diff --git a/working/1426-extension-types/feature-specification-views.md b/working/1426-extension-types/feature-specification-views.md index 104efd5622..3bf9da6a7e 100644 --- a/working/1426-extension-types/feature-specification-views.md +++ b/working/1426-extension-types/feature-specification-views.md @@ -7,7 +7,7 @@ Status: Draft ## Change Log -2021.05.06 +2021.05.12 - Initial version. @@ -30,15 +30,14 @@ any). Hence, using a supertype as the static type allows us to see only a subset of the members, but using a view type allows us to _replace_ the set of members, with subsetting as a special case. -The functionality is entirely static. View types rely on the resolution and -scoping mechanisms of the extension methods feature which was added to Dart -in version 2.6. In particular, the semantics of extension method invocation -is also the semantics of invocations of a member of a view type (but the -rules about which call sites will invoke an extension member respectively a -view member are very different). This is important because it implies that -the language Dart has a single and consistent semantics for all statically -resolved member invocations, rather than having one set of rules for -extension methods, and a subtly different set of rules for view members. +The functionality is entirely static. Invocation of a view member is +resolved at compile-time, based on the static type of the receiver. Inside +the view declaration, the scoping and the type and meaning of `this` is the +same as for extension methods (a feature which was added to Dart in version +2.6). This is important because it implies that the language Dart has a +single and consistent semantics for all statically resolved member +invocations, rather than having one set of rules for extension methods, and +a different set of rules for view members. ## Motivation @@ -210,7 +209,7 @@ downcast, and hence it must be written explicitly. This cast corresponds to "exiting" the view type (allowing for violations of the discipline associated with `V`), and the fact that the cast must be written explicitly helps developers maintaining the discipline as intended, rather than -dropping out of it by accident, silently. +dropping out of the view type by accident, silently. ```dart view ListSize on List { @@ -236,8 +235,8 @@ void main() { ## Syntax -The rule for `Declaration>` in the grammar is replaced by the -following: +A rule for `` is added to the grammar, along with some +rules for elements used in view declarations: ```ebnf ::= @@ -277,7 +276,7 @@ following: ::= ``` -The token `view` becomes a built-in identifier. +The token `view` is made a built-in identifier. *In the rule ``, note that `` derives ``, which makes `` nearly redundant. However, @@ -286,41 +285,90 @@ be the name of a type but can be the basename of a member, e.g., the built-in identifiers.* -## Static Analysis +## Primitives + +This document needs to refer to explicit view method invocations, so we +will add a special primitive, `invokeViewMethod`, to denote invocations of +view methods. + +`invokeViewMethod` is used as a specification device and it cannot occur in +Dart source code. (*As a reminder of this fact, it uses syntax which is not +derivable in the Dart grammar.*) + + +### Static Analysis of invokeViewMethod + +We use +invokeViewMethod(V, 1, .. Tk>, o).m(args) +where `V` is a view to denote the invocation of the view method `m` on `o` +with arguments `args` and view type arguments +T1, .. Tk. +Similar +constructs exist for invocation of getters, setters, and operators. + +*For instance, `invokeViewMethod(V, , o).myGetter` and +`invokeViewMethod(V, , o) + rightOperand`.* + +*We need special syntax because there is no syntax which will unambiguously +denote a view member invocation. We could consider the syntax of explicit +extension member invocations, e.g., +V1, .. Tk>(o).m(args), +but this is ambiguous since +V1, .. Tk>(o) +can be a view constructor invocation. Similarly, +V1, .. Tk>.m(o, args) +quite well, but that is also confusing because it looks like actual source +code, but it couldn't be used in an actual program.* + +*Let us compare view methods to extension methods, noting that they are +similar in many ways. With an extension declaration `E`, +E1, .. Tk>(o).m(args) +denotes an explicit invocation of the extension member +named `m` declared by the extension `E`, with `o` bound to `this`, the type +parameters bound to T1, .. Tk, +and value parameters bound to the values of `args`. If `V` is a view with +the same declaration of a member `m`, +invokeViewMethod(V, 1, .. Tk>, o).m(args) +denotes an invocation of the view method `m` with the same bindings.* + +The static analysis of `invokeViewMethod` is that it takes exactly three +positional arguments and must be the receiver in a member access. The first +argument must be a name that denotes an extension, the next argument must +be a type argument list, together yielding a view type _V_. The third +argument must be an expression whose static type is _V_ or the +corresponding instantiated on-type (defined below). The member access must +be a member of `V`. -This document needs to refer to explicit view method invocations. We use -`invokeViewMethod(V, o).m(args)` where `V` is a view to denote -the invocation of the view method `m` on `o` with arguments `args`. Note -that `invokeViewMethod` is used as a specification device, it cannot occur -in Dart source code. +If the member access is a method invocation (including an invocation of an +operator that takes at least one argument), it is allowed to pass an actual +argument list, and the static analysis of the actual arguments proceeds as +with other function calls, using a signature where the formal type +parameters of `V` are replaced by +T1, .. Tk. +The type of the entire member access is the return type of said member if +it is a member invocation, and the function type of the method if it is a +view member tear-off, again substituting +T1, .. Tk +for the formal type parameters. -*This is needed because there is no syntax which will unambiguously denote -a view method invocation. We could consider `V(o).m(args)`, but -this is ambiguous since `V(o)` can be a view constructor -invocation.* -*Let us compare to extension methods, given that they are similar in many -ways. With an extension declaration `E`, `E(o).m(args)` denotes -an explicit invocation of the extension member named `m` declared by the -extension `E`, with `o` bound to `this`, the type parameters bound to `T1, -.. Tk`, and value parameters bound to the values of `args`. -If `V` is a view with the same declaration of a member `m`, -`invokeViewMethod(V, o).m(args)` denotes an invocation of -the view method `m` with the same bindings.* +### Dynamic Semantics of invokeViewMethod + +Let `e0` be an expression of the form +invokeViewMethod(View1, .. Sk>, e).m(args) +Evaluation of `e0` proceeds by evaluating `e` to an object `o` and +evaluating `args` to an actual argument list, and then executing +the body of `View.m` in an environment where `this` is bound to `o`, +the type variables of `View` are bound to the actual values of +S1, .. Sk, +and the formal parameters of `m` are bound to `args` in the same way +as they would be bound for a normal function call. If the body completes +returning an object `o2`, then `e0` completes with the object `o2`; if the +body throws then the evaluation of `e0` throws the same object with the +same stack trace. -The static analysis of `invokeViewMethod` is that it takes exactly two -positional arguments and must be the receiver in a member access. The first -argument must be a ``, denoting a view type _V_, and the second -argument must be an expression whose static type is _V_ or the -corresponding instantiated on-type. The member access must be a member of -`V`. If the member access is a method invocation (including an invocation -of an operator that takes at least one argument), it is allowed to pass an -actual argument list, and the static analysis of the actual arguments -proceeds as with other function calls, using a signature where the formal -type parameters of `V` are replaced by `T1, .. Tk`. The type of the entire -member access is the return type of said member if it is a member -invocation, and the function type of the method if it is a view member -tear-off, again substituting `T1, .. Tk` for the formal type parameters. + +## Static Analysis of Views Assume that _V_ is a view declaration of the following form: @@ -365,21 +413,21 @@ class or mixin, or if a view type is used to derive a mixin. has two errors.* If `e` is an expression whose static type is the view type -Ext1, .. Sk> +View1, .. Sk> and the basename of `m` is the basename of a member declared by `V`, then a member access like `e.m(args)` is treated as -invokeViewMethod(Ext1, .. Sk>, e).m(args), +invokeViewMethod(View1, .. Sk>, e).m(args), and similarly for instance getters and operators. Lexical lookup for identifier references and unqualified function invocations in the body of a view declaration work the same as the same lookup in an extension declaration with the same type parameters and on-type and members: -In the body of a view declaration `V` with type parameters +In the body of a view declaration `V` with name `View` and type parameters X1, .. Xk, for an invocation like -`m(args)`, if a declaration named `m` is found in the body of `V` +`m(args)`, if a declaration named `m` is found in the body of `V` then that invocation is treated as -invokeViewMethod(Ext1, .. Xk>, this).m(args). +invokeViewMethod(View1, .. Xk>, this).m(args). If there is no declaration in scope whose basename is the basename of `m`, `m(args)` is treated as `this.m(args)`. *See a later section for the lookup rule when an `extends` clause is present.* @@ -401,7 +449,7 @@ void qux() { print('qux'); } view V2 on V1 { void foo() { print('V2.foo); } - void bar() { + void bar() { foo(); // Prints 'V2.foo'. this.foo(); // Prints 'V1.foo'. 1.foo(); // Prints 'E1.foo'. @@ -424,7 +472,7 @@ later section).* Let `V` be a view declaration named `View` with type parameters X1 extends B1, .. Xk extends Bk and on-type clause `on T`. Then we say that the _declared on-type_ of `View` -is `T`, and the _instantiated on-type_ of +is `T`, and the _instantiated on-type_ corresponding to View1, .. Sk> is [S1/X1, .. Sk/Xk]T. @@ -454,10 +502,10 @@ type promotion. In the body of a member of a view `V`, the static type of `this` is the on-type of `V`. -*Compared to the extension methods feature, there is no change to the type -of `this` in the body of a view type _V_. Similarly, members of _V_ invoked -in the body of _V_ are subject to the same treatment as members of an -extension, which means that view members of the enclosing view can be +*Compared to the extension methods feature, there is no difference wrt the +type of `this` in the body of a view type _V_. Similarly, members of _V_ +invoked in the body of _V_ are subject to the same treatment as members of +an extension, which means that view members of the enclosing view can be invoked implicitly, and view members are given higher priority than instance methods on `this`, when `this` is implicit.* @@ -659,7 +707,9 @@ This section describes the implicitly induced `box` getter of a view type. *It may be helpful to equip each view with a companion class whose instances have a single field holding an instance of the on-type. So it's a -wrapper with the same interface as the view type.* +wrapper with the same interface as the view type, except that the view type +may have an implicitly induced getter named `box` and the companion class +may have an implicitly induced getter named `unbox`.* Let `V` be a view type. The declaration of `V` implicitly induces a declaration of a class `V.class`, with the same type parameters and members @@ -706,11 +756,13 @@ _the view extends clause_ to refer to this clause, or just _the extends clause_ when no ambiguity can arise. *The rationale is that the set of members and member implementations of a -given view type may need to overlap with that of other view types. The -extends clause allows for implementation reuse by putting shared members in -a "super-view" `V0` and putting `V0` in the extends clause of several -view declarations `V1 .. Vk`, thus "inheriting" the members of -`V0` into all of `V1 .. Vk` without code duplication.* +given view may need to overlap with that of other views. The extends clause +allows for implementation reuse by putting shared members in a "super-view" +`V0` and putting `V0` in the extends clause of several view declarations +V1 .. Vk, +thus "inheriting" the members of `V0` into all of +V1 .. Vk +without code duplication.* *Note that there is no subtype relationship between `V0` and `Vj` in this scenario, only code reuse. This also implies that there is no need to @@ -732,11 +784,11 @@ arguments*). Assume that `S` is the instantiated on-type corresponding to *This ensures that it is sound to bind the value of `this` in `V` to `this` in `V0` when invoking members of `V0`.* -Consider an `` of the form `V0 -`. The _associated members_ of said extends element -are computed from the instance members of `V0` in the same way as we -compute the included instance members of the on-type using the -`` that follows the on-type in the declaration. +Consider a `` of the form `V0 `. The +_associated members_ of said extends element are computed from the instance +members of `V0` in the same way as we compute the included instance members +of the on-type using the `` that follows the on-type in +the declaration. Assume that `V` is a view declaration and that the view type `V0` is a superview of `V`. Let `m` be the name of an associated member of `V0`. A @@ -778,6 +830,11 @@ basename of `n`, the lexical lookup yields nothing (*which implies that `this.` will be prepended to the expression, following the existing rules*). +In the body of `V`, the syntax of an explicit extension method invocation +can be used to invoke a member of a superview which is hidden. *For +instance, `V3(this).foo();` can be used to call the `foo` of `V3` in the +case where the extends clause has `extends ... V3 hide foo, ...`.* + *This means that the declarations that occur in the enclosing syntax, i.e., in an enclosing lexical scope, get the highest priority, as always in Dart. Those declarations may be top-level declarations, or they may be @@ -790,27 +847,17 @@ an extension `E1`, as long as the type of `this` matches the on-type of `E1`.* -## Dynamic Semantics +## Dynamic Semantics of Views The dynamic semantics of view member invocation follows from the code -transformation specified in the section about the static analysis. +transformation specified in the section about the static analysis: -*So, if `e` is an expression whose static type `V` is the view type -View1, .. Sk>, -then a member access like `e.m(args)` is executed as -invokeViewMethod(View1, .. Sk>, e).m(args) +Let `e` be an expression whose static type `T` is the view type +View1, .. Sk>. +A member access like `e.m(args)` is then executed as +invokeViewMethod(View, 1, .. Sk>, e).m(args), and similarly for instance getters and operators.* -Let `e0` be this `invokeViewMethod` expression. The semantics of `e0` -is that `e` is evaluated to an object `o`, the argument list denoted by -`(args)` is evaluated to an actual argument list value `(o1, .. ok, x1: -ok+1, .. xn: ok+n)`, and then the body of `E.m` is executed in an -environment where `this` is bound to `o`, the type variables `X1, .. Xk` -are bound to the actual values of `S1, .. Sk`, and the formal parameters -are bound to the actual arguments. If the body completes returning an -object `o2`, then `e0` completes with the object `o2`; if the body -throws then `e0` throws the same object and stack trace. - The dynamic semantics of an invocation of an instance method of the on-type which is enabled in a view type by the show/hide part is as if a forwarder were implicitly induced in the view, with the same signature as that of the