The Chapel team is implementing a new approach to user-defined intialization of variables with record type or instances of class type. This approach relies on methods known as initializers rather than the original methods known as constructors.
A discussion of the current design and rationale is provided in CHIP 10.
Release Chapel 1.15.0 provides a strong preview implementation of this new feature. This initial implementation is not of production quality and has not been heavily tested, but the basic features are working and it is believed that adventurous developers will have success using init methods in new code.
It is anticipated that the implementation will continue to advance steadily after this release and that many aspects of the internal implementation of Chapel will transition from constructor methods to initializer methods during this release.
The functionality of an initializer method has four major aspects
- user-defined code that runs in phase 2
- user-defined code that runs in phase 1
- compiler generated code that runs in phase 1
- the generation of effective user-facing diagnostic messages
in approximate order of robustness and maturity.
The user-defined code that executes in phase 2 of an initializer is comparable to the user-defined code in any method and is likely to behave in the expected way.
The user-defined code that executes in phase 1 of an initializer has many of the same properties and is also expected to behave well. However there are more restrictions on user-defined code for phase 1 and the compiler may fail to apply these restrictions correctly. The most likely error is that the compiler will accept phase 1 code that it should reject but it is possible that there will be situations in which valid code is rejected.
The design for initializer methods attempts to define predictable default behaviors within phase 1 and then rely on the compiler to insert these defaults when appropriate. The intent is to reduce the amount of code that the type designer is responsible for. This includes the generation of code using field-level default declarations. Defining and implementing these default behaviors relies on relatively less mature code within the compiler.
We believe that initializer methods are ready for use in application code but we also expect that the implementation will mature significantly during the 1.15 release. We encourage their use but recognize that they may prove to be less robust than we would like at the start of the release. If you do choose to experiment with this feature then please do not hesitate to seek help, report bugs, or offer suggestions.
Generic records and classes, i.e. records or classes in which the definition of the type of one or more fields is parameterized are less well developed than records and classes for which the type of every fields can be inferred directly from the type declaration.
As of the 1.15.0 release, support for initializers includes the following:
- Methods named "init" on a type are utilized during the creation of an instance of that type.
- An error message is generated if both a constructor and an initializer is defined on a type.
- User-defined initializers whose body consists of only Phase 2 statements appear to be fully supported
- User-defined initializers with Phase 1 statements that explicitly initialize
all of the fields appear to be well supported
- Some errors remain with omitted field initialization in Phase 1, notably
paramfields in the omitted initialization of another field, and with utilizing some record fields.
- Some errors remain with omitted field initialization in Phase 1, notably with utilizing
- Many checks against Phase 1 and Phase 2 rules are implemented, including:
- Forbidding the update of
constfields during Phase 2
- Forbidding the initialization of fields in parallel statements during Phase 1
- Forbidding the initialization of fields in loops during Phase 1
this.init()calls in parallel statements
this.init()calls in loops
- Forbidding access to a field prior to its initialization in Phase 1
- Forbidding the initialization of fields prior to
- Limiting the access of parent fields during Phase 1 of initialization
- Forbidding the update of
- Support for initializers on generic classes when instances infer their type
from the result of a
newcall or another instance.
- As a result, generic fields no longer require an argument sharing their name in the initializer argument list.
- Support for copy initializers
- Support for deinitializers, as a replacement of destructors.
- Library types were converted to use deinitializers.
Remaining todo items include:
- Support for compiler generated initializers
- Improve the error message for out of order field initialization during Phase 1
- Forbid the utilization of methods on the type during Phase 1 of
initialization, as well as the use of
thisas an argument to a function.
- Full support for
- Resolve an issue with secondary initializers
- Support for instance declarations of generic instantiations, i.e.
var x: Foo(t);
- Resolve an issue with generic initializers and records (relates to records
being passed by
refto the initializer method)
- Convert library types to utilize initializers instead of constructors