1 INTRODUCTIONThe Theory of Classification is an informal series of articles considering the formal notions of type and class in objectoriented languages. The series began by constructing models of objects, types, and inheritance, then branched out into interesting areas such as mixins, multiple inheritance and generic classes. Most recently, we returned to the core argument that simple types and subtyping are formally different concepts from polymorphic classes and subclassing. The previous article [1] described how objectoriented languages were unclear about the distinction between simple and polymorphic types, so we described one possible approach in which class names (which are used like type identifiers) were interpreted unambiguously either as simple types, or polymorphic classes, according to context. In the current article, we consider in more detail the kinds of manipulations performed upon polymorphic classtypes. These are expressed using functionbounded type parameters of the form: <: F[], where F is a type function, describing the shape of the interface that the type is expected to satisfy [2]. In the following, we motivate the need for parameters and bounded parameters, then examine what happens when we require a language to express all of its polymorphism in this way. The result is a proliferation of parameters and constraints, which has both good and bad consequences. On the positive side, it is clear over what types the polymorphic variables may range. On the negative side, the syntax of such languages becomes inflated with parameters and is therefore somewhat unwieldy. 2 POLYMORPHISM NEEDS PARAMETERSThroughout this series, we have made it clear that wherever polymorphism is intended, this formally requires the use of a type parameter, to stand for the group of types over which the function is defined [3]. For example, consider how the identity function applies to a value of any type and returns an identical value, and the result is of the same type as the argument. To define such a function in C++, we must use the template mechanism: in which the line “template <class T>” introduces the type parameter T, which stands for any type. This function can be applied to values of many different types in C++, including primitive types, pointers and structured object types: The polymorphic function appears to be “smart” because it somehow detects the type of its argument and returns a value of the exact same type. In C++ this is accomplished by the compiler, which statically detects the argument’s type and creates a specific instantiation of the template function at compile time. In fact, for every distinct instantiation of the identity function, the compiler must generate a new copy of the identity function. In the end, it is as if the above program contained four different identity functions, with the overloaded type signatures: and then C++’s normal rules for selecting one or other overloaded function were used to determine which function to apply. In the calculus, which is a very simple symbolmanipulation system, we bind the type parameter explicitly, supplying the intended type. The calculus cannot detect by itself that a value has any particular type unless we tell it so. All polymorphic functions therefore accept type arguments and value arguments, in that order [3]. The identity function is defined: and we apply it to values of different types in the following style, in which we supply first the type argument, then a value argument of that type: Readers will recall that a function simply binds its arguments in a lefttoright fashion and so doesn’t need to put the arguments in parentheses. In earlier articles, we have sometimes adopted the convention of wrapping type arguments in brackets [] and value arguments in parentheses () to help the reader visualise what is going on inside the function. Secondorder functions like the above are really two functions, one nested inside the other. The outer function is a typefunction and the inner function is a valuefunction. In the above examples, we supplied both the type argument and then the value argument. If we only supply the first type argument, then the result we get back is the second function, which is the body of the first function, in which the type parameter has been replaced: and this models the effect of C++’s type instantiation, since it returns a specific version of the identity function, ready to be applied to a value of one particular type. 3 CLASSES NEED BOUNDED PARAMETERSThroughout this series, we have argued that classtypes are also polymorphic things and therefore need to be modelled using type parameters [3, 4, 1]. The main difference between a universal polymorphic function, such as identity, and the kinds of function that belong to a restricted class, such as plus, minus, times and divide, is that we want these functions to apply not just to any old type, but only to those types which are considered to be “at least some kind of number”. In earlier articles, we demonstrated that this required the introduction of constraints, or bounds on the type parameter. We defined the interface of a number class using a type function: and then used this to constrain a type parameter, in the functionbounded style [2, 4]: How does this constraint express what we mean by a class? The intention is that may only range over certain types in a type family [3]. What the constraint literally says is “all types that are subtypes of the type you get when you apply GenNumber to the type”.
So, returning to the original problem, imagine that we expect the Integer type to belong to the class of numbers. Therefore, Integer must be one of the types that satisfies the above constraint, in other words, we expect the following to be true: by applying GenNumber[Integer] and seeing what kind of recordtype we get when we substitute {Integer / } in the body of the generator. So, what this constraint is really saying is that the Integer type must have at least as many methods as the record type on the righthand side. This is easy enough to satisfy if we give Integer all of those methods; and we could possibly give it more methods, such as: modulo : Integer Integer, which returns a remainder. So, this kind of bounded type parameter captures exactly the sort of constraint we need when defining groups of functions that apply to all the types in a given class and only to those types. We may write the polymorphic type of plus and minus in the style of methods: This says that, for all numeric types , selecting the plus or minus methods from an object of type will return a function that accepts the remaining argument of the same type and this will also return a result of the type . Alternatively, we can write the polymorphic type of plus and minus in the style of regular functions: which are now clearly seen to accept two arguments, the first of which is the receiver object, from which the method is to be selected. The remaining argument and result type are as above. 4 BOUNDED PARAMETERS IN FUNCTIONAL LANGUAGESOur notion of a class is exactly the same as the notion of type classes in the stronglytyped functional programming languages. The language Haskell [6] defines polymorphic functions using type parameters and sometimes needs to assert that these parameters range over certain restricted classes of types. In the following example of a polymorphic function to compute the length of a list, “[]” denotes a listtype and “a” denotes a type parameter: The first line is a type signature, saying that length takes a list of any polymorphic element type “[a]” and returns a result of the Int type. This is a universal polymorphic function, rather like identity earlier. The length can be computed, irrespective of what type of element we choose for the list. However, the polymorphic function for list membership cannot be defined in quite the same way. To determine list membership, the body of the elem function needs to be able to compare the supplied value with successive elements of the supplied list, using the equal function “==”. So, the polymorphic element type must be one that is in the class of types possessing an equal function. In Haskell, this is represented by the constraint “Eq a”, which has the sense “any polymorphic type a in the Eq class”. Elsewhere, Haskell defines the Eq class as the class of all those types possessing “==” and “/=” (not equal). The definition of elem is given by: in which the constraint “Eq a =>” on the first line has the sense: “provided that the type a is a member of the Eq class, then this function takes an element of the type a and a list of the type [a] and returns a Bool result”. The effect of this class constraint is to ensure that elem can only apply to lists, whose elements have a welldefined equal function. If this constraint were not present, then the Haskell compiler would refuse to compile the definition of elem, because it could not guarantee that “x == head” was a welltyped expression. This, by the way, contrasts with the approach taken in C++, which allows you to write arbitrary expressions involving variables with parametric types, because C++ doesn’t compile or check any of its template definitions. It simply waits for these to be instantiated, and then checks that all the calls are welltyped for each instantiation, separately. In the calculus, we can express such a class of types with equality, using a type generator to constrain the polymorphic type parameter to accept only types that have at least the two functions equal and notEqual: The above Fbound provides the exact meaning of Haskell’s “Eq a => …” constraint. It was quite satisfying to find this convergence between the notion of class put forward in the Theory of Classification, and the notion of “type classes” in functional programming languages, because it means that we are all probably on the right track! 5 CLASSES WITHIN CLASSES NEED NESTED PARAMETERSAfter a while, the realisation comes that everywhere you want polymorphism, you formally need another type parameter. This has interesting consequences when you consider more complex classes that are built up out of a number of other classes as their “elements”. Simple examples include the kind of generic classes we considered in an earlier article [7]. For example, if we have a List class whose selftype is expressed as the parameter , and if this List has a method insert accepting elements of some other class, whose polymorphic type is expressed as the parameter , then in which order should these be declared? The solution to this quandary is found by thinking about the notion of type dependency. The elementtype may exist by itself, but the listtype depends in some way upon the elementtype in its definition. That is, for whatever element type we imagine, the list selftype somehow depends on the type of . This is natural, since we would expect an IntegerList to depend on its Integer elements, whereas a RealList would have Reals as its elements. We therefore introduce before , since this keeps within the secondorder calculus, in which type parameters only range over simple types [3]. That is, we introduce in a context in which is already bound to some actual type, so can range over simple types also. To see how the type parameters stack up, let us construct the type signature for a generic List class with a method elem to test whether the inserted elements are members of the list. From section 4 above, we know that this puts a constraint on the element type, which must have an equal method to compare itself with other elements. Therefore, we will first require a type generator to express the shape of the element type: The shape of the list’s interface is expressed through a second type generator: which now recognises that there are two parameters involved. The first is for the elementtype. We can see this by applying GenList to some actual element type, say Integer, to see what kind of typeexpression this produces: The result substitutes {Integer/} in the body of the generator, returning a nested type function beginning: .{…}. This looks exactly like the shape of a regular type generator for a nongeneric class, and so it is. The second type parameter stands for the selftype of this class. We need this because we are dealing with polymorphic classes, not simple types. That is, the above definition is the minimum common interface for a variety of different listtypes, which might include more specialised listtypes, such as sorted lists. What is the form of the Fbound constraint that ensures that our list selftype ranges over only those lists which (i) have elements with an equalmethod and (ii) have a listinterface that includes all of the methods: insert, head, tail, length and elem? This is a constraint constructed using both of the above two type generators: What is interesting here is the double quantification. On the outside, we assert that the element type may only range over subtypes of GenEqual[t], that is, types with at least the methods: equal and notEqual. Then, on the inside, we assert that the selftype may only range over subtypes of GenList[, ], that is, types with at least the methods: insert, head, tail, length and elem, provided that ? is of the earlier specified type. This is the way in which the constraint for the listtype depends on the elementtype. Translating this into objectoriented programming terms, the polymorphic type of a class depends on the polymorphic types of the other classes which it references internally. More precisely, it depends on those classes which affect the shape of its external interface. 6 SUBCLASSING USES PARAMETER SUBSTITUTIONAssume now that we wish to derive a more specialised kind of List class, say a SortedList of elements which are inserted automatically in ascending order. SortedList has two new operations least and greatest, to return the smallest and largest elements and otherwise has all the operations of a List. Below, we define the polymorphic type of the SortedList class by inheritance, constructing the new constraint partly from information present in the old one. Afterwards, we show how the new constraint preserves the old constraint. To permit the sorting of elements when they are inserted, elements must be comparable with each other using lessThan. This means that the constraint on the elementtype will be stronger than the one required for a plain List element. The new type generator must at least have the interface of Equal, otherwise the old List membership method elem would not work. The new generator GenComparable is therefore defined to extend the interface of GenEqual: In the above, the new element type parameter is . This is introduced on the outside, then the result is formed internally as the union of the old interface and the extra methods. Note that the inherited interface is given by: GenEqual[]. This essentially applies the old generator to the new parameter, and creates a record type in which {} has been substituted throughout: and this inherited record type can be safely unioned with the additional method signatures. A similar trick is used to extend the List generator to produce the SortedList generator: Note again how this creates an inherited version of the old List interface by applying the old generator to the new parameters: GenList[, ]. This returns a record type in which {, } have been substituted throughout: and this inherited record type can be safely unioned with the new method signatures. Previously, we noted that the reason for substituting parameters like this was to ensure, in the inheriting class, that the selftype was consistently denoted by (rather than a mixture of new ) [3, 4]. Now that we are dealing with nested classes, the rules must be applied recursively for the element type, which must be consistently ? throughout. The stronger constraint for our new SortedList class, which ensures that the type paramter ranges over only those types which are at least SortedLists of Comparable elements, is given by a nested Fbound that is written in terms of the two new generators, but is otherwise similar to the nested Fbound for a List: 7 SUBCLASSING IS THE INCREASING OF TYPE CONSTRAINTSWhat is even more sophisticated in this model of inheritance is the preservation of the constraints on all the type parameters. Technically, we are substituting parameters bounded by one set of constraints with new parameters bounded by a different set of constraints. Are the substitutions all legal? To do this, we check the expectations made internally by the type generators. When we apply: GenEqual[], we substitute {}. Now, the parameter expects to receive a type satisfying: <: GenEqual[], in other words, a type with at least the interface of the Equal class. It so happens that we are replacing one parameter with another parameter, rather than an actual type. So instead, we have to consider all the types that might be allowed by the new parameter. The new parameter expects to receive a type satisfying: <: GenComparable[], in other words, any type with at least the interface of the Comparable class. So, we have to ensure that all types that we could substitute for will also be acceptable types for the parameter . Formally, we determine this using the pointwise subtyping rule [4], checking the assertion: By inspection, the Comparable interface includes the Equal interface, no matter what value we supply for , so this substitution is proven legitimate. A similar process happens when we apply: GenList[, ], causing the substitutions {, }. We follow the same argument for , and then a similar argument for as it replaces the parameter . Eventually, we conclude that any type which we could substitute for will also be an acceptable type for the parameter , as a result of the pointwise rule: This expresses the assertion that the interfaces of SortedList and List stand in a pointwise subtyping relationship, no matter what types we substitute for and . By inspection of the two interfaces, we conclude this holds, so the substitution is proven legitimate. In general, the derivation of subclasses follows a process of monotonic restriction (steadily increasing, or stable constraint) on all the type parameters involved. The kinds of restriction that are permitted can be modelled as a kind of commuting diagram, shown in figure 1.
Figure 1: Commuting diagram of increasing constraints This shows that you can start with a polymorphic list of elements with equality, and can choose to restrict the elementtype, giving a similar list ? of comparable elements with lessthan ordering, and then constrain the list further to a sorted list with comparable elements . Alternatively, both restrictions on the elementtype and listtype may be carried out simultaneously, which is illustrated by the diagonal path. A fourth substitution path, changing a list to a sorted list without changing the element type is not possible, according to the diagram. This is because a sorted list depends on having a comparable elementtype. One new thing that the diagram shows is that substituting one of the “type elements” of the main class, here, the elementtype of the list, leads to a change in the selftype also. Merely choosing to substitute {} with a more constrained elementtype entails the substitution of the selftype {?/} in figure 1. This is something rarely considered in informal treatments of objectoriented type systems. Why has the selftype of the list changed? Well, the result of one access method is typed: head : for the original list; but after we have substituted {}, then we would expect this method to return a different type: head : . Therefore, this must be the head of a different type of list. Jens Palsberg and Michael Schwartzbach were the first to explore the consequential effects of type substitutions to any significant degree in objectoriented type systems [8]. This is still a relatively new area, the subject of continuing research. 8 CONCLUSIONSWe have shown how a proper treatment of objectoriented polymorphism involves the use and manipulation of constrained type parameters. In particular, the polymorphic type intended by a class identifier in objectoriented programs is quite a complex notion. It is a parametric type, restricted to receive only types with a certain interface structure. But more interestingly, the parametric type depends in turn on all the type elements of the class in question. So, the polymorphic aliasing of one of these elementtypes may also affect the type of the whole class. During the operation of inheritance, many type substitutions are performed, both of the elementtypes and the class’s selftype. This follows a pattern which gradually increases the constraints on all type parameters monotonically, restricting the types which may eventually be valid members of the subclass. Very few attempts have been made so far to build a practical objectoriented programming language based on entirely parametric treatments of polymorphism. One attempt was by Simons et al. in the early 1990s [9, 10]. In the experimental language Brunel, simple types were written in the usual way as: x:Integer; y:Boolean; and polymorphic classtypes were written using explicit type parameters: p:P, where #Point[P] introduced the parameter and expressed the Fbound constraint that P is in the class of Points. However, the language eventually proved unwieldy, due to the stacking up of type parameters. Every class that itself contained further classelements had to declare the elementtypes up front. For larger classes, this was a considerably high overhead and eventually was judged impractical for a real programming language. It seems that the most practical way ahead should be to design languages that can keep track automatically of all the complex, and interdependent type substitutions. This might eventually lead to a whole new kind of compiler technology. REFERENCES[1] A J H Simons, “The theory of classification, part 18: Polymorphism through the looking glass”, Journal of Object Technology, 4(4), MayJune 2005, 718. http://www.jot.fm/issues/issue_2005_05/column1 [2] P Canning, W Cook, W Hill, W Olthoff and J Mitchell, “Fbounded polymorphism for objectoriented programming”, Proc. 4th Int. Conf. Func. Prog. Lang. and Arch. (Imperial College, London, 1989), 273280. [3] A J H Simons, “The theory of classification, part 7: A class is a type family”, Journal of Object Technology, 2(3), MayJune 2003, 1322. http://www.jot.fm/issues/issue_2003_05/column2 [4] A J H Simons, “The theory of classification, part 8: Classification and inheritance”, Journal of Object Technology, 2(4), JulyAugust 2003, 5564. http://www.jot.fm/issues/issue_2003_07/ [5] A J H Simons, “The theory of classification, part 4: Object types and subtyping”, Journal of Object Technology, 1(5), NovemberDecember 2002, 2735 http://www.jot.fm/issues/issue_2002_11/column2 [6] S PeytonJones et al., The Haskell 98 Language and Libraries: the Revised Report (Cambridge, UK: CUP, 2003), 270pp. Also pub. as special edn. Journal of Functional Programming, 13(1), January, 2003. Online version: http://www.haskell.org/onlinereport/ [7] A J H Simons, “The theory of classification, part 13: Template classes and genericity”, Journal of Object Technology, 3 (7), JulyAugust 2004,, 1525. http://www.jot.fm/issues/issue_2004_07/ [8] J Palsberg and M I Schwartzbach, ObjectOriented Type Systems (Chichester: John Wiley, 1994). [8] A J H Simons, Low EK and Ng YM, “An optimising delivery system for objectoriented software”, ObjectOriented Systems, 1 (1) (1994), 2144. [9] A J H Simons, A Language with Class: The Theory of Classification Exemplified in an ObjectOriented Programming Language, PhD Thesis, Department of Computer Science, University of Sheffield (Sheffield, 1995), 255pp. About the author
