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 class
  • ClassName, TypeInfo Name
    • Delphi, pas2js: full path with param names, e.g. 'TAnt<System.Word>' and 'TAnt<System.Word>.TLeg'
    • FPC: instead of param names uses crc, e.g. 'TAnt$1$crcHexNumber', full paths for classnames, e.g. 'TAnt$1$crcHexNumber.TLeg', last name for TypeInfo: e.g. 'TLeg'
  • JS specialize code in unit of generic
  • forward generic class, e.g. type TAnt<T>=class; TBird=class a: TAnt<word>; end; TAnt<T>=class a:T; end;
    • Delphi/pas2js: supported
    • 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, e.g. type generic TAnt<T> = class Parent: TAnt; end;
  • Delphi/FPC: descendants cannot refer to type parameters of ancestors, pas2js allows it within same unit (strict private).
  • Delphi/FPC does not allow accessing local symbols. pas2js allows it.
  • class constructor
• generic external class
• generic record
• generic dynamic 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
  • Procedure local scope:
    • pas2js: allowed, except for typeinfo
    • FPC: allowed, including typeinfo
    • Delphi: Error: parameterized type can not be declared in procedure local scope
• anonymous type in specialize: TBird<array of word>
• Delphi type overloads A, A<T> and A<S,T>
• 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>;
• 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
• 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 can refer prior templates: type TBird<X, Y: TAnt<X>> = record a: X; b: Y; end;. This is Delphi compatible. FPC 3.3.1 does not support this (23th Aug 2020).
• statements:
  • specialize (cloning) all kinds of statements and expressions
  • for-loop, if-then-else, repeat-until, while-do, try-finally, try-except, case-of
  • assignments
  • primitive expression like identifiers and constants
  • operators: +, -, *, /
  • 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 T do -> T must have constraints
  • 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)
  • parameter type inline specialization: procedure Run<T>(List: TList<T>);
• 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)
  • $modeswitch implicitfunctionspecialization: mode Delphi: default enabled, ObjFPC: disabled
  • array of T: procedure Run<T>(a: array of T)
  • any order: procedure Run<S,T>(a:T; b:S)
  • infer types: widen types, e.g. Run(1,1000000) specializes Run<T>(a,b:T) with T as longint. Not supported by Delphi/FPC, supported by pas2js to reduce number of specializations
• typecast between specializations gives a warning, not an error
• typecast TGenType<jsvalue> from/to TGenType<SomeType> without warning
• allowing ATExtClass<jsvalue> := ATExtClass<SomeType>
• function GetTypeKind(aType): TTypeKind
• RTTI
  • RTTI names use FPC/Delphi like TList<word> instead of the JS name TList$G1

RTL Units

• generics.defaults.pas
• generics.strings.pas
• generics.collections.pas

RTL Classes

• TArray<T>
• IComparer<T>
• TOnComparison<T>
• TComparer<T>
• TDefaultComparer<T>
• IEnumerator<T>
• IEnumerable<T>
• TCollectionNotifyEvent<T>
• TCustomArrayHelper<T>
• TArrayHelper<T>
• TEnumerator<T>
• TEnumerable<T>
• TCustomList<T>
• TCustomListEnumerator<T>
• TCustomInvertedListEnumerator<T>
• TList<T>
• TObjectList<T: class>
• TThreadList<T>
• TQueue<T>
• TObjectQueue<T: class>
• TStack<T>
• TObjectStack<T: class>
• TPair<TKey,TValue>
• TDictionary<TKey,TValue>

ToDos

• anonymous record, e.g. type TAnt<T> = class a: record end; end;
• anonymous set, e.g. type TAnt<T> = class a: (red, blue); end;
• type alias type as parameter: create separate specialization, at the moment TList<integer> and TList<TColor> are the same specialization in pas2js
• generic class:
  • nested generic type: i.e. generic inside a generic. Allowed by Delphi, not allowed by FPC
  • cascaded specialize: TBird<word>.TWing<byte>
  • generic ancestor, allowed in FPC, not allowed in Delphi: TExample<T: class> = class(T) end;
• nicer error messages
  • write a hint where it was specialized
• Redefinition. Example: type TBird<T> = class i: integer; end; integer = string; TEagle = TBird<integer>; FPC+pas2js: create i as string, Delphi: creates i as system.integer
• optimization: reuse specialized functions
• constants as templates
• precompiled js files. At the moment all specializations are added to the unit where the generic is defined. That means using a new specialization require to recompile the js file.

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From the Free Pascal wiki, page "pas2js Generics", as it stood on 2022-12-19 after a change by Mattias2, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.