@function font-size($key) {
  @if map-has-key($font-sizes, $key) {
    @return map-get($font-sizes, $key);
  }
  @warn "Unknown `#{$key}` in $font-sizes.";
  @return null;
}

@function font-weight($key) {
  @if map-has-key($font-weights, $key) {
    @return map-get($font-weights, $key);
  }
  @warn "Unknown `#{$key}` in $font-weights.";
  @return null;
}

@function line-height($key) {
  @if map-has-key($line-heights, $key) {
    @return map-get($line-heights, $key);
  }
  @warn "Unknown `#{$key}` in $line-heights.";
  @return null;
}

@function letter-spacing($key) {
  @if map-has-key($letter-spacings, $key) {
    @return map-get($letter-spacings, $key);
  }
  @warn "Unknown `#{$key}` in $letter-spacings.";
  @return null;
}