Dec, 2010 the diagnostic messages produced by the ocaml compilers for all type errors, not just the structural subtyping errors, can be a bit oblique. A structural type system or propertybased type system is a major class of type system, in which type compatibility and equivalence are determined by the types actual structure or definition, and not by other characteristics such as its name or place of declaration. On julias efficient algorithm for subtyping union types. Jan 29, 2017 theres also structural subtyping, where a being a subtype of b is determined by the structure of both a and b. The best example i could come up with of a language with structural subtyping is go, and ive shamelessly stolen the following example from go by example. Polymorphic variant types 7 were introduced in ocaml version 3. The process described in the post seems vastly different than the reflective and generative techniques described for scalas optional structural type system and for the white oak extension of the java language. Subtyping definition subtyping refers to a process whereby people come to view individuals who dont fit a stereotype as exceptions or as poor members of a group. The whiteoak programming language is an extension to java that introduces a greater degree of flexibility into the language. Such functions can accept arguments of types which will only be defined. This permits structural subtyping, where object types are compatible if their method signatures are compatible, regardless of their declared inheritance an unusual feature in statically typed languages.
The modulewise structure of our implementation is shown on fig. Structural subtyping is popular in research languages, but all mainstream objectoriented languages use nominal subtyping. However, adding support for structural subtyping shouldnt be too difficult, once we figure. The other mechanism used to create subtyping relations is structural subtyping. This is also possible in ocaml, but the use of subtyping must be indicated explicitly. Structural systems are used to determine if types are equivalent and whether a type is a subtype of another. Subtyping involves psychologically fencing off deviant group.
Subtyping relaxes the requirement that functions take arguments of a given type, by allowing arguments of any subtype of that type to be given. Pdf we report on implementing graph grammars for intelligence analysis in ocaml. Ocaml top is a gtkbased editor coupled with an ocaml toplevel, providing straight forward evaluation controls, builtin syntax coloring and forced visual indentation. For example, ocaml uses structural typing on methods for compatibility of object types. On julias efficient algorithm for subtyping union types and. Structural subtyping is arguably more flexible than nominative subtyping, as it permits the creation of ad hoc types and protocols. Many statically and dynamically typed languages attempt to achieve flexibility in their type discipline by some notion of subtyping. Combinators and typedriven transformers in objective caml. Structural subtyping defines an implicit relationship between types that have the same structure.
It has been publicly announced on 20030701 it consists in an extension of the ocaml with a type system tracing information flow. We are interested in type inference in the presence of structural subtyping from a pragmatic perspective. Note that the inferred type for the argument of describes a subtype of the. An analytic is a generalpurpose ocaml function whose output. The diagnostic messages produced by the ocaml compilers for all type errors, not just the structural subtyping errors, can be a bit oblique. In addition to algebraic data types, we use other concepts from functional programming languages to implement features of graph grammars.
Under structural subtyping or any kind of subtyping, for that matter, the subtyping relation admits a rule called subsumption, which can lead to information loss. There are some situations where you might prefer row types, and some where you. In contrast, structural subtyping implicitly allows any type with appropriate structure to be used in a given context. Besides, the framework has been implemented in objective caml, yielding a generic.
Elaboratetype classes, a kind of bounded quanti cation where the subtyping relation needs not be internalized. The javascript language is a bunch of objectoriented ideas and functional ideas mixed together. The most notable other approach to structural typing is structural subtyping, which im not going to go over here, but which also exists in ocaml. Signature based polymorphism, structural subtyping and duck typing hello all, i could really use some help in enumerating the various languages which provide support for signature based polymorphism, structural subtyping andor duck typing. Implementing a language with flowsensitive and structural. This strategy is less common than nominal subtyping.
Since languages with structural subtyping are not in widespread use, the empirical questions of whether and how structural subtyping. But this options is very hard to apply existing project. Certifying ocaml type inference and other type systems. Structural subtyping is popular in research languages, but all mainstream object oriented languages use. Run the installer and follow the directions therein. Motivating the development of whiteoak is the understanding that due to extensive use of libraries, components, and external data sources, the developer has limited control on how the objects in her program are created. On julias efficient algorithm for subtyping union types and covariant tuples artifact introduction. Of course, doing so causes a loss of structural subtyping.
This property, called subtyping, would then allow log to be used with type out, eg. Ocaml subtyping ocaml comes with a structural subtyping relation generated by object types and polymorphic variants subtyping and extended structurally by considering the variance of ml type constructors. Go and ocaml have structural subtyping where objects typed as some interface can be dynamically dispatched on. The idea of deriving the general structural subtyping rules from the inductive schemata was. The code below, written in this language, defines two objects, x and y. Its pragmatic approach allows us to allocate without much fear of efficiency loss. Introduction to functional programming in ocaml hacker news. Structural subtyping for inductive types with functorial. Subtyping concept is generally based on the principle called liskov substitution principle where type b is a subtype of a if every function invoked on an object type of a is also be invoked in object type of b.
Ocamls definition of what is an expression is just a little wider than cs. The garbage collector probably is ocamls greatest unsung hero. Integrating nominal and structural subtyping springerlink. Although ive been wrapping my mind around structural subtyping in java for quite some time now, there is one argument in favor of structural typing that didnt get the attention it deserves. We report on implementing graph grammars for intelligence analysis in ocaml. Signature based polymorphism, structural subtyping and. This permits structural subtyping, where object types are compatible if their method signatures are. The inferred type of arrayofdogs will be an object with a method named bark of type unit unit not necessarily a dog.
Structural sub typing in haxe is allowed when unifying. Integrating nominal and structural subtyping donna malayeri jonathan aldrich structural vs. Piggybacking on the battletested ocaml type checker would allow me get off the ground fast and more easily experiment with my languagetype system. An important attribute of every type system is whether they are structural or nominal, they can even be mixed within a single type system. Nominal and structural subtyping in componentbased. Subtyping in ptb and pt is non structural, that is, two types can be related by even in cases where they have di erent structures. On the one hand, kuncak and rinard kr03 recently showed that the firstorder theory of structural subtyping of nonrecursive types is decidable. This is obviously how most dynamically typed languages work duck typing and i also think that you find some of the same ideas in the ocaml module. Sep 15, 2016 an important attribute of every type system is whether they are structural or nominal, they can even be mixed within a single type system. Integrating nominal and nominal subtyping t structural subtyping. Look at just about any ocaml code that uses the structurallytyped object system.
Hi all, im trying to achieve a compiler enforced checking of equality of structural types. Equality of structural types the scala programming language. Contribute to rehanmalaktpl development by creating an account on github. A structural type system is a major class of type system in which type compatibility and. The design decision in flow around mixing nominal and structural typing was chosen based on how objects, functions, and classes are already used in javascript.
Graph transformations are defined as implicit coercions derived from structural subtyping proofs, subset types, lambda abstractions, and analytics. Structural subtyping of nonrecursive types is decidable. The subtyping relation tells wether an object of some type can safely be considered as an object of another type. It extends caml with objectoriented features, and is a member of the ml family the ocaml toolchain includes an interactive toplevel interpreter, a. Luo luo 2005, it has become clear that in an intensional type theory, the structural subtyping rules for some of. This is actually row polymorphism rather than structural. You can now run an ocaml toplevel from the windows command line by typing ocaml. An analytic is a generalpurpose ocaml function whose output is required to match a graph pattern described by an element of an algebraic data type. Structural types and duck typing infinite negative utility. It provides a convention how to store libraries, and a file format meta to describe the properties of libraries. Graph grammars are represented as elements of an algebraic data type in ocaml. Citeseerx document details isaac councill, lee giles, pradeep teregowda. Subtyping and inclusion polymorphism subtyping makes it possible for an object of some type to be considered and used as an object of another type. Introduction to programming languagesuniversal polymorphism.
Pearce1 james noble2 school of engineering and computer science victoria university of wellington, nz abstract dynamically typed languages are. Since contracts are typically expressed as refinements of types, contracts in nominally typed languages introduce additional obstacles to reuse. The following example is part of the lambda class in the haxe. Oh, and the other neat thing is that ocaml cleanly separates. Structural subtyping of course uses subtyping to achieve the above. While researching about structural typing i found the following post describing how interfaces in go are translated to method lookup tables at runtime. Ocaml uses structural subtyping, so its type system works differently when it comes to type inference. We use type checking to perform graph pattern matching. In most objectoriented languages, an object with a larger interface may be used in place of an object with a smaller one.
Sep 17, 2006 ocaml subtyping ocaml comes with a structural subtyping relation generated by object types and polymorphic variants subtyping and extended structurally by considering the variance of ml type constructors. It results in a huge overlap of functionality with the nominal system that youll want most of the time for enforcing invariants, providing explicit program structure, et cetera. It is quite similar to abcs, but neither is a replacement for the other. Languages like javascala we can use inheritance as of subtype perspective. In fact, c has the concept of statements but all of cs statements are just expressions in ocaml of type unit combined with the. In this paper, we describe a core calculus for a language that integrates the key aspects of nominal and structural subtyping in a unified framework. What are the novel ideas and profound insights in the. Languages like ocaml and scala allow generics to be. Structural subtyping with control flow based type analysis. Ok, i now see that this is mainly a matter of terminology. Its crossplatform and specially tailored for students and practicals. But scala is a mixture of both oop and fp, so we can use a bit of structural subtyping via duck typing design.
On the other hand, i want to control the backend of my compiler for example, generating c code in the end, so pure translation into ocaml code and usage of the ocaml compiler on an elaborated. Mbt subtyping software module fast microorganism identification combined with instant, automated typing a major reason for the broad application of the maldi biotyper for identification of microorganisms is the superb discriminatory power of malditof mass spectrometry. The use of this subtyping relation in programs is explicit. Ocaml is notable for extending mlstyle type inference to an object system in a generalpurpose language. Ocaml subtyping ocaml comes with a structural subtyping rela tion generated by object types and polymorph ic variants subtyping and extended structurally by considering the v ariance of ml type. Carnegie mellon university, pittsburgh, usa pa 152. Are there any object oriented programming languages that. The flow caml system is a prototype implementation of an information flow analyzer for the caml language. This is the artifact for the pearl paper on julias efficient algorithm for subtyping union types and covariant tuples. There is also a tool ocamlfind for interpreting the meta files, so that it is very easy to use libraries in programs and scripts. This is actually row polymorphism rather than structural subtyping. Structural subtyping haxe the crossplatform toolkit. When you specify a type it will not match with the subtype, it will only match literally, and you need to use an explicit upcasting operator to make a value of type t to be applicable in a context where supert is expected.
The details of how to use protocol are still poorly defined, as the mypy type system does not know about structural subtyping yet. In nominal systems, the sub typing relation is a subset of the structural subtyping relation. Graph grammars are represented as elements of an algebraic data type. Apart from the previously mentioned prototypebased oo systems javascript, self, theres several distinct cases. One of the most wellknown programming languages that foster structural subtyping is ocaml. Languages like ocaml, haskell, and elm have primarily structural type systems. In structural subtyping, a wider record type is subsumed by a narrower one. On the other hand, structural subtyping is flexible and compositional, allowing unanticipated reuse. To date, nearly all objectoriented languages fully support one subtyping paradigm or the other. Structural subtyping is popular in research languages, but all mainstream objectoriented languages use.
Ocaml is a generalpurpose, multiparadigm programming language which extends the caml. Jacques garrigue certifying ocaml type inference 1 whats in ocamls type system core ml with relaxed value restriction recursive types polymorphic objects and variants structural subtyping with variance annotations modules and applicative functors private types. Implementing a language with flowsensitive and structural typing on the jvm david j. Implementing graph grammars for intelligence analysis in ocaml. In contrast, type systems with atomic subtyping 4 have the property that if sis a subtype of t, then sand thave the same structure. The concept is important because it explains why people often do not change their stereotypes in the face of disconfirming information. Pierce, types and programming languages, mit press, 2002, isbn 0262162091, chapter 15 subtyping of record types, 19.
470 1310 1380 709 33 521 321 1583 42 215 497 1405 1006 1210 112 265 742 204 716 989 222 1576 196 73 1605 422 110 800 427 580 623 1116 1039 1120 37 1482 142