@function time($value)
  @if unit_number($value) and unit($value) == s
    @return true
  @else
    @return null

@function timing_function($value)
  @if string($value)
    @if $value == 'ease' or $value == 'linear' or $value == 'ease-in' or $value == 'ease-out' or $value == 'ease-in-out' or str-index($value, cubic-bezier)
      @return true
    @else
      @return null
  @else
    @return null

@function delay($value)
  @if list($value) and nth($value, 1) == delay
    @return true
  @else
    @return null

//////////////////
// transition
//////////////////

@function transition_duration($value)
  @return time($value)

@function transition_timing_function($value)
  @return timing_function($value)

@function transition_delay($value)
  @return delay($value)

@function transition-property($value)
  @if string($value)
    @if not transition_timing_function($value) or not transition_duration($value) or not transition_delay($value)
      @return true
    @else
      @return null
  @else if list($value)
    @return true
  @else
    @return null

//////////////////
// animation
//////////////////

@function animation_duration($value)
  @return time($value)

@function animation_timing_function($value)
  @return timing_function($value)

@function animation_delay($value)
  @return delay($value)

@function animation_iteration_count($value)
  @if unitless_number($value) or (string($value) and $value == 'infinite')
    @return true
  @else
    @return null

@function animation_direction($value)
  @if string($value) and ($value == 'normal' or $value == 'alternate')
    @return true
  @else
    @return null

@function animation_fill_mode($value)
  @if string($value) and ($value == 'nonel' or $value == 'forwards' or $value == 'backwards' or $value == 'both')
    @return true
  @else
    @return null

@function animation_play_state($value)
  @if string($value) and ($value == 'running' or $value == 'paused')
    @return true
  @else
    @return null

@function animation_name($value)
  @if string($value)
    @if animation_timing_function($value) or animation_duration($value) or animation_delay($value) or animation_iteration_count($value) or animation_direction($value) or animation_fill_mode($value) or animation_play_state($value)
      @return null
    @else
      @return true
  @else
    @return null