@function __is-length($value) {
    @return type-of($value) == 'number'
    and $value > -1
    and $value % 1 == 0;
}


@function __is-strict-comparable($value) {
    @return not __is-map-like($value);
}


@function __is-iteratee-call($value: null, $index: null, $map: null, $args...) {
    @if not __is-map-like($map)
    or length($args) > 0 {
        @return false;
    }

    $type: type-of($index);
    $prereq: false;

    @if ($type == 'number') {
        $length: length($map);
        $prereq: (__is-length($length) and __is-index($index, $length));
    } @else {
        $prereq: ($type == 'string');
    }

    @return if($prereq, __get($map, $index) == $value, false);
}


@function __is-arglist($value, $args...) {
    @return type-of($value) == 'arglist';
}


@function __is-list($value, $args...) {
    @return type-of($value) == 'list';
}


@function __is-boolean($value, $args...) {
    @return type-of($value) == 'bool';
}


@function __is-falsey($value) {
    @if $value == null
    or $value == ''
    or $value == ""
    or $value == $__undefined__
    or $value == 0
    or (
        type-of($value) == 'number'
        and unit($value) == '%'
        and $value == 0%)
    or not $value {
        @return true;
    }

    @return false;
}


@function __is-truthy($value) {
    @return $value and not __is-falsey($value);
}


@function __is-finite($value, $args...) {
    @return __is-number($value);
}


@function __is-function($value, $args...) {
    @return __function-exists($value);
}


@function __is-map($value, $args...) {
    @return type-of($value) == 'map';
}


@function __is-null($value, $args...) {
    @return type-of($value) == 'null';
}


@function __is-number($value, $args...) {
    @return type-of($value) == 'number';
}


@function __is-integer($value, $args...) {
    @return __is-number($value) and round($value) == $value;
}


@function __is-plain-map($value, $args...) {
    @return (type-of($value) == 'map')
    and not (map-has-key($value, '_type'));
}


@function __is-string($value, $args...) {
    @return type-of($value) == 'string';
}


@function __is-undefined($value, $args...) {
    @return if(global-variable-exists('__undefined__'), $value == $__undefined__, false);
}


@function __is-index($value, $length: null) {
    $value: __parse-float($value);

    @return $value > 0
    and $value % 1 == 0
    and ($length == null or $value <= $length);
}


@function __is-iterable($value, $args...) {
    @return __is-map-like($value) or __is-string($value);
}


@function __is-map-like($value, $args...) {
    @return $value and index('map' 'list' 'arglist', type-of($value)) != null;
}


@function __is-list-like($value, $args...) {
    @return __is-list($value) or __is-arglist($value);
}


@function __is-native($value, $args...) {
    @return __is-function($value) and index(__const('NATIVE_FUNCTIONS'), $value) != null;
}


@function __is-color($value, $args...) {
    @return type-of($value) == 'color';
}


@function __is-time($value, $args...) {
    @return type-of($value) == 'number' and index(__const('TIME_UNITS'), unit($value)) != null;
}

/// Checks if `$value` is an `Arglist`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is an `Arglist`, else `false`.
/// @example scss
///     @function get-args($args...) {
///         @return $args;
///     }
///     
///     $foo: _is-arglist(get-args(1, 2, 3));
///     // => true
///     
///     $foo: _is-arglist((1, 2, 3));
///     // => false

@function _is-arglist($args...) {
    @return call(get-function('__is-arglist'), $args...);
}

/// Checks if `$value` is a `List`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `List`, else `false`.
/// @example scss
///     
/// $foo: _is-list(1 2 3);
/// // => true
/// 
/// $foo: _is-list((1, 2, 3));
/// // => true

@function _is-list($args...) {
    @return call(get-function('__is-list'), $args...);
}

/// Checks if `$value` is a `Boolean`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `Boolean`, else `false`.
/// @example scss
///     
/// $foo: _is-boolean(false);
/// // => true
/// 
/// $foo: _is-boolean(1 + 1 == 2);
/// // => true
/// 
/// $foo: _is-boolean(null);
/// // => false

@function _is-boolean($args...) {
    @return call(get-function('__is-boolean'), $args...);
}

/// Checks if `$value` is a standard Sass `Function` or a Sassdash
/// callback `Function`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `Function`, else `false`.
/// @example scss
///     
/// $foo: _is-function('lighten');
/// // => true
/// 
/// $foo: _is-function(_partial('lighten', #C0FF33));
/// // => true
/// 
/// $foo: _is-function('not-a-function');
/// // => false

@function _is-function($args...) {
    @return call(get-function('__is-function'), $args...);
}

/// Checks if `$value` is `null`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is `null`, else `false`.
/// @example scss
///     
/// $foo: _is-null(null);
/// // => true
/// 
/// $foo: _is-null(map-get(('a': 1), 'b'));
/// // => true
/// 
/// $foo: _is-null(false);
/// // => false
/// 
/// 
@function _is-null($args...) {
    @return call(get-function('__is-null'), $args...);
}

/// Checks if `$value` is a `Number`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `Number`, else `false`.
/// @example scss
///     
/// $foo: _is-number(30);
/// // => true
/// 
/// $foo: _is-number('30');
/// // => false

@function _is-number($args...) {
    @return call(get-function('__is-number'), $args...);
}


/// Checks if `$value` is an integer number.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is an integer number, else `false`.
/// @example scss
///     
/// $foo: _is-integer(31);
/// // => true
/// 
/// $foo: _is-integer('31');
/// // => false
/// 
/// $foo: _is-integer(31.25);
/// // => false

@function _is-integer($args...) {
    @return call(get-function('__is-integer'), $args...);
}


/// Checks if `$value` is a `Map`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `Map`, else `false`.
/// @example scss
///     
/// $foo: _is-map(('a': 1));
/// // => true
/// 
/// $foo: _is-map(('a', 1));
/// // => false

@function _is-map($args...) {
    @return call(get-function('__is-map'), $args...);
}


/// Checks if `$value` is a plain `Map`, not from a constructor.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a plain `Map`, else `false`.
/// @example scss
/// // todo

@function _is-plain-map($args...) {
    @return call(get-function('__is-plain-map'), $args...);
}


/// Checks if `$value` is a `String`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `String`, else `false`.
/// @example scss
///     
/// $foo: _is-string('hello');
/// // => true
/// 
/// $foo: _is-string(hello);
/// // => true
/// 
/// $foo: _is-string(31);
/// // => false

@function _is-string($args...) {
    @return call(get-function('__is-string'), $args...);
}


/// Checks if `$value` is the Sassdash `undefined` value.
/// Using `undefined` (`$__undefined__`) can be useful in distinguishing
/// between `null` and undefined values.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is `undefined`, else `false`.
/// @example scss
/// // todo

@function _is-undefined($args...) {
    @return call(get-function('__is-undefined'), $args...);
}


/// Checks if `$value` is iterable. A value in Sassdash is considered iterable
/// if it contains a distinct set of elements and acts as a container for these
/// elements - i.e. a `Map`, `List`, `Arglist`, or `String`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is iterable, else `false`.
/// @example scss
///     
/// $foo: _is-iterable('hello');
/// // => true
/// 
/// $foo: _is-iterable(1 2 3 4);
/// // => true
/// 
/// $foo: _is-iterable(('a': 1, 'b': 2));
/// // => true
/// 
/// $foo: _is-iterable(1234);
/// // => false

@function _is-iterable($args...) {
    @return call(get-function('__is-iterable'), $args...);
}


/// Checks if `$value` is similar to a `Map`, in that it has
/// (implicit or explicit) keys bijectively paired with values.
/// In Sassdash, a `Map`, `List`, or `Arglist` can be considered map-like.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is similar to a `Map`, else `false`.
/// @example scss
///     
/// $foo: _is-map-like(1 2 3 4);
/// // => true
/// 
/// $foo: _is-map-like(('a': 1, 'b': 2));
/// // => true
/// 
/// $foo: _is-map-like('1234');
/// // => false

@function _is-map-like($args...) {
    @return call(get-function('__is-map-like'), $args...);
}


/// Checks if `$value` is a `List` or `Arglist`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `List` or `Arglist`, else `false`.
/// @example scss
///     
/// $foo: _is-list-like((1, 2));
/// // => true

@function _is-list-like($args...) {
    @return call(get-function('__is-list-like'), $args...);
}


/// Checks if `$value` is a `Color`.
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if `$value` is a `Color`, else `false`.
/// @example scss
///     
/// $foo: _is-color($C0FF33);
/// // => true
/// 
/// $foo: _is-color(green);
/// // => true
/// 
/// $foo: _is-color('green');
/// // => false

@function _is-color($args...) {
    @return call(get-function('__is-color'), $args...);
}


/// Checks if `$value` is a Time value (i.e. seconds or milliseconds).
/// 
/// @access public
/// @group Lang
/// @param {*} $value The value to check.
/// @returns {boolean} Returns `true` if unit of numerical `$value` is 's' or 'ms', else `false`.
/// @example scss
/// 
/// $foo: _is-time(13s);
/// // => true
/// 
/// $foo: _is-time(0.3ms);
/// // => true
/// 
/// $foo: _is-time('13s');
/// // => false

@function _is-time($args...) {
    @return call(get-function('__is-time'), $args...);
}