pas2js Generics

Overview

Pas2js will support FPC and Delphi like generics.

Working

• generic record
• specialize:
  • ObjFPC: type TFoo = specialize TGen<word>;
  • Delphi: type TFoo = TGen<word>;
  • ObjFPC anonymous: var r: specialize TBird<word>;
  • Delphi: anonymous var r: TBird<word>;
• generic class
  • specialize in ancestor type
  • forward generic class (FPC 3.3.1 does not support it yet)
  • enumtype inside generic is not propagated
• generic external class
• generic array
• generic interface
• generic procedural type
• Delphi type overload, e.g. type TArr = array of word; TArr<T> = array of T; TArr<T,S> = record i:T; j:S end;
• anonymous type in specialize: TBird<array of word>
• record/class field
• property
• method (not confused with generic method)
  • Delphi: implementation must repeat type params, must omit constraints
  • FPC: implementation must not repeat type params
• for-loop, if-then-else, repeat-until, while-do, try-finally, try-except, case-of
• some assignments
• primitive expression like identifiers and constants
• operators: +, -
• constraints:
  • keyword record, keyword class checked at specialization
  • class type checked at specialization
• constraints:
  • "class", "record": test if param fits
  • class type: test if param fits
  • list of interface types: test if param fits
  • forward class must repeat constraints
• statements:
  • specialize (cloning) all kinds of statements and expressions
  • inline specialize expression, e.g. TList<word>.Create
    • Delphi: TFoo<Integer>.Create
    • FPC: specialize TFoo<Integer>.Create

ToDos

• cascaded specialize: TBird<word>.TWing<byte>
• nested specialize: TBird<TWing<byte>>
• generic class:
  • default classname "TTest<System.Longint>"
  • pas2js classname "TTest$G1" and overloads
  • descendants cannot refer to type parameters of ancestors
  • members can refer to itself without parameters
  • nested generic: allowed by Delphi, not allowed by FPC
• generic static array: Note: delphi wiki says "no static arrays", but 10.3 compiles it, see http://docwiki.embarcadero.com/RADStudio/Rio/en/Declaring_Generics
• generic function (aka parametrized method):
  • Delphi does not allow global generic functions, only generic methods
  • methods: virtual, message, constructor and destructors cannot have type parameters
• constraints:
  • "constructor" (class+default constructor, can be combined with class)
  • one class type (Delphi: not TObject)
• specialize method:
  • explicitly specifying Fly<word>(aWord)
  • automatic inferring types Fly(aWord), $modeswitch implicitfunctionspecialization MyProc(1) calls MyProc<T>(a: T)
• specialized proc types can assign normal proc
• Statements:
  • for-in, enumerators
  • inline specialization
  • T is TFoo<Integer>
  • T as TFoo<Integer>
  • TFoo<Integer>(expr)
  • "obj is T": allowed with constraint "class" or class type
  • typecast T(): allowed with constraint "class" or class type
  • typeinfo(T)
  • using implementation function in generic function:
    • Delphi: Error: Method of parameterized type declared in interface section must not use local symbol '%s'
    • FPC: Error: Global Generic template references static symtable
  • anInt:=GenericVar
    • Delphi: always Error: Incompatible types: 'Integer' and 'T'
    • FPC: allowed for valid combinations -> operators are checked on specialization, Note: because of implementation cross uses, check must be delayed until used unit implementation is complete
  • call
    • Delphi: must fit the constraint. For example without constraint it only fits untyped args.
    • FPC: if only one function in scope preselect it, overloads are selected by constraints alone. Later checked by specialization.