Abstract type: Difference between revisions
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You can declare a virtual function to not have an implementation by using <code>F.virtual.abstract</code> keyword. A type containing at least one abstract virtual function cannot be instantiated.
<syntaxhighlight lang="11l">T AbstractQueue
F.virtual.abstract enqueue(Int item) ->
T PrintQueue(AbstractQueue)
F.virtual.assign enqueue(Int item) ->
print(item)</syntaxhighlight>
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Then say we have a structure ListQueue which implements queues as lists. If we write <tt>ListQueue :> QUEUE</tt>
then the queue type will be abstract, but if we write <tt>ListQueue : QUEUE</tt> or <tt>ListQueue : LIST_QUEUE</tt> it won't.
=={{header|Swift}}==
Swift uses Protocols to provide abstract type features. See [https://docs.swift.org/swift-book/documentation/the-swift-programming-language/protocols/ the docs]
A trivial example showing required properties and methods, and the means of providing a default implementation.
<syntaxhighlight lang="sml">
protocol Pet {
var name: String { get set }
var favouriteToy: String { get set }
func feed() -> Bool
func stroke() -> Void
}
extension Pet {
// Default implementation must be in an extension, not in the declaration above
func stroke() {
print("default purr")
}
}
struct Dog: Pet {
var name: String
var favouriteToy: String
// Required implementation
func feed() -> Bool {
print("more please")
return false
}
// If this were not implemented, the default from the extension above
// would be called.
func stroke() {
print("roll over")
}
}
</syntaxhighlight>
=={{header|Tcl}}==
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The Go example, when rewritten in Wren, looks like this.
<syntaxhighlight lang="
class Beast{
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