BDD vs xyzUnit style unit-tests

test.t

describe "StringCalculator" => sub {
    it "returns sum of comma delimited numbers" => sub {
        is(add("$a,$b"), $a + $b);
    };

    it "treats newline as delimiter" => sub {
        is(add("$a\n$b"), $a + $b);
    };

    describe 'delimiter marker //<delimiter-spec>\n' => sub {
        it "is used to specify custom delimiter" => sub {
            is(add("//$delimiter\n$a$delimiter$b"), $a + $b);
        };

        it "could be used to specify multiple delimiters" => sub {
            is(add("//[$delimiter][$other_delimiter]\n$a$delimiter$b$other_delimiter$c"),
               $a + $b + $c);
        };

        it "could specify delimiter marker longer than one char" => sub {
            is(add("//$multichar_delimiter\n$a$multichar_delimiter$b"), $a + $b);
        };
    };

    describe "invalid input exception" => sub {
        it "is thrown when input contains negative number" => sub {
            ...
        };

        it "contains list of all invalid numbers in given string" => sub {
            ...
        };
    };
};

## When something fails I get meaningful and readable test description:
#
# StringCalculator
#     returns sum of comma delimited numbers
#     treats newline as delimiter
#
#     delimiter marker //<delimiter-spec>\n
#         is used to specify custom delimiter
#         could be used to specify multiple delimiters
#         could specify delimiter marker longer than one char
#
#     invalid input exception
#         is thrown when input contains negative number
#         contains list of all invalid numbers in given string

class StringCalculatorTest {
    sub returnsSumOfCommaDelimitedNumbers : Tests {
        is(add("$a,$b"), $a + $b);
    }

    sub treatsNewlineAsDelimiter : Tests {
        is(add("$a\n$b"), $a + $b);
    }

    sub delimiterMarkerIsUsedToSpecifyCustomDelimiter : Tests {
        is(add("//$delimiter\n$a;$b"), $a + $b);
    }

    sub delimiterMarkerCouldBeUsedToSpecifyMultipleDelimiters : Tests {
        is(add("//[$delimiter][$other_delimiter]\n$a$delimiter$b$other_delimiter$c"),
           $a + $b + $c);
    }

    sub delimiterMarkerCouldSpecifyDelimiterMarkerLongerThanOneChar : Tests {
        is(add("//$multichar_delimiter\n$a$multichar_delimiter$b"), $a + $b);
    }

    sub invalidInputExceptionIsThrownWhenInputContainsNegativeNumber : Tests {
        ...
    }

    sub invalidInputExceptionContainsListOfAllInvalidNumbersInGivenString : Tests {
        ...
    }
}

# Ruby - http://rspec.info
# Perl - https://metacpan.org/module/Test::Spec
# JavaScript - http://pivotal.github.com/jasmine/