Leafo\ScssPhp\Parser::value PHP Method

    protected function value(&$out)
    {
        $s = $this->seek();
        if ($this->literal('not', false) && $this->whitespace() && $this->value($inner)) {
            $out = [Type::T_UNARY, 'not', $inner, $this->inParens];
            return true;
        }
        $this->seek($s);
        if ($this->literal('not', false) && $this->parenValue($inner)) {
            $out = [Type::T_UNARY, 'not', $inner, $this->inParens];
            return true;
        }
        $this->seek($s);
        if ($this->literal('+') && $this->value($inner)) {
            $out = [Type::T_UNARY, '+', $inner, $this->inParens];
            return true;
        }
        $this->seek($s);
        // negation
        if ($this->literal('-', false) && ($this->variable($inner) || $this->unit($inner) || $this->parenValue($inner))) {
            $out = [Type::T_UNARY, '-', $inner, $this->inParens];
            return true;
        }
        $this->seek($s);
        if ($this->parenValue($out) || $this->interpolation($out) || $this->variable($out) || $this->color($out) || $this->unit($out) || $this->string($out) || $this->func($out) || $this->progid($out)) {
            return true;
        }
        if ($this->keyword($keyword)) {
            if ($keyword === 'null') {
                $out = [Type::T_NULL];
            } else {
                $out = [Type::T_KEYWORD, $keyword];
            }
            return true;
        }
        return false;
    }