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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/
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,, and were exceptionally good. My favorite was this one from, 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?


  1. 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.

  2. 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.


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