2019년 5월 17일 금요일

Type Conversion in Rust

Type conversion is not special in Rust. It's just a function that takes ownership of the value and returns the other type. So you can name convert functions anything. However, it's a convention to use as_, to_, and into_ prefixed name or to use from_ prefixed constructor.

From

You can create any function for type conversion. However, if you want to provide generic interfaces, you'd better implement the From trait. For instance, you should implement From<X> for Y when you want the interface that converts the X type value to the Y type value.
The From trait have an associated function named from. You can call this function like From::from(x). You also can call it like Y::from(x) if the compiler cannot infer the type of the destination type.

Into

From have an associated function, it makes you be able to specify the destination type. It's why From has an associated function instead of a method, but on the other hands, you cannot use it as a method, like a.from().
You should implement the Into trait to use a method for type conversion. This trait allows a variable to be converted to another type with the method. You can use into as X::into(x) or Into::into, but no one needs to do it. It's merely a verbose code. Use x.into() as long as the compiler can infer the destination type. Otherwise, use From.

TryFrom

From and Into are traits to provide a conversion function that would not fail. However, some conversions can fail. For instance, converting from i128 to i32 can fail because some values of i128 are not in the range of i32. To do these conversions, you should use TryFrom and TryInto. For instance, Rust uses TryFrom that returns TryFromIntError on the failure for the above example.

TryInto

The relation between TryFrom and TryInto is the same as From and Into. TryFrom has an associated function named try_from and TryInto has a method named try_into.

From Implies Into

from and into should have the same behavior. The different behavior confuses the users; in fact, you cannot implement them differently. If you simultaneously implement From<Y> for X and Into<X> for Y, you would see the below error message.

error[E0119]: conflicting implementations of trait `std::convert::Into<X>` for type `Y`:

It's because From implies Into. It's called a blanket implementation.
Because of this blanket implementation, you'd better implement From instead of Into when you need a type converting method.
As far as I know, there is only one exception that you should implement Into instead of From. If the output type of the conversion function is a generic type that is not declared in the current crate, you cannot implement From. It's the only case(at least as far as I know). The same rule is applied to TryFrom and TryInto.