Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$items: (
  small: 320,
  medium: 600,
  large: 1008
);


// Imperative style, but without mutation
$new-items-imperative: ();
@each $key, $value in $items {
  $new-value: $value *2;
  $new-items-imperative: map-merge($new-items-imperative, ($key: $new-value))
}


// Function Programming style, enforcing no mutation

// Generic map function, currently this only handles Sass maps, but we could make
// it handle lists too later
@function map-map($map, $function) {
  @if not function-exists($function) {
    @error "There is no `#{$function}` function.";
  }
  
  $new-map: ();
  
  @each $key, $value in $map {
    $new-map: map-merge($new-map, ($key: call($function, $value)));
  }
  
  @return $new-map;
}


// Sass doesn't do anonymous functions, so you need to define the function separatly
// to the call to map
@function double($value) {
  @return $value *2;
}
$new-items-functional: map-map($items, 'double');


// Demoing the output of these three variables to prove no mutation has occured
html {
  items: $items;
  new-items-imperative: $new-items-imperative;
  new-items-functional: $new-items-functional;
}

SassMeister-output.css

html {
  items: (small: 320, medium: 600, large: 1008);
  new-items-imperative: (small: 640, medium: 1200, large: 2016);
  new-items-functional: (small: 640, medium: 1200, large: 2016);
}