32 lines
1.2 KiB
32 lines
1.2 KiB
### Design Doc: Switch
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### Background
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Many programming languages provide `switch` as a generalization of `if-elif-else`. We want to add it to Fluid.
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The following example shows the usage of `fluid.switch`.
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```python
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a = fluid.Var(10)
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b = fluid.Var(0)
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with switch() as switch:
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with switch.case(fluid.less_equal(a, 10)):
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fluid.print("Case 1")
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with switch.case(fluid.larger(a, 0)):
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fluid.print("Case 2")
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with switch.default():
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fluid.print("Case 3")
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```
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### The Semantics
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1. A `switch` control-flow checks cases one-by-one.
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1. 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.
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1. It runs the first matched case, or the default case if there is one.
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1. 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.
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The above program should print and print only "Case 1".
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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.
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