Mutation

Value bindings in Grain are immutable, which means that they do not change after they have been declared. While there’s a ton we can get done with regular bindings, it can sometimes be helpful to change the data already associated with a name. We can opt into mutable values by declaring a binding mut when defined.

Using mut

Take a look at the following example:

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let mut b = "foo"

print(b) // "foo"

b = "bar"

print(b) // "bar"

A couple things to note from this example:

1. When defining our value binding, we added the mut keyword.
2. We can use the value of our mutable binding like any other binding!
3. We can also assign values to it using = and the name will be associated with the new value.
4. If we forget mut but try to assign to the binding, we’ll get a type error.

Working with Mutable Numbers

When using mutable values that are Numbers, we can use some built-in operators to make working with them easier. Grain provides +=, -=, *=, /=, and %=, which perform the math operation on the value and re-assign the result.

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let mut count = 7

count += 9
count -= 6
count *= 5
count /= 25

print(count) // 2

These operators return void to help alleviate ambiguity and make code easier to understand—use the name of the binding to get its current value.

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let mut count = 7

print(count += 9) // void

// Instead, use
count += 9
print(count) // 25