Choosing the source urls for a set of font formats in SASS

browser-support.scss

$support-ie8: true !default;
$use-opentype: true !default;

// A function to return a list of values associated with a list of keys
@function map-select-values($map, $selection) {
   $values: ();
   @each $key in $selection {
     $values: append($values, map-get($map, $key));
   }
   @return $values;
}

@function supported-font-sources($sources) {
   $chosen-formats: ();
   @if $support-ie8 {
      $chosen-formats: append($chosen-formats, embedded-opentype);
   }

   @if $use-opentype {
      $chosen-formats: join($chosen-formats, otf-woff otf-woff2 opentype);
   } @else {
      $chosen-formats: join($chosen-formats, ttf-woff ttf-woff2 truetype);
   }

   @return map-select-values($sources, $chosen-formats);

}

/* Extra Light (200) */
 @font-face {
   font-family: 'Source Sans Pro';
   font-weight: 200;
   font-style: normal;
   font-stretch: normal;
   src: supported-font-sources((
      embedded-opentype: url('../fonts/SourceSansPro/EOT/SourceSansPro-ExtraLight.eot') format('embedded-opentype'),
      ttf-woff2: url('../fonts/SourceSansPro/WOFF2/TTF/SourceSansPro-ExtraLight.ttf.woff2') format('woff2'),
      ttf-woff: url('../fonts/SourceSansPro/WOFF/TTF/SourceSansPro-ExtraLight.ttf.woff') format('woff'),
      otf-woff2: url('../fonts/SourceSansPro/WOFF2/OTF/SourceSansPro-ExtraLight.otf.woff2') format('woff2'),
      otf-woff: url('../fonts/SourceSansPro/WOFF/OTF/SourceSansPro-ExtraLight.otf.woff') format('woff'),
      opentype: url('../fonts/SourceSansPro/OTF/SourceSansPro-ExtraLight.otf') format('opentype'),
      truetype: url('../fonts/SourceSansPro/TTF/SourceSansPro-ExtraLight.ttf') format('truetype')
   ));
 }

map-select-values.scss

// A function to return a list of values associated with a list of keys
@function map-select-values($map, $selection) {
   $values: ();
   @each $key in $selection {
     $values: append($values, map-get($map, $key));
   }
   @return $values;
}

$chosen-formats: woff woff2;
$formats: (embedded-opentype: url('../fonts/SourceSansPro/EOT/SourceSansPro-ExtraLight.eot') format('embedded-opentype'),
      woff2: url('../fonts/SourceSansPro/WOFF2/TTF/SourceSansPro-ExtraLight.ttf.woff2') format('woff2'),
      woff: url('../fonts/SourceSansPro/WOFF/TTF/SourceSansPro-ExtraLight.ttf.woff') format('woff'),
      opentype: url('../fonts/SourceSansPro/OTF/SourceSansPro-ExtraLight.otf') format('opentype'),
      truetype: url('../fonts/SourceSansPro/TTF/SourceSansPro-ExtraLight.ttf') format('truetype'));

@font-face {
 src: map-select-values($formats, $chosen-formats);
}

// CSS Output 
@font-face {
  src: url("../fonts/SourceSansPro/WOFF/TTF/SourceSansPro-ExtraLight.ttf.woff") format("woff"),
       url("../fonts/SourceSansPro/WOFF2/TTF/SourceSansPro-ExtraLight.ttf.woff2") format("woff2");
}