint operator Y (see notes of Lecture 2) has no type.
the inhabited types are exactly the formulas which are valid in the intuitionistic propositional logicwhere -> is to be interpreted as logical implication.
Definition Given a set of variables Var, and a set of function symbols Fun (possibly including constant symbols) the first-order terms are defined by the following grammar:
Term ::= Var | Fun(Term,...,Term)
In the case of type expressions, we have only one binary function symbol (represented in infix notation): the arrow ->.
Definition A substitution theta is any mapping theta : Var -> Term.
A substitution theta will be denoted by listing explicitly the result of its application to every variable (usually we are interested only in finite substitutions, i.e. substitutions which affect only a finite number of variables). We will use the notation x |-> theta(x).
The application of a substitution theta to a term t, denoted by t theta, is the term obtained from t by replacing each variable x by theta(x).
The composition of two substitutions sigma and theta is the substitution sigma theta s.t. for every variable x, (sigma theta)(x) = (x sigma)theta.
Definition Given a set of equations on terms E = {t1 = u1, ... , tn = un}, a substitution theta is a unifier (solution) for E iff for each i we have that ti theta is identical to ui theta. A substitution theta is the most general unifier of E if it is a unifier for E and, for any other unifier sigma, there exists sigma' s.t. sigma = theta sigma' (i.e. sigma can be obtained by instantiating theta).
Example Consider the set of equations
E = {A = B->C , B->C = C->B}We have that