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# What `#[derive(FromStr)]` generates
Deriving `FromStr` only works for enums with no fields
or newtypes, i.e structs with only a single
field. The result is that you will be able to call the `parse()` method on a
string to convert it to your newtype. This only works when the type that is
contained in the type implements `FromStr`.
## Example usage
```rust
# use derive_more::FromStr;
#
#[derive(FromStr, Debug, Eq, PartialEq)]
struct MyInt(i32);
#[derive(FromStr, Debug, Eq, PartialEq)]
struct Point1D{
x: i32,
}
assert_eq!(MyInt(5), "5".parse().unwrap());
assert_eq!(Point1D{x: 100}, "100".parse().unwrap());
```
## Tuple structs
When deriving `FromStr` for a tuple struct with one field:
```rust
# use derive_more::FromStr;
#
#[derive(FromStr)]
struct MyInt(i32);
```
Code like this will be generated:
```rust
# struct MyInt(i32);
impl derive_more::core::str::FromStr for MyInt {
type Err = <i32 as derive_more::core::str::FromStr>::Err;
fn from_str(src: &str) -> Result<Self, <i32 as derive_more::core::str::FromStr>::Err> {
return Ok(MyInt(i32::from_str(src)?));
}
}
```
## Regular structs
When deriving `FromStr` for a regular struct with one field:
```rust
# use derive_more::FromStr;
#
#[derive(FromStr)]
struct Point1D {
x: i32,
}
```
Code like this will be generated:
```rust
# struct Point1D {
# x: i32,
# }
impl derive_more::core::str::FromStr for Point1D {
type Err = <i32 as derive_more::core::str::FromStr>::Err;
fn from_str(src: &str) -> Result<Self, <i32 as derive_more::core::str::FromStr>::Err> {
return Ok(Point1D {
x: i32::from_str(src)?,
});
}
}
```
## Enums
When deriving `FromStr` for an enums with variants with no fields it will
generate a `from_str` method that converts strings that match the variant name
to the variant. If using a case insensitive match would give a unique variant
(i.e you dont have both a `MyEnum::Foo` and a `MyEnum::foo` variant) then case
insensitive matching will be used, otherwise it will fall back to exact string
matching.
Since the string may not match any variants an error type is needed, so the
`derive_more::FromStrError` will be used for that purpose.
e.g. Given the following enum:
```rust
# use derive_more::FromStr;
#
#[derive(FromStr)]
enum EnumNoFields {
Foo,
Bar,
Baz,
}
```
Code like this will be generated:
```rust
# enum EnumNoFields {
# Foo,
# Bar,
# Baz,
# }
#
impl derive_more::core::str::FromStr for EnumNoFields {
type Err = derive_more::FromStrError;
fn from_str(src: &str) -> Result<Self, derive_more::FromStrError> {
Ok(match src.to_lowercase().as_str() {
"foo" => EnumNoFields::Foo,
"bar" => EnumNoFields::Bar,
"baz" => EnumNoFields::Baz,
_ => return Err(derive_more::FromStrError::new("EnumNoFields")),
})
}
}
```
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