// Takes a value and checks to ensure it's expressed correctly then returns
// the type.
//
// @param number|string|list $x
//   A multiple of $base-line-height.
//   A px value.
//   A size from the $font-size map.
//   A space seperated list container multiples and/or px values.
//
// @return string
//   multiplier, px, font-size, list
@function typey-check-value($x) {
  @if type-of($x) == "number" {
    @if unitless($x) {
      @return "multiplier";
    }
    @if not unitless($x) {
      @if unit($x) == px {
        @return "px";
      }
      @else {
        @error "All units must be expressed in px";
      }
    }
  }
  @if type-of($x) == "string" {
    @if $x == "auto" {
      @return "auto";
    }
    @if map-has-key($font-size, $x) {
      @return "font-size";
    }
    @else {
      @error "'#{$x}' not found in $font-size map";
    }
  }
  @if type-of($x) == "list" {
    @if list-separator($x) == space {
      @each $value in $x {
        @if type-of($value) == "number" or $value == "auto" {
          @if type-of($value) == "number" {
            @if not unitless($value) and unit($value) != px {
              @error "All units must be expressed in px";
            }
          }
        }
        @else {
          @error "Values specified inside lists must be a number or 'auto'";
        }
      }
      @return "list";
    }
    @else {
      @error "All lists must use a space as their seperator";
    }
  }
  @else {
    @return type-of($x);
  }
}

// Takes a value and validates it against a specified type.
//
// @param number|string|list $x
//   A multiple of $base-line-height.
//   A px value.
//   A size from the $font-size map.
//   A space seperated list container multiples and/or px values.
// @param string|list $allowed-types
//   Either multiplier, px, font-size, list, or a comibation specified in a list.
//
// @return string
//   The values type.
@function typey-validator($x, $allowed-types) {
  $type: typey-check-value($x);
  @if index($allowed-types, $type) != null {
    @return $type;
  }
  @else {
    @error "'#{$type}' is not a valid type for this function (allowed types are: #{$allowed-types})";
  }
}