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Add explanation about when you should use each of the Fn* traits (#1579)

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Beginners are confused by this often.

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Co-authored-by: Dustin J. Mitchell <dustin@v.igoro.us>
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Chayim Refael Friedman
2023-12-15 17:22:59 +02:00
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6
src/std-traits/closures.md
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@ -48,6 +48,12 @@ captured values.
`FnMut` wherever an `FnOnce` is called for, and you can use an `Fn` wherever an `FnMut` or `FnOnce` `FnMut` wherever an `FnOnce` is called for, and you can use an `Fn` wherever an `FnMut` or `FnOnce`
is called for. is called for.
When you define a function that takes a closure, you should take `FnOnce` if you can (i.e. you call it once), or `FnMut`
else, and last `Fn`. This allows the most flexibility for the caller.
In contrast, when you have a closure, the most flexible you can have is `Fn` (it can be passed everywhere), then `FnMut`,
and lastly `FnOnce`.
The compiler also infers `Copy` (e.g. for `add_3`) and `Clone` (e.g. `multiply_sum`), The compiler also infers `Copy` (e.g. for `add_3`) and `Clone` (e.g. `multiply_sum`),
depending on what the closure captures. depending on what the closure captures.