Define ValueCastable.shape()


Require value-castable objects to have a method that returns their high-level shape.


RFC #15 advanced the extensibility of the language significantly, but broke constructs like In addition, not having a well-defined point for returning the high-level shape (i.e. a shape-castable object from which this value-castable object was creaed rather than a Shape instance) causes workarounds such as data.Layout.of to be added to the language and standard library.


The ValueCastable interface has a method .shape(). This method returns a shape-castable object. Where possibe, this object should, when passed to Signal, create a value-castable object of the same type. is removed immediately.


  • Increased API surface area
    • At one point a commitment was made that the only method ValueCastable will ever define will be as_value. However, radical changes to the language such as RFC #15 make it reasonable to revisit this.

Rationale and alternatives

This change is the minimal possible one that fixes the problem systemically. Some minor variations in the design are possible:

  • Instead of requiring shape() to be defined (which is a breaking change), this method can be added optionally in the next release and be required in the release after that.
    • This is difficult to do with ValueCastable and will require workarounds both in the core language implementation and in downstream code that operates on ValueCastable objects.
    • ValueCastable is not very widely used yet and the breakage will likely be minimal.

Prior art

It is typical for a programming language to have a way of retrieving the type of a value. The mechanism being added here is equivalent.

Unresolved questions


Future possibilities