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https://github.com/jbranchaud/til
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40 lines
1.3 KiB
Markdown
40 lines
1.3 KiB
Markdown
# Defining Variants With Constructor Arguments
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In [Helping The Compiler Help Us With
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Variants](https://github.com/jbranchaud/til/blob/master/reason/helping-the-compiler-help-us-with-variants.md),
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I introduced the concept of variants with a basic example of how to define
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and use one. The fun doesn't stop there.
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We can take variants a step further by defining them with constructor
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arguments.
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```reason
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type listActions =
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| Length
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| Nth(int);
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```
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The second variant is defined such that it is paired with some extra data --
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a single `int` argument.
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Here is how we use that variant in our code:
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```reason
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let performListAction = (l: list(int), action: listActions) => {
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switch(action) {
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| Length => List.length(l)
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| Nth(n) => List.nth(l, n)
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}
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};
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performListAction([7,8,9], Nth(1)); /* 8 */
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performListAction([1,2,3], Length); /* 3 */
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```
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Our switch statement not only matches on that variant, but it makes the
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`int` argument available as a value we can consume in that step of the
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switch.
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[source
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code](https://reasonml.github.io/en/try.html?reason=C4TwDgpgBANglgZ2AQQMbDgewHYKgXgCgooAfKAGQmwHNgALYsqAOQYAo5tgBKAbkKEYEYFEgAnAGaZxAWwqIU6LNgJR2MAFyxFnbjwA0UAIbKc2+EjQYcCHgQB8UAN5MEAdzjBU9dqZvY9q4k5FS0DI6UigB0wuG+MDxM5Gy+gZEKSNHcCUaBTAC+hAUChABSCLGYNOwS0nKZSgHsANoA7AYAHAYAnAC6RqnsAIw8YwIVVTV1MvKK1iqtwwYATAYAzAOU1HT040A)
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