package org.simantics.scl.runtime.lazy;

import org.simantics.scl.runtime.function.Function;
import org.simantics.scl.runtime.function.FunctionImpl1;
import org.simantics.scl.runtime.tuple.Tuple0;

public class Lazy {
    private static final Function CYCLIC_EVALUATION_TRAP = new FunctionImpl1() {
        @Override
        public Object apply(Object p0) {
            throw new RuntimeException("Cyclic dependency on lazy value.");
        }
    };
    
    volatile Function thunk;
    volatile Object value;
    
    public Lazy(Function thunk) {
        this.thunk = thunk;
    }

    public Object force() {
        if(thunk == null)
            return value;
        else {
            synchronized(this) {
                Function curThunk = thunk;
                if(curThunk != null) {
                    thunk = CYCLIC_EVALUATION_TRAP;
                    value = curThunk.apply(Tuple0.INSTANCE);
                    thunk = null;
                }
            }
            return value;
        }
    }
}