#[macro_export]
macro_rules! system {
    ($name:ident $pat:tt => $($rest:tt)* ) => {
        struct $name;
        impl<'a> specs::System<'a> for $name {
            type SystemData = args_to_systemdata!($pat);
            fn run(&mut self, args_to_pat!($pat): Self::SystemData) {
                for args_to_pat!($pat) in args_to_join!($pat).join() {
                    $($rest)*
                }
            }
        }
    };
}

#[macro_export]
macro_rules! args_to_systemdata {
    ( ( $name:ident : $ty:ty ) ) => {
        ( specs::ReadStorage<'a, $ty> ,)
    };
    ( ( mut $name:ident : $ty:ty ) ) => {
        ( specs::WriteStorage<'a, $ty> ,)
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( specs::ReadStorage<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
    ( ( mut $name:ident : $ty:ty , $( $tok:tt )* ) ) => {
        ( specs::WriteStorage<'a, $ty>, args_to_systemdata!( ( $( $tok )* ) ) )
    };
}

#[macro_export]
macro_rules! args_to_pat {
    ( ( $name:ident : $ty:ty ) ) => {
        ( $name ,)
    };
    ( ( mut $name:ident : $ty:ty ) ) => {
        ( mut $name ,)
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( $name, args_to_pat!( ( $($tok)* ) ) )
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( mut $name, args_to_pat!( ( $($tok)* ) ) )
    };
}

#[macro_export]
macro_rules! args_to_join {
    ( ( $name:ident : $ty:ty ) ) => {
        ( & $name ,)
    };
    ( ( mut $name:ident : $ty:ty ) ) => {
        ( &mut $name ,)
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( & $name, args_to_join!( ( $($tok)* ) ) )
    };
    ( ( $name:ident : $ty:ty , $($tok:tt)* ) ) => {
        ( &mut $name, args_to_join!( ( $($tok)* ) ) )
    };
}

#[cfg(test)]
mod tests {
    use specs::join::Join;

    struct Pos {
        x: usize,
    }
    impl specs::Component for Pos {
        type Storage = specs::VecStorage<Pos>;
    }

    struct Mov;
    impl specs::Component for Mov {
        type Storage = specs::VecStorage<Mov>;
    }

    system!{ Foo (_x: Mov, mut y: Pos) =>
        y.x += 1;
    }
}