Created by: wangkuiyi

After talking with @zchen0211 and @Canpio , here is the very early draft of the idea of IfThenElse operator and layer:

A Simple Example

The Python Application Program

cost = paddle.layer.mse(
    paddel.layer.fc(
        paddle.layer.data(name="features"),
        output_size = 100),
    paddle.layer.data(name="label"))
parameters = paddle.train(cost, minist_reader(), {"features":0, "label":1})
print paddle.inf(cost, parameters, inference_input_reader())

FC Layer as a Python Function

def fc(input):
    output = paddle.framework.tensor()
    W = paddle.framework.tensor(label="parameter")
    b = paddle.framework.tensor(label="parameter")
    paddle.operator.fc(input, W, b, output)
    return output

FC Operator as a C++ Class

class FC : public framework:Operator {...};

The IfThenElse Layer and Operator

Another Example Using IfThenElse Layer

def a_network(input):
    A = paddle.layer.fc(input, parameter_name="A")
    B = paddle.layer.fc(input, parameter_name="B")
    return paddle.layer.if_then_else(
        paddle.layer.less_than(A, B),
        paddle.layer.fc(
            paddle.layer.fc(
                paddle.layer.fc
                paddle.layer.fc(A, output_size = 100))),
        paddle.layer.fc(
            paddle.layer.fc(
                paddle.layer.fc(B, output_size = 100))))

The IfThenElse Layer

def if_then_else(cond, left, right) :
    ret = paddle.framework.tensor()
    paddle.operator.if_then_else(cond, left, right, ret)
    return ret

The IfThenElseOp C++ Class

class IfThenElseOp : public framework::Operator {
 // Check that cond returns a minibatch of boolean values.
 // Check that left and right outputs are of the same size.
};