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32 lines
988 B
Markdown
32 lines
988 B
Markdown
# Making Things Mutable
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In [ReasonML](https://reasonml.github.io/en/), things that we create with
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`let` are immutable -- which means that we can't change them.
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```reason
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let num = 5;
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```
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Once `num` is _bound_ to `5` it is stuck with that value for the duration
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of it's scope.
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ReasonML doesn't completely restrict us to immutability though. The
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`ref` construct allows us to bind a variable to a sort of box that holds a
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value. We can then look in the box and change what is in the box.
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```reason
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let num = ref(5); /* put 5 in the box */
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Js.log(num^); /* use `^` to look in the box */
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num := 3; /* remove 5, put 3 in the box */
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```
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We use `ref` to bind our variable to a box with some initial value. The `:=`
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assignment operator allows us to change what's in the box. Anytime we want
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to refer to what's in the box, we postfix our variable with `^`.
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Also of note: while `list` instances are not mutable, `array` instances are.
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[source](https://reasonml.github.io/docs/en/mutation)
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