pas2js Generics

From Lazarus wiki

Overview

Pas2js 1.5 supports generic types and functions in either the ObjFPC or Delphi like way.

Next goal is to test and fix bugs.

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
    • ObjFPC: members can refer to parent type without type parameters
    • Delphi: descendants cannot refer to type parameters of ancestors, pas2js allows it.
    • Delphi/FPC does not allows accessing local symbols. pas2js allows it.
    • class constructor
  • generic external class
  • generic array
  • 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 interface
  • generic procedural type
    • specialized proc types can assign normal proc
  • anonymous type in specialize: TBird<array of word>
  • nested specialize: TBird<TWing<byte>>
  • 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
  • assignments
  • primitive expression like identifiers and constants
  • operators: +, -, *, /
  • helper for generic type
  • constraints:
    • keyword record, class, constructor checked at specialization
    • class type checked at specialization
  • constraints:
    • "class":
      • Delphi/FPC: T is TObject
      • pas2js: test if T is a class, either TObject or an external class
    • "constructor": test if T is TObject and Create resolves to TObject.create
    • "record": test if T is a record type
    • class type: test if param fits
    • list of interface types: test if param fits
    • forward class must repeat constraints
    • Delphi does not allow TObject as constraint. pas2js allows it, because it has other root classes.
    • constraint refers to prior template name: type TBird<X, Y: TAnt<X>> = record a: X; b: Y; end;
  • 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
    • "obj is T": allowed with constraint "class" or class type
    • typecast T(): allowed with constraint "class" or class type
    • 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
      • Pas2js: allowed
    • anInt:=GenericVar
      • Delphi: always Error: Incompatible types: 'Integer' and 'T'
      • FPC/pas2js: allowed for valid combinations -> operators are checked on specialization, Note: because of implementation cross uses, check is delayed until used unit implementation is complete
    • for-in TWithConstraint
    • typeinfo(T)
    • T is TFoo<Integer>
    • T as TFoo<Integer>
    • TFoo<Integer>(expr)
    • call
      • pas2js: Later checked by specialization.
      • Delphi: must fit the constraint. For example without constraint it only fits untyped args.
      • FPC: if there is only one function in scope select it, overloads are selected by constraints alone. Later checked by specialization.
  • generic function (aka parametrized method):
    • ObjFPC: generic procedure Fly<T>(a: T)
    • Delphi: procedure Fly<T>(a: T) Note: Delphi 10.3 only supports parametrized method, not global procedure, pas2js allows it
    • forward
    • unit interface/implementation
    • overloads
    • generic procedure inside a procedure is forbidden, same as FPC/Delphi
    • local procedures inside a generic function: supported by FPC and pas2js, not supported by Delphi
    • calling self
    • constraints, see above, same as type constraints
    • ObjFPC explicitly spezialize: specialize Fly<word>(aWord)
    • Delphi explicitly spezialize: Fly<word>(aWord)
  • generic method (aka parametrized method):
    • methods: virtual, message, constructor and destructors cannot have type parameters
    • class interface methods cannot have type parameters
    • proc types
    • overloads
  • automatically inferring types of generic method/function:
    • MyProc(1) calls MyProc<T>(a: T)
    • $modeswitch implicitfunctionspecialization: mode Delphi: default enabled, ObjFPC: disabled
    • array of T: procedure Run<T>(a: array of T)
    • any order: procedure Run<S,T>(a:T; b:S)

ToDos

  • nested generic type: allowed by Delphi, not allowed by FPC, not planned for 1.5
  • cascaded specialize: TBird<word>.TWing<byte>, not planned for 1.5
  • generic class:
    • Delphi type overloads A, A<T> and A<S,T>, not planned for 1.5
    • Delphi/FPC classname "TTest<System.Longint>", pas2js uses shorter classname "TTest$G1"
  • generic method (aka parametrized method):
    • infer types: widen types, e.g. Run(1,1000000) specializes Run<T>(a,b:T) with T as longint. Not supported by Delphi/FPC, supported by pas2js to reduce number of specializations
  • nicer error messages
    • instead of TList$G1 write TList<word>
    • write a hint where it was specialized
  • filer
  • optimization: reuse specialized functions, not planned for 1.5
  • constants as templates: Not planned