Constant definition allows you to define an array as a constant

bgrommes

You can define a constant of type Array(Of String) which I believe is intended to be illegal. The default value remains zero. You can set the default to ["Foo","Bar","Whatever"] and it all looks great but then when you try to add another constant the new MY_CONSTANT replaces what you just defined.

If constants are to be constrained to scalars then attempts by the user to define a non-scalar type should "fail fast" with an informative error message, perhaps suggesting a read-only property as an alternative.

Garry

Thanks for reporting this. Arrays are intentionally allowed as constants in Objo. Const prevents the name from being assigned a different value; it does not restrict the value to a scalar or make an array’s contents immutable.

The apparent replacement is a Studio bug. Studio was only recognising single-word constant types, so a type such as Array(OfArray(Of String) stopped appearing in the Inspector when the source was reparsed—for example, after adding another constant. The underlying source remained, but the constant disappeared from the structured editor.

This is now fixed for generic, nested-generic, and qualified type names and will be included in the next release:

https://feedback.objo.dev/bug/1381

bgrommes

Garry Thanks, Garry.

This makes me wonder -- is there any practical difference between a constant and a read-only property? Why would I use one vs the other? Both have fixed design-time values, do they not? Or is it legal to define the value of a read-only property with a method call, or to set it from a constructor?

ETA: I see that now Analyser is reporting "Constant VALID_NATIVE_TYPES is already declared in this scope". Search finds two declarations on adjacent lines but will not navigate to either. This is 2 copies of the "disappearing constant" described in my original post. I assume this will fix itself in the next release -- or do I need to hand edit it out of the code?

Garry

Good questions. Const and Readonly Property both prevent reassignment, but they have different lifetimes and uses.

A class or module Const is shared: there is one value rather than one per instance. Its initializer may be a runtime expression, including a method call. With reference types such as arrays, the reference cannot be replaced, but the referenced object can still be modified.

A Readonly Property is instance state by default, so every instance receives its own value. Its initialiser must be a compile-time-safe default, and the property cannot subsequently be assigned, even from a constructor. Shared Readonly Property provides one class-wide value instead.

For a value calculated by a method or constructor, use a get-only computed property, optionally backed by a private writable property:

Class Example
    Private Property StoredValue As String

    Constructor()
        Self.StoredValue = MakeValue()
    End Constructor

    Property Value As String
        Get
            Return Self.StoredValue
        End Get
    End Property
End Class

Regarding the duplicate VALID_NATIVE_TYPES declarations: the Analyser is correct; there are now two physical declarations in the source. The next release will recognise and display them correctly, but it will not remove either automatically. You can either delete one manually now or wait for the update and delete one through the Inspector. The failure to navigate from Search is another consequence of the current Studio parser not recognising that generic constant declaration; that should also be resolved by the fix.