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// This trait creates a `flat_map` method for `Option` (equivalent to
// `Option::and_then`) so that it can be used in the `map_for` macro.
pub trait FlatMap<Item> {
   type Item;
   fn flat_map<U, F> (self, f: F) -> Option<U>
      where F: FnOnce (Self::Item) -> Option<U>;
}
impl<Item> FlatMap<Item> for Option<Item> {
   type Item = Item;
   fn flat_map<U, F> (self, f: F) -> Option<U>
      where F: FnOnce (Self::Item) -> Option<U>
   { self.and_then (f) }
}

// This trait creates a `filter` method for `Option` so that it can be
// filtered on when used in the `map_for` macro.
pub trait Filter<Item> {
   type Item;
   fn filter<P> (self, predicate: P) -> Option<Item>
      where P: FnOnce (&Self::Item) -> bool;
}
impl<Item> Filter<Item> for Option<Item> {
   type Item = Item;
   fn filter<P> (self, predicate: P) -> Option<Item>
      where P: FnOnce (&Self::Item) -> bool
   { self.and_then (|item| if predicate (&item) { Some (item) } else { None }) }
}

// Scala-like for comprehension similar to those described in
// https://stackoverflow.com/questions/3754089/scala-for-comprehension#3754568
// The main difference is that since rust does not have partial
// functions, we do not support general patterns in the left-hand
// sides, but only single identifiers and list of identifiers (that
// will match a tuple).
#[macro_export]
macro_rules! map_for {
   // Process tuple patterns. Note that this must come first due to
   // rust issue #27832.
   (($($n:ident),+) <- $e:expr; => $e0:expr) => (
      $e.map (move |params| {
         #[allow(unused_variables)] let ($($n),+) = params;
         $e0 }));
   (($($n:ident),+) <- $e:expr; if $g:expr; $($tail:tt)+) => (
      map_for! {
         ($($n),+) <- $e.filter (|params| {
            #[allow(unused_variables)] let ($($n),+) = *params;
            $g });
         $($tail)+ });
   (($($n:ident),+) <- $e:expr; ($($n0:ident),+) <- $e0:expr; $($tail:tt)+) => (
      $e.flat_map (move |params| {
         #[allow(unused_variables)] let ($($n),+) = params;
         map_for!{ ($($n0),+) <- $e0; $($tail)+ } }));
   (($($n:ident),+) <- $e:expr; $n0:ident <- $e0:expr; $($tail:tt)+) => (
      $e.flat_map (move |params| {
         #[allow(unused_variables)] let ($($n),+) = params;
         map_for!{ $n0 <- $e0; $($tail)+ } }));
   (($($n:ident),+) <- $e:expr; $n0:ident = $e0:expr; $($tail:tt)+) => (
      map_for!{
         ($($n),+, $n0) <- map_for!{$($n),+ <- $e; => {
            let $n0 = $e0; ($($n),+, $n0) } };
         $($tail)+ });

   // Process single-identifier patterns.
   ($n:ident <- $e:expr; => $e0:expr) => ($e.map (move |$n| { $e0 }));
   ($n:ident <- $e:expr; if $g:expr; $($tail:tt)+) => (
      map_for! { $n <- $e.filter (|$n| { $g }); $($tail)+ });
   ($n:ident <- $e:expr; $n0:ident <- $e0:expr; $($tail:tt)+) => (
      $e.flat_map (move |$n| { map_for!{ $n0 <- $e0; $($tail)+ } }));
   ($n:ident <- $e:expr; $n0:ident = $e0:expr; $($tail:tt)+) => (
      map_for!{
         ($n, $n0) <- map_for!{$n <- $e; => {
            let $n0 = $e0; ($n, $n0) } };
         $($tail)+ });
}

#[cfg(test)]
mod tests {
   use super::*;

   #[test]
   fn tuple() {
      let c = map_for!{
         (a, b) <- Some ((1, 2));
         => a + b };
      assert_eq!(c, Some (3));
   }

   #[test]
   fn option() {
      let c = map_for!{
         a <- Some (1);
         b <- Some (2);
         => { a + b } };
      assert_eq!(c, Some (3));
   }

   #[test]
   fn range() {
      let l = map_for!{
         x <- 0..4;
         y <- 0..x;
         => y
         // x == 0 -> empty
         // x == 1 -> 0
         // x == 2 -> 0, 1
         // x == 3 -> 0, 1, 2
      }.collect::<Vec<_>>();
      assert_eq!(l, vec![ 0, 0, 1, 0, 1, 2 ]);
   }

   #[test]
   fn capture() {
      let count = 3;
      let offset = 2;
      let l = map_for!{
         x <- 0..4;
         y <- 0..count;
         => x + y + offset
      }.collect::<Vec<_>>();
      assert_eq!(l, vec![ 2, 3, 4,
                          3, 4, 5,
                          4, 5, 6,
                          5, 6, 7 ]);
   }

   #[test]
   fn regular_binding() {
      let l = map_for!{
         x <- 0..4;
         y = 2*x;
         z <- 0..1;
         => y+z
      }.collect::<Vec<_>>();
      assert_eq!(l, vec![ 0, 2, 4, 6 ]);
   }

   #[test]
   fn filtering() {
      let l = map_for!{
         x <- 0..10;
         if (x%2) == 0;
         => x }.collect::<Vec<_>>();
      assert_eq!(l, vec![ 0, 2, 4, 6, 8 ]);

      let l = map_for!{
         x <- 0..10;
         y = x/2;
         if (y%2) == 0;
         => y }.collect::<Vec<_>>();
      assert_eq!(l, vec![0, 0, 2, 2, 4, 4]);
   }

   #[test]
   fn filtering_options() {
      let c = map_for!{
         a <- Some (1);
         b <- Some (2);
         if (b%2) == 0;
         => { a + b } };
      assert_eq!(c, Some (3));
      let c = map_for!{
         a <- Some (1);
         if (a%2) == 0;
         b <- Some (2);
         => { a + b } };
      assert_eq!(c, None);
   }
}