either.rs

//! A type that represents one of two alternatives

import result::result;

/// The either type
enum either<T, U> {
    left(T),
    right(U)
}

fn either<T, U, V>(f_left: fn(T) -> V,
                   f_right: fn(U) -> V, value: either<T, U>) -> V {
    /*!
     * Applies a function based on the given either value
     *
     * If `value` is left(T) then `f_left` is applied to its contents, if
     * `value` is right(U) then `f_right` is applied to its contents, and the
     * result is returned.
     */

    alt value { left(l) { f_left(l) } right(r) { f_right(r) } }
}

fn lefts<T: copy, U>(eithers: ~[either<T, U>]) -> ~[T] {
    //! Extracts from a vector of either all the left values

    let mut result: ~[T] = ~[];
    for vec::each(eithers) |elt| {
        alt elt { left(l) { vec::push(result, l); } _ {/* fallthrough */ } }
    }
    ret result;
}

fn rights<T, U: copy>(eithers: ~[either<T, U>]) -> ~[U] {
    //! Extracts from a vector of either all the right values

    let mut result: ~[U] = ~[];
    for vec::each(eithers) |elt| {
        alt elt { right(r) { vec::push(result, r); } _ {/* fallthrough */ } }
    }
    ret result;
}

fn partition<T: copy, U: copy>(eithers: ~[either<T, U>])
    -> {lefts: ~[T], rights: ~[U]} {
    /*!
     * Extracts from a vector of either all the left values and right values
     *
     * Returns a structure containing a vector of left values and a vector of
     * right values.
     */

    let mut lefts: ~[T] = ~[];
    let mut rights: ~[U] = ~[];
    for vec::each(eithers) |elt| {
        alt elt {
          left(l) { vec::push(lefts, l); }
          right(r) { vec::push(rights, r); }
        }
    }
    ret {lefts: lefts, rights: rights};
}

pure fn flip<T: copy, U: copy>(eith: either<T, U>) -> either<U, T> {
    //! Flips between left and right of a given either

    alt eith {
      right(r) { left(r) }
      left(l) { right(l) }
    }
}

pure fn to_result<T: copy, U: copy>(
    eith: either<T, U>) -> result<U, T> {
    /*!
     * Converts either::t to a result::t
     *
     * Converts an `either` type to a `result` type, making the "right" choice
     * an ok result, and the "left" choice a fail
     */

    alt eith {
      right(r) { result::ok(r) }
      left(l) { result::err(l) }
    }
}

pure fn is_left<T, U>(eith: either<T, U>) -> bool {
    //! Checks whether the given value is a left

    alt eith { left(_) { true } _ { false } }
}

pure fn is_right<T, U>(eith: either<T, U>) -> bool {
    //! Checks whether the given value is a right

    alt eith { right(_) { true } _ { false } }
}

#[test]
fn test_either_left() {
    let val = left(10);
    fn f_left(&&x: int) -> bool { x == 10 }
    fn f_right(&&_x: uint) -> bool { false }
    assert (either(f_left, f_right, val));
}

#[test]
fn test_either_right() {
    let val = right(10u);
    fn f_left(&&_x: int) -> bool { false }
    fn f_right(&&x: uint) -> bool { x == 10u }
    assert (either(f_left, f_right, val));
}

#[test]
fn test_lefts() {
    let input = ~[left(10), right(11), left(12), right(13), left(14)];
    let result = lefts(input);
    assert (result == ~[10, 12, 14]);
}

#[test]
fn test_lefts_none() {
    let input: ~[either<int, int>] = ~[right(10), right(10)];
    let result = lefts(input);
    assert (vec::len(result) == 0u);
}

#[test]
fn test_lefts_empty() {
    let input: ~[either<int, int>] = ~[];
    let result = lefts(input);
    assert (vec::len(result) == 0u);
}

#[test]
fn test_rights() {
    let input = ~[left(10), right(11), left(12), right(13), left(14)];
    let result = rights(input);
    assert (result == ~[11, 13]);
}

#[test]
fn test_rights_none() {
    let input: ~[either<int, int>] = ~[left(10), left(10)];
    let result = rights(input);
    assert (vec::len(result) == 0u);
}

#[test]
fn test_rights_empty() {
    let input: ~[either<int, int>] = ~[];
    let result = rights(input);
    assert (vec::len(result) == 0u);
}

#[test]
fn test_partition() {
    let input = ~[left(10), right(11), left(12), right(13), left(14)];
    let result = partition(input);
    assert (result.lefts[0] == 10);
    assert (result.lefts[1] == 12);
    assert (result.lefts[2] == 14);
    assert (result.rights[0] == 11);
    assert (result.rights[1] == 13);
}

#[test]
fn test_partition_no_lefts() {
    let input: ~[either<int, int>] = ~[right(10), right(11)];
    let result = partition(input);
    assert (vec::len(result.lefts) == 0u);
    assert (vec::len(result.rights) == 2u);
}

#[test]
fn test_partition_no_rights() {
    let input: ~[either<int, int>] = ~[left(10), left(11)];
    let result = partition(input);
    assert (vec::len(result.lefts) == 2u);
    assert (vec::len(result.rights) == 0u);
}

#[test]
fn test_partition_empty() {
    let input: ~[either<int, int>] = ~[];
    let result = partition(input);
    assert (vec::len(result.lefts) == 0u);
    assert (vec::len(result.rights) == 0u);
}

//
// Local Variables:
// mode: rust
// fill-column: 78;
// indent-tabs-mode: nil
// c-basic-offset: 4
// buffer-file-coding-system: utf-8-unix
// End:
//