cache.groovy

@Grab(group='org.codehaus.groovy.modules.http-builder', module='http-builder', version='0.7' )
abstract class Cache {
    Cache parent
    Closure loader
    
    Cache(Closure loader) {
        this.loader = loader
    }
    
    Cache(Cache parent) {
        this.parent = parent
    }
    
    abstract getHere(key)
    abstract putHere(key, value)
    
    def get(key) {
        def value = getHere(key)
        if (!value) {
            if (parent) {
                value = parent.get(key)
            } else {
                value = loader(key)
            }
            putHere(key, value)
        } 
        return value;
    }
}

loader = { it -> 
    println("Computing for $it")
    return it * 2
}

level2 = new Cache(loader) {
    private def map = [:]
    def getHere(key) {
        return map[key]
    }
    def putHere(key, value) {
        println("level 2 stores $key => $value")
        map[key] = value
    }
}

level1 = new Cache(level2) {
    private def map = [:]
    def getHere(key) {
        return map[key]
    }
    def putHere(key, value) {
        println("level 1 stores $key => $value")
        map[key] = value
    }
}

assert level1.get(2) == 4
assert level1.get(2) == 4