Higher-order functions for Sass 3.3 -- now that we've got the `call()` function in Sass, we can start to compose functions. We don't have the benefit of anonymous functions, of course, but this is real functional programming, folks!

higher-order-functions.scss

// 'map', 'transform', 'select', or 'project' function. Iterate over a list,
// performing a function on each item, and collecting the new items. Each
// function takes an item as a first argument and returns a transformed item.
// You can pass additional arguments to the function too, which is a decent poor
// man's function composition.
// (I didn't call this f-map because the term 'map' is already used in Sass to
// denote a hash or dictionary.)
@function f-apply($func, $list, $args...) {
    $new-list: ();
    @each $item in $list {
        $new-list: append($new-list, call($func, $item, $args...));
    }
    @return $new-list;
}

// Reduce a list to a single value. Each function takes the previous value, an
// item, and additional arguments, and returns a transformed value. Sadly, you
// can't leave out the initial value (can't have default arguments before rest
// arguments), so if you don't want an initial value, set it to null. 
@function f-fold($func, $list, $initial, $args...) {
    $start: 1;
    @if $initial == null {
        // If there's no initial value, use the first in the list.
        $start: 2;
        $initial: nth($list, 1);
    }
    $prev: $initial;
    @for $i from $start through length($list) {
        $next: nth($list, $i);
        $prev: call($func, $prev, $next, $args...);
    }
    @return $prev;
}

// Filter items from a list. Each function takes an item (and additional
// arguments) and returns a boolean value indicating whether the item passed the
// filter.
@function f-filter($func, $list, $args...) {
    $new-list: ();
    @each $item in $list {
        @if call($func, $item, $args...) {
            $new-list: append($new-list, $item);
        }
    }
    @return $new-list;
}

Fucking A, man. :)

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