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A Comparative Study Of Language Support for Generic Programming Oopsla 2003

This study compares the language support for generic programming in different programming languages and identifies areas where improvement is needed. It discusses concepts such as associated types, concepts, and multiple constraints on type parameters.

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A Comparative Study Of Language Support for Generic Programming Oopsla 2003

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  1. A Comparative Study Of Language Support for Generic ProgrammingOopsla 2003 • Roland Garcia • Jaakko Jarvi • Andrew Lumsdaine • Jeremy Siek • Jeremiah Wilcock Presented by: Regine Issan

  2. Generics… • What’s the point ? • reusable algorithms , reusable data structures • Expressed using type parameter • No programming language does this right ! • Previous attempts • C++ STL ?! • Java type safe containers ?! • LEDA ?! • This work: • Generic graph library in 6 PL http://www.osl.iu.edu/research/comparing

  3. Research Results • 8 criterions indicating generics support extended research …

  4. Our Graph Library all we want is: BFS (G g,Vertex s, ColorMap c,Visitor vis)

  5. Iterator element Iter.next() element is associated type What about BFS ReadWriteMap ? Terminology To features understanding associated typessimple example

  6. type parametes: G, C, vis what are G associated types ? G models VertexListGraph and IncidenceGraph interfaces IncidenceGraph { “interface”} G::out_edge_iter out_edges (const G&, vertex ) vertex src (edge, const G& ) VertexListGraph {“interface”} G::vertex_iter vertices (const G&) 4 associated types Terminology to feature understanding:associated typesgraph library example

  7. BFS (G g, Vertex s, ColorMap c,Visitor vis) Any restrictions ? C.key type == G.Vertex type G::vertex type == Vertex type C A.T: {key, value} G A.T : {Vertex, edge, out_edge_iter, vertex_iter } Constraint associated types feature C.key type == G.Vertex type access associated types feature call: BFS (G g, G::vertex s, ColorMap c,Visitor vis) Terminology to features understanding:associated types restriction extended research…

  8. Terminology to features understanding:concept, model algorithm restrict type parameters using Concepts • C is a ColorMap Concept set of requirements on a type or on a set of types • method signatures • colorMap: value_t get (key_t) • support associated types access • C.key_t • A.T relationship • (C.key type == G.Vertex type) • Almost as Interface A type models a concept by meeting its requirements. • Explicit model: class C implements I • Implicit model: structural conformance multiple constraints on type parameter • G models vertexListGraph and IncidenceGrpah concepts multi type concept • concept set restrictions on several types

  9. Terminology: concept refinement • Concept A is said to refine concepts c1,c2, … if its requirements includes concepts c1,c2, …. Requirements • Usage: • G models vertexListGraph and IncidenceGrpah concepts • What if P.L doesn’t support multiple constraints on G ? • Create a single concept from scratch • Or … refine! • ML: includeJava: extends • EiffelL inheritC#: (:)

  10. Introduction to generics in ML ML Structure ~ OOP Class ~ type ML Signature ~ OOP Interface ~ Concept Structure f1Model = struct type f1_field = int fun calc (x: int ) = x*x end End Signature f1Concept = Sig type f1_field val calc: f1_feild -> f1_feild End Signature S = sig structure f1: f1Concept End Functor makeInstance (Params: S ) = struct fun go x = Params.f1.calc (Params.f1.calc ( x) ) End Structure F= makeInstance ( struct structure f1= f1Model end ) F.go 5 Signature restrict type Represents ML concept Structure meet signature S is abstract type algorithm expressed with abstract type Instantiate algorithm with concrete type (f1Model)

  11. Apple Example: C++ ComparableConceptdocumented no constrainton type parameter //bool better (const T&, const T&) Template <class Comparable> const Comparable& go (const Comparable& x, const Comparable& y) { if ( better (x,y)) return x; return y; } Struct Apple { int rating; Apple (int r) : rating (r) {} }; Bool better (const Apple& a, const Apple& b) { return b. rating < a. rating; } Int main (char*[]){ Apple a1(3),a2(5); Apple& a3 =go (a1,a2) Generic algorithm as function template with type parameter Appleimplicitly modelsComparable Implicit instantiation

  12. Comparableconceptas interface with type parameter Apple Example: Java InterfaceComparable<T>{ boolean better(T x); } Class pick { static <T extendsComparable<T>> T go (T a, T b) { if ( a. better (b)) return a; return b; } Class AppleimplementsComparable<Apple> { Apple (int r) { rating = r; } public boolean better (Apple x) {return x .rating < rating; } Int rating; } Public class Main() { public static void main (String[] args) { Apple a1 = new Apple (3); Apple a2 = new Apple (5); Apple a3 =pick. go (a1,s2) } } constraint extend a class and implement a set of interfaces Multiple constraints on type parameter Generic algorithmin class method with type parameter Appleexplicitly modelsComparable Implicit instantiation

  13. Comparable concept as Java Apple example: C# constraint No more than one constraint on type parameter (Gyro, not .NET) InterfaceComparable<T>{ bool better(T x); } Class Pick{ Static T go<T>(T a, T b)where T: Comparable<T>{ if (a. better (b)) return a; return b;} } Class Apple:Comparable<Apple> { public Apple (int r) {rating = r;} public boolbetter(Apple x) { return x .rating < rating; } private int rating; } Public class Main_eg { public static int Main (string[] args) { Apple a1 = new Apple(3); a2 = new Apple(5); Apple a3 =pick.go <Apple> (a1,a2); return 0; } Generic algorithm as Java Apple explicitly models Comparableas in Java Explicit instantiation Programmer specifies type parameters (and associated type parameter)

  14. Apple Example: Eiffel Comparable concept as deferred class with type parameter Deferred classComparable [T]Feature better (a : T) :BOOLEAN is deferred end Class Pick[T -> Comparable [T] ] Feature go (a: T, b: T) is do if a. better (b) Result:=a else Result:= b; end End End Class AppleinheritComparable [Apple] end Create make Feature make (r: INTEGER) is do rating:= r end better (a :Apple) : BOOLEAN is do Result:= rating < a. rating; end End feature {Apple} rating: INTEGER end Class ROOT_CLASS create main Feature main is local a1, a2, a3 :Apple; picker :Pick [Apple] docreate picker; create a1.make (3); create a2.make(5); a3 :=picker. Go (a1,a2) End end constraint No more than one constraint on type parameter Apple explicitly models Comparable Explicit instantiation

  15. If you build new generic algorithm T is constrained by completely new concept Can T be substituted by existing types ? Can existing types model the new concept ? ML: yes If you add: Signature newS = sig … end structures implicitly model newS Java, C#, Eiffel: If you add : Interface newI = { … } no class models it until it’s explicitly declared Evaluation: retroactive modeling feature support

  16. No Type parameters in generic instantiation avoid verbose code Evaluation: implicit instantiation feature support C++: go (a1,a2) Java: pick. go (a1,s2) C#: pick. go <Apple> (a1,a2); Eiffel: picker :Pick [Apple] a3 := picker. Go (a1,a2) ML: Structure F= calcF10F2 ( struct structure f1= f1Model structure f2= f2Model end ) F.go 5

  17. Can generic algorithm be type checked and compiled independently from its call site ? Benefits catch type errors as early as possible fast compilation and small executable type arguments constraints => separate compilation support Evaluation: Separate compilation feature support

  18. Evaluation: Separate compilation feature support in C++ • No constraints on type parameter • Pitfalls • executable size • Type checking against template body , not parameters • Invocation with improper type • non informative error messages • expose template internals Template <class Comparable> const Comparable& go (const Comparable& x, const Comparable& y) {if ( better (x,y)) …} … go (a1,a2) …

  19. Multiple constraints feature: what do we want ? BFS: G models VertexListGraph & IncidenceGraph multiple constraints on G ?! what if PL don’t’ support ?

  20. Evaluation: Multiple constraints feature support Can you set more than one constraint on a type parameter ? • C++: no constraints on type parameters … • Java: extend a class, implement a set of interfaces • Eiffel: not supported, build new concept (ie, deferred class) ML: not as we would want. lets see …

  21. Evaluation: Multiple constraints in ML simple solution : Refinement Signature GraphSig = Sig Type graph_t Eqtype vertex_t End Signature IncidenceGraphSig =Sig Include GraphSig …end Signature VertexListGraphSig =Sig Include GraphSig…end Signature vertexListAndIncidenceGraph = Sig Include IncidenceGraphSig Include VertexListGraphSig End Signatures Include expressed Concept refinement What is wrong ?!

  22. Evaluation: Multiple constraints in ML workaround when refinement fails – duplicate the structure • idea: • G is constrained multiple times ? Let it appear multiple times ! • Each time it will be constrained by different concept ! Signature GraphSig = Sig Type graph_t Eqtype vertex_t End Signature IncidenceGraphSig = Sig Include GraphSig Type edge_t Val out_edges: graph_t -> vertex_t -> edge_t list end Signature VertexListGraphSig = Sig Include GraphSig Val vertices: graph_t -> vertex_t list End Signature BFSSig =Sig Structure G1: InceidenceGraphSig Structure G2: VertexListGraphSig Structure C: ColorMapSig Structure Vis: BFSVisitor Sig Sharing G1 = G2 …. End Sharingcommon nested elements refer the same entity

  23. Evaluation: multiple constraints in ML- workaround when refinement fails result in awkward code Signature BFSSig =Sig Structure G1: InceidenceGraphSig Structure G2: VertexListGraphSig Structure C: ColorMapSig Structure Vis: BFSVisitor Sig Sharing G1 = G2 …. End functor MakeBFS (Params: BFSPSg) =struct fun breadth_first_search graph root visitor map = … end End Structure ALGraph =Struct Datatype graph_t = Data of int * int list Array.array Type vertex_t = int type edge_t = int*int Fun out_edges Data(n,g) vertex = …. Fun vertices Data(n,g) = …. End; Structure BFS = MakeBFS ( struct Structure G1 = ALGraph Structure G2 = ALGraph Structure C = ALGColorMap Structure Vis = VisitImpl End) BFS.breadth_first_search … models both restrictions !

  24. Evaluation: associated type access in C++ Look at BFS generic algorithm : template <class G, Class C, Class Vis> void breadth_first_search (const G& graph, ????? sourceNode, C colorMap, Vis visitor); Is it possible to express that ????? Is graph node type ? traits + typedefs just another class template provide Compile time A.T access

  25. Evaluation:associated type access in C++using traits GT must have vertex typedef Class AdjacencyList { //models IncidenceGraph and vertexListGraph Public: Typedef int vertex; … } Template <typename G> Struct graph_traits { typedef G::vertex vertex; … } } template <class G, Class C, Class Vis> void breadth_first_search (const G& graph, typename graph_traits<G> :: vertex sourceNode, C colorMap, Vis visitor); AdjacencyList g; … Breadth_first_search (g,0, ….) G vertex A.T accessed graph_trait Typename : unknown identifier is considered as type

  26. Evaluation: associated type access in ML So easy: associated types encapsulated within the type signature ColorMapSig = sig type key_t type val_t Val get key_t ->value_t … end Signature BFSSig = Sig Structure Vis: BFSVisitorSig Sturcure G: VertexListGraphAndIncidenceGraph Structure C: ColorMapSig access C.key_t C.val_t when needed End • constraint on associated types: sharing type C.key_t = C.val_t !

  27. Evaluation: associated type accesswhat about Java, C#, Eiffel ? • Ideally , associated are encapsulated within the type • ML: • Structures naturally encapsulate types • C : • Force encapsulation using typedefs • Java,C#,Eiffel • Classes/interfaces cant encapsulate types • only methods and member variables “Solution”: add associated types to the type parameter list !!! lets see …

  28. Evaluation: associated type access in Java adding A.T to interface declaration Vertex associated type in interface parameter list public interface GraphEdge <Vertex> { Vertex source(); Vertex target(); } Public interface VertexListGraph <Vertex, VertexIterator extendIterator<Vertex>> { VertexIterator vertices();… } public interface IncidenceGraph <Vertex, Edge, OutEdgeIterator extends Iterator<Edge>>{ OutEdgeIterator out_edges(Vertex v);… } Edge associated type in interface parameter list

  29. Evaluation: associated type access in Javareferring interfaces AT in generic method Public class breadth_first_search { Public static < Vertex, Visitorextends BFSVisitor, ColorMapextends ReadWriteMap <Vertex, Integer> Edgeextends GraphEdge<Vertex>, VertexIterator extends Iterator <Vertex>, OutEdgeIteratorextends IIterator<Edge>, G extendsVertexListGraph <Vertex,VertexIterator> , IncidenceGraph <Vertex, Edge, OutEdgeIterator> > void go ( G g, Vertex s, ColorMap c, Visitor vis); } 4 method parameters. 3 associated types specified in interface type list Capture A.T relationships colorMap key is same type as graph vertex S tart vertex is same type as graph vertex

  30. Evaluation: associated type access in Javareferring interfaces AT in implementing classes Public interface VertexListGraph <Vertex, VertexIterator extendIterator<Vertex>> { VertexIterator vertices(); } public interface IncidenceGraph <Vertex, Edge, OutEdgeIterator extends Iterator<Edge>>{ OutEdgeIterator out_edges(Vertex v); } public class adjacency_list implements VertexListGraph < Integer, Iterator<Integer> >, IncidenceGraph < Integer, simpleEdge<integer>, Iterator<simpleEdge<integer> > > { …} breadth_first_search.go (g, src,color_map,visitor)

  31. Evaluation: associated type access in Java,C#,Eiffel summery A.T as part of type parameters list ?! • We can access then • within generic algorithm • we must access them every time we refer a concept • Even if algorithm does not use it • lengthy code • Constraints repetition • Breaking encapsulation Expose potentially helper types encapsulate associated types !!!

  32. Motivation: express relation betwee A.T Evaluation: associated type access ML: encapsulate within structure, sharing C++: encapsulated within class, traits used to access them captures associated types relations Java,c#,eiffe: A.T explicitly specified in type parameter list of a concept

  33. Evaluation: multi type concept • Can one concept constrain several types ? • ML • Signature  • Encapsulate structures and methods • OOP: • Interface  • constrains a single class OOP concepts are not as strong as concepts ! , why ? Lets see …

  34. Evaluation: multi type conceptmulti type concept split into several interfaces • Algorithm ConceptA: { Vector<V> mult Scalar<S> => Vector<V> Scalar<S> mult Vector<V> => Vector<V> } • Oop interpretation • Vector<V> has: • Vector<V> mult (Scalar<S>) • Scalar<S> has: • Vector<V> mult (Vector<V>) two interfaces required to express ConceptA ! interfaces cant express multi types concept ! • Restrict implementing class only, not multiple classes What if conceptA is refined ? (2^n interfaces)

  35. Research Results • criterions to indicate PL generics support

  36. Questions !?

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