# Design Doc: Switch

## Background

Many programming languages provide switch as a generalization of if-elif-else. We want to add it to Fluid.

The following example shows the usage of fluid.switch.

a = fluid.Var(10)
b = fluid.Var(0)

with switch() as switch:
with switch.case(fluid.less_equal(a, 10)):
fluid.print("Case 1")
with switch.case(fluid.larger(a, 0)):
fluid.print("Case 2")
with switch.default():
fluid.print("Case 3")


## The Semantics

1. A switch control-flow checks cases one-by-one.
2. The condition of each case is a boolean value, which is a scalar, and differs from the fluid.if_else control-flow, which condition could be a vector of boolean values.
3. It runs the first matched case, or the default case if there is one.
4. Once it matches a case, it runs the corresponding branch and only that branch. It's like there is a C's break keyword at the end of each case.

The above program should print and print only "Case 1".

The implementation of the backward pass of the switch control-flow is easier than the backward of the if_else, because switch runs at most one branch, whereas if-else could run more than one branches.