Macro as_variant

macro_rules! as_variant {
    ( $enum:expr, $( $variants:path )|* ) => { ... };
    ( $enum:expr, $pattern:pat $( if $guard:expr )? => $inner:expr $(,)? ) => { ... };
    ( $( $variants:path )|* ) => { ... };
    ( $pattern:pat $( if $guard:expr )? => $inner:expr $(,)? ) => { ... };
}

Convert the given enum value to Option.

There are two main syntactic forms to this macro:

1. Variants: Enum expression followed by a comma and then one or more |-separated newtype (one-element tuple) variants paths that should be converted to Some(_). Any other variants of the enum will be converted to None.

   enum Result<T, E> {
       Ok(T),
       Err(E),
   }
   
   impl<T, E> Result<T, E> {
       pub fn ok(self) -> Option<T> {
           as_variant!(self, Self::Ok)
       }
   }

2. Match arm: Enum expression followed by a comma, then a pattern matching one or more variants of that enum and possibly capturing variables, then a fat right arrow followed by an expression that will be put inside Some(_) if the pattern matches. If the pattern doesn’t match, None will be returned.

   enum ListenConfig {
       Ipv4 { addr: Ipv4Addr, port: u16 },
       Ipv6 { addr: Ipv6Addr, port: u16 },
       Unix { path: PathBuf },
   }
   
   impl ListenConfig {
       fn port(&self) -> Option<u16> {
           as_variant!(self, Self::Ipv4 { port, .. } | Self::Ipv6 { port, .. } => *port)
       }
   
       fn privileged_port(&self) -> Option<u16> {
           as_variant!(
               self,
               // using a guard after the pattern also works:    vvvvvvvvvvvvvvv
               Self::Ipv4 { port, .. } | Self::Ipv6 { port, .. } if *port < 1024 => *port,
           )
       }
   }

The enum expression at the start can also be left out, which causes that as_variant! invocation to expand to a closure that does the same thing. That is, as_variant!(<variants or match arm>) is the same as |val| as_variant!(val, <variants or match arm>). This is especially useful for combinators, for example Option::and_then or Iterator::filter_map:

let optional_ip_addr = Some("127.0.0.1".parse::<IpAddr>().unwrap());
let optional_ipv4_addr = optional_ip_addr.and_then(as_variant!(IpAddr::V4));

More Examples

use std::ops::Deref;

use as_variant::as_variant;

enum Value {
    Integer(i64),
    String(String),
    Array(Vec<Value>),
}

impl Value {
    pub fn as_integer(&self) -> Option<i64> {
        as_variant!(self, Self::Integer).copied()
    }

    pub fn as_str(&self) -> Option<&str> {
        as_variant!(self, Self::String).map(Deref::deref)
    }

    pub fn as_array(&self) -> Option<&[Value]> {
        as_variant!(self, Self::Array).map(Deref::deref)
    }

    pub fn into_string(self) -> Option<String> {
        as_variant!(self, Self::String)
    }

    pub fn into_array(self) -> Option<Vec<Value>> {
        as_variant!(self, Self::Array)
    }
}

use as_variant::as_variant;

enum Either3<A, B, C> {
    A(A),
    B(B),
    C(C),
}

impl<T, U> Either3<T, T, U> {
    fn as_a_or_b(&self) -> Option<&T> {
        as_variant!(self, Self::A | Self::B)
    }

    fn into_a_or_b(self) -> Option<T> {
        as_variant!(self, Self::A | Self::B)
    }
}

let a: Either3<_, _, &str> = Either3::A(1);
assert_eq!(a.as_a_or_b(), Some(&1));
assert_eq!(a.into_a_or_b(), Some(1));

let b: Either3<_, _, u8> = Either3::B("hello");
assert_eq!(b.as_a_or_b(), Some(&"hello"));
assert_eq!(b.into_a_or_b(), Some("hello"));

let c: Either3<char, _, _> = Either3::C('c');
assert_eq!(c.as_a_or_b(), None);
assert_eq!(c.into_a_or_b(), None);