Section 15: Implicit Polymorphism




Implicit Polymorphism

In the previous lecture, we introduced polymorphism and distinguished between its explicit and implicit forms. When confronted with the former, the programmer writes down the types of all variables and explicitly abstracts over both values and types. For the latter, the programmer simply omits all type information from the program and tells the language implementation to derive the types. However, we left this process of derivation unspecified.

Consider the following examples:

  1. (lambda (x _) x): we can use any type in the type language in place of _.
  2. (lambda (x _) (+ x 0)): we can only put in int, since the argument is used in an addition.
  3. (lambda (x _) (x 0)): since the argument is applied, we know its type must be of the form a -> b. The type accepted by the argument, a, must be a number; its result type can still be any legal type. Hence, we can assign the type (int -> _) for this expression.

Is it still true that (lambda (x _) (x x)) cannot be typed? Yes. The type _ must be some type definable in our type language.

We can work around the restriction on self application by introducing
(datatype circtype
  ([circproc circproc? ((circtype -> T) proc)])

where T is any type and rewriting the procedure application (x x) as

((proc x) x)

Of course, the latter is cheating in some sense since we no longer have true self application anymore. But we can then type (lambda (x _) ((proc x) x)) by letting _ be the type circtype. In this case, the return type of our lambda expression is T. Any argument bound to x must be of type circtype but any expression of the form

(circproc (lambda (x circtype) <any value of type T>))

obviously works. In general, what can we do if we are given an expression of the form (lambda (x _) <body>)? We can infer a type for _ in two stages:

  1. We shall name and conquer: first, we assign _ a type variable (harkening back to explicit polymorphism). Thus, we introduce type variables into the language. Now every variable in the program has a type taken from the following language:
    t_alpha = tv | int | (t_alpha -> t_alpha)
    where the tv are Greek alphabet representing type variables.
  2. Once we have types for all variables, we need to write down equations constraining these type variables and solve this set of equations. Where do these equations come from? They arise naturally from the typing rules.
First, we show the typing of (lambda (x _) x). Initially, we infer a type for the procedure based on two fresh type variables:
T |- (lambda (x alpha) x) : alpha -> beta

Now we type the body of the procedure. Since x is bound in the body, we have

T [ x/alpha ] |- x : alpha

However, since the type of the result of the procedure is that of x, and this result type has been indicated as beta, we also have

T [ x/alpha ] |- x : beta

Combining these facts, we see that the type of the procedure is alpha -> beta subject to the constraint that alpha = beta, ie, alpha -> alpha. Put differently, the programmer could have written down any arbitrary TLC type, and the program would have typed correctly.

This example illustrates the process of deriving a type. We use the type language t_alpha to play the guessing game, but the types given to the programmer are still in terms of the old types we had.

We now consider a more involved case:

((lambda (x _) x) (lambda (y _) y))

We begin by assuming a new type for the whole expression:

T |- ((lambda (x _) x) (lambda (y _) y)) : gamma

Now we type the sub-terms. Each one is a procedure, so each is given an ``arrow'' type.

T |- (lambda (x alpha) x) : delta -> epsilon
T |- (lambda (y beta) y) : phi -> psi

From this, we derive two equations:

gamma = epsilon
delta = phi -> psi

The former holds since the type of the overall expression is that returned by the applied procedure, the latter by the typing of arguments to procedures.

Next, we can conclude that

T [ x/alpha ] |- x : alpha
T [ y/beta ] |- y : beta

Each of these induces several more equations:

alpha = delta since both represent the argument,
alpha = epsilon since both represent the result;
beta = phi and
beta = psi similarly.

From these, we can in turn conclude that

delta = phi -> psi = beta -> beta
epsilon = alpha = delta = beta -> beta
gamma = epsilon = beta -> beta

which gives the type beta -> beta for the entire expression, where beta is the type assigned to the argument of the procedure passed in as the argument.

The process of solving symbolic constraints is similar to other forms of linear equation solving including Gaussian elimination. In fact, solving symbolic constrains is a surprisingly easy because the algebraic structure of abstract syntax (called free algebras by mathematicians) is so simple. The general algorithm for solving symbolic constraints is called unification and was invented by Robinson at Rice (in the Philosophy department) in the mid-60s.

In spite of all this machinery, expressions like

(let ((f (lambda (x _) x)))
  (if (f true)
    (f 5)
    (f 6)))

still cannot be typed. The problem is that the rule we have adopted for let requires us to make a guess before we check the body; no matter which type we choose, we will run afoul. So we propose that, instead, we use the copying rule for let that was proposed in the previous lecture. What is the property of this rule that enables us to type this expression? It is that it copies the blanks. The ``copying'' let typing rule copies places for guessing types.

However, this new rule has one very deleterious effect. There could be a let in the binding for f; there could be another let in its binding; and so forth. When we copy the code, this leads to an exponential blow-up in the resulting (copied) code that needs to be typed. Thus, in practice, we want this typing rule, but would like to avoid having to duplicate code. In effect, this means that we want to put the _'s in the environment, ie, we want to bind type variables at lets and free them at uses.

Idea: Introduce type schemas with close and open operations.

Our type schemas will take the following shapes:

ts = t_alpha | (forall (alpha ... beta) t_alpha)

In the above expression, we would get the type alpha -> alpha for f, which is (forall (alpha) (alpha -> alpha)). This also shows that type inference for implicit polymorphism can be thought of as a translation process from TLC syntax without types into XPolyLC.

In the following example, we quantify over multiple type variables:

(let ((k (lambda (x alpha)
           (lambda (y beta)

The shape of type inferred for k is then (forall (alpha beta) (alpha -> (beta -> alpha))).

With type schemas in hand, we try to produce a better let rule.

T |- E : t    T[ x/CLOSE(t) ] |- B : s
        T |- (let (x E) B) : s

When we look up identifiers in the type environment, we might sometimes get a CLOSEd object instead of a regular type. Hence, we need another typing rule, OPEN, to cover these cases:

T |- x : t [ alpha.../beta... ]

This rule is read as follows. Say an identifier x is looked up in the type environment T, and it has the type t. If t is a type schema, inserted into the environment through a CLOSE expression, then we replace the type variables quantified over in the schema (the alpha...) with fresh type variables (the beta...), then proceed as before. Since a new set of type variables is instantiated at each use, the same code can be given different types in different uses, so our canonical example can be typed.

Unfortunately, these simple versions of CLOSE and OPEN are flawed. Suppose we had this program:

(lambda (y _)
  (let ((f (lambda (x _) y)))
    (if (f true)
      (+ (f true) 5)

Say y is assigned type alpha and x is assigned beta. Then the type of f is (alpha -> beta). Our typing rule has naïvely closed over all the type variables lexically visible. At each use of f, new type variables are created. In the first, beta_1 = bool and alpha_1 = bool; in the second, alpha_2 = int and beta_2 = bool. The type checker is satisfied, and an expression that ``type-faults'' at run-time has passed the type checker.

In the rule for let, if we restrict CLOSE to only close over variables introduced by the polymorphic let, then we get the desired behavior. The introduction rule is now

T |- E : t    T[ x/CLOSE(t,T) ] |- B : s
         T |- (let (x E) B) : s

with the same elimination rule, and we can formally write down a specification of CLOSE:

CLOSE (t,T) = (forall (alpha...) t)

where alpha... = ftv(t) - ftv(T) (ftv computes the set of free type variables in expressions and type environments). With this new typing rule, the type schema for f is (forall (beta) (beta -> alpha)), so we now have to unify alpha with both int and bool, which fails and results in an error.

A more detailed discussion of implicit parametric polymorphism appears in the PowerPoint presentations entitled Types I and Types I in the Readings section of this web site.

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