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Delegation |
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The pattern seen earlier
match [verb, args] {
E.call(target-expression, verb, args)
}
may often be replaced by the more convenient, which has a similar meaning.
extends super-expression
placed in the head of the object definition. There are two differences between the two:
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Whereas the target-expression is evaluated on each delegation, the super-expression gets evaluated once at the time the object is evaluated / instantiated. This means that the match / target-expression form is more flexible -- it can determine dynamically where the message will be delegated. Such flexibility is rarely needed, but when it is, the match / target-expression form should be used.
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The value of the super-expression is bound to the special variable name "super" within the outer scope box.
***Need to show expansion
***Need examples
Here is another alternative category implementation that uses extends to delegate all messages it doesn't understand to the wrapped predicate.
? pragma.syntax("0.8") ? def makeCategory(pred) :any { > def category extends pred { > to and(other) :any { > makeCategory(def _(specimen) :any { > pred(specimen) && other(specimen) > }) > } > > # ... similarly for "or" and "xor" > > to not(other) :any { > makeCategory(def _(specimen) :any { > !(pred(specimen)) > }) > } > } > } # value: <makeCategory>
Which creates the same behaviors as before:
? def evenPred { > to (num) :any {num %% 2 == 0} > to foo() :any {"bar"} > } # value: <evenPred> ? def even := makeCategory(evenPred) # value: <category> ? even(3) # value: false ? even.foo() # value: "bar"
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