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Smalltalk Challenge: Post 9 - Koans

Besides tinkering with turtles and hashes in Squeak, I secretly went back to GNU Smalltalk and went through some of the Smalltalk Koans. Sssh... don't tell Josh!

Programming koans are a series of failing unit tests that a student reads through and corrects. Each test demonstrates a particular concept in the language. They can be a fun way to review one's understanding of a language, and occasionally learn something new. Here's an of a koan:
testSingleCharacterFromString [
    | string |

    string := 'Smalltalk'.

    self expect: (self fillMeIn) toEqual: (string at: 1).
    self expect: (self fillMeIn) toEqual: (string at: 6).
]
When the test suite is run, it displays:
Do not lose hope.  Expected value should equal actual value.

Expected : FILL ME IN
Actual   : $S  (an instance of Character)

TestString#testSingleCharacterFromString has damaged your karma
(in src/koans/TestString.st)
The name of the method indicates it is possible access the characters that make up a string. The test shows how the at: message is passed to a string to obtain a character at the given index. The programmer must replace (self fillMeIn) with the correct value which will allow the test to pass and the student to proceed.
self expect: $S toEqual: (string at: 1).
self expect: $t toEqual: (string at: 6).
This demonstrates that in Smalltalk character instances are preceded by a dollar-sign, and indexes start at 1, not 0 as in many other languages.

Going through the koans, I thought the ones from TestString.st, TestMessage.st, and TestDictionary.st were exceptionally good. My favorite was this one from TestMessage.st, which demonstrates an unintuitive edge-case resulting from Smalltalk's everything-is-an-object and message passing philosophies.
testMessageCascading [
    | value |

    value := 3 + 2; * 100.  "';' separates each message sent to '3'"

    self expect: (self fillMeIn) toEqual: value.

    "Think about it: we are sending multiple messages to '3'."
]
The correct answer is 300. Pretty evil, eh?
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Comments

  1. skimJuly 3, 2011 at 2:20 PM

    Hey Timothy, thanks for the write up! Just want to say I recently updated the koans to avoid explicit self on fillMeIn. Now you only have to replace fillMeIn in the tests.

    ReplyDelete
  2. Timothy BoronczykJuly 5, 2011 at 12:45 PM

    You're welcome.., and thank you for the time and effort you put in to write some really awesome koans. I look forward to seeing your Squeak koans if/when you get around to them.

    ReplyDelete

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