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Review response for 3961 #3978

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Jul 12, 2024
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Review response for 3961 #3978

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55 changes: 42 additions & 13 deletions working/2364 - primary constructors/feature-specification.md
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Expand Up @@ -490,15 +490,17 @@ default value.
If there are any assertions following the formal parameter list _L_ then
_k_ has an initializer list with the same assertions in the same order.

The current scope of the assertions in _D_ is the formal parameter list
scope of the enclosing class *(that is, they can see the parameters, the
type parameters, and the library scope)*.

The expressions in the assertions are subject to a transformation similar
to the one which is used with default values, such that the meaning of the
expression is the same when it occurs in the class header and when it
occurs in the initializer list of _k_ *(in particular, an identifier in an
assertion expression cannot resolve to a declaration in the class body)*.
The current scope of the assertions in _D_ is the formal parameter
initializer scope of the formal parameter list *(that is, they can see the
parameters including any initializing formals, the type parameters, and
everything in the library scope that isn't shadowed by the scopes in
between)*.

The expressions in the assertions are subject to a transformation that
preserves the resolution of every identifier in said expressions when they
occur as part of the initializer list of _k_. *(In particular, an
identifier in an assertion expression cannot resolve to a declaration in
the class body)*.

Finally, _k_ is added to _D2_, and _D_ is replaced by _D2_.

Expand Down Expand Up @@ -559,6 +561,20 @@ constructor using `D.named`, and that would fail if we use the approach
where it occurs as `new.named` or `const.named` because that particular
constructor has been expressed as a primary constructor.

It has been argued that a primary constructor parameter should be able to
introduce an instance variable (which is already true in this proposal),
and also to be a regular parameter (that doesn't introduce anything extra,
which is not supported by this proposal). The point would be that this
makes primary constructors more expressive. In particular, if we also
generalize the proposal to allow a full initializer list with a
superinitialization then we could allow the primary constructor to invoke
any superconstructor, with any actual argument list.

This would provide a considerable enhancement of the expressive power of
primary constructors. At the same time, it would make primary constructors
considerably more verbose, which seems to lessen a crucial motivating
factor for primary constructors in the first place.

A proposal which was mentioned during the discussions about primary
constructors was that the keyword `final` could be used in order to specify
that all instance variables introduced by the primary constructor are
Expand All @@ -582,10 +598,23 @@ class Point {
class final Point(int x, int y); // Not supported!
```

Most likely, there is an easy workaround: Make the constructor `const`. It
is very often possible to make the constructor `const`, even in the case
where the class isn't necessarily intended to be used in constant
expressions: There is no initializer list, no superinitialization, no
body. The only way it can be an error to use `const` on a primary
constructor is if the superclass doesn't have a constant constructor, or if
the class has a mutable or late instance variable, or it has some
non-constant expressions in instance variable declarations. (Those issues
can only be created by instance variables that are declared explicitly in
the class body whereas the ones that are created by primary constructor
parameters will necessarily satisfy the `const` requirements).

Finally, we could allow a primary constructor to be declared in the body of
a class or similar declaration, using the modifier `primary`, in which case
it could have an initializer list and a body, and it would still have the
ability to introduce instance variable declarations implicitly:
a class or similar declaration, possibly using a modifier like `primary`,
in which case it could have an initializer list and a body, and it would
still have the ability to introduce instance variable declarations
implicitly:

```dart
// Current syntax.
Expand Down Expand Up @@ -676,7 +705,7 @@ where it is a compile-time error to not have them, but that is a

### Changelog

1.3 - July 4, 2024
1.3 - July 12, 2024

* Add support for assertions in the primary constructor. Add support for
inferring the declared type of an optional parameter based on its default
Expand Down