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SassMeister-input.scss

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// ----

/// Map of existing generators
/// @access private
$__generators__: () !global;

/// Current generator stored in `generator`
/// @access private
$__current__: null !global;

/// Initialize a new generator
/// @access public
/// @param {String} $name - generator's name
/// @content - `yield` directives
/// @require $__generators__
/// @require $__current__
@mixin generator($name) {
  $__current__: $name !global;
  $__generators__: map-merge($__generators__, ($__current__: (
    'values': (),
    'position': 0
  ))) !global;
  @content;
  $__current__: null !global;
}

/// Yield a new value for the generator
/// @param {*} $value
/// @require $__generators__
/// @require $__current__
@mixin yield($value) {
  @if $__current__ == null {
    @error "You cannot yield outside of a generator.";
  }
  
  $current: map-get($__generators__, $__current__);
  $current: map-merge($current, ('values': append(map-get($current, 'values'), $value)));
  $__generators__: map-merge($__generators__, ($__current__: $current)) !global;
}

/// Return next value for generator
/// @param {String} $name
/// @require $__generators__
/// @throw Unknown $generator `#{$generator}`.
@function generator-next($name) {
  $generator: map-get($__generators__, $name);
  
  @if $generator == null {
    @error "Unknown $generator `#{$generator}`.";
  }
  
  $position: map-get($generator, 'position') + 1;
  $values: map-get($generator, 'values');
  $done: $position > length($values);
  $__generators__: map-merge($__generators__, ($name: map-merge($generator, ('position': $position)))) !global;
  
  @return (
    'done': $done, 
    'value': if($done, null, nth($values, $position))
  );
}

/// Check whether an generator is still valid
/// @param {String} $name
/// @require $__generators__
/// @throw Unknown $generator `#{$generator}`.
@function generator-valid($name) {
  $generator: map-get($__generators__, $name);
  @if $generator == null {
    @error "Unknown $generator `#{$generator}`.";
  }
  @return map-get($generator, 'position') < length(map-get($generator, 'values'));
}

/// Reset an generator
/// @param {String} $name
/// @require $__generators__
/// @throw Unknown $generator `#{$generator}`.
@mixin generator-rewind($name) {
  $generator: map-get($__generators__, $name);
  @if $generator == null {
    @error "Unknown $generator `#{$generator}`.";
  }
  $__generators__: map-merge($__generators__, ($name: map-merge($generator, ('position': 0)))) !global;
}

// Test
@include generator('foo') {
  @each $item in 1 2 3 4 5 {
    @include yield($item * $item);
  }
}

.test {
  @while generator-valid('foo') {
    content: inspect(generator-next('foo'));
  }
  content: inspect(generator-next('foo')); // Done
}

@include generator-rewind('foo');

.test {
  @while generator-valid('foo') {
    content: inspect(generator-next('foo'));
  }
  content: inspect(generator-next('foo')); // Done
}

SassMeister-output.css

.test {
  content: ("done": false, "value": 1);
  content: ("done": false, "value": 4);
  content: ("done": false, "value": 9);
  content: ("done": false, "value": 16);
  content: ("done": false, "value": 25);
  content: ("done": true, "value": null);
}

.test {
  content: ("done": false, "value": 1);
  content: ("done": false, "value": 4);
  content: ("done": false, "value": 9);
  content: ("done": false, "value": 16);
  content: ("done": false, "value": 25);
  content: ("done": true, "value": null);
}