Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// ----

/// Deep set function to set a value in nested maps
/// @param {Map} $map - map
/// @param {List} $keys - list of keys to reach
/// @param {Literal} $value - value to assign
/// @return {Map}
@function map-set($map, $keys, $value) {
  $maps: ($map,);
  $result: null;
  
  // If the last key is a map already
  // Warn the user we will be overriding it with $value
  @if type-of(nth($keys, -1)) == "map" {
    @warn "The last key you specified is a map; it will be overrided with `#{$value}`.";
  }
  
  // If $keys is a single key
  // Just merge and return
  @if length($keys) == 1 {
    @return map-merge($map, ($keys: $value));
  }
  
  // Loop from the first to the second to last key from $keys
  // Store the associated map to this key in the $maps list
  // If the key doesn't exist, throw an error
  @for $i from 1 through length($keys) - 1 {
    $current-key: nth($keys, $i);
    $current-map: nth($maps, -1);
    $current-get: map-get($current-map, $current-key);
    @if $current-get == null {
      @error "Key `#{$key}` doesn't exist at current level in map.";
    }
    $maps: append($maps, $current-get);
  }
  
  // Loop from the last map to the first one
  // Merge it with the previous one
  @for $i from length($maps) through 1 {
    $current-map: nth($maps, $i);
    $current-key: nth($keys, $i);
    $current-val: if($i == length($maps), $value, $result);
    $result: map-merge($current-map, ($current-key: $current-val));
  }
  
  // Return result
  @return $result;
}

$map: (
	"config": (
		"desktop": ( 
			"columns": 16,
			"breakpoint": 1280px,
		),
		"laptop": (
			"columns": 12,
			"breakpoint": 960px,
		),
		"tablet": (
			"columns": 8,
			"breakpoint": 640px,
		),
		"mobile": (
			"columns": 4,
			"breakpoint": 320px,
		),
	)
);

sass {
  test: inspect(map-set($map, config desktop columns, 42));
}

SassMeister-output.css

sass {
  test: ("config": (("desktop": (("columns": 42, "breakpoint": 1280px)), "laptop": (("columns": 12, "breakpoint": 960px)), "tablet": (("columns": 8, "breakpoint": 640px)), "mobile": (("columns": 4, "breakpoint": 320px)))));
}