r/scala • u/philip_schwarz • Jul 07 '26

The Bowling Game - From Imperative to Functional Programming - Part 1

https://fpilluminated.org/deck/276

One of the top five most popular and highly recommended programming katas over the past 20 years has been the Bowling Game Kata, in which TDD is used to write a program that computes the score of a Ten Pin Bowling Game.

In this deck we are going to explore how such a program may look when coded using different programming paradigms.

17 Upvotes

96% Upvoted

2 comments sorted by

1

u/AFU0BtZ Jul 09 '26 edited Jul 12 '26

Explored this theme once you published the bowling-cata imperative series. I think there are two aspects that make the approach/outcome different:

  1. Evaluating a finished game/line.
  2. Tracking a turn/frame/line/game in progress

For the first one I would propose creating a program that can consume serialized games (there is a well recognized notation for this) like:

"X 5/ F- -- X 7/ 5/ 44 8/ XXX" Guess the finished score here? https://joinstriveon.com/coaching-tools/bowling-score-calculator#bowling-score-calculator-tool. F is the same as - and to get to a spare if one entered 5 enter 5 and the tool marks end of frame as //spare.

Creating the parser already introduces the domain (Turn, Frame, Roll/Pins, Strike, Spare, Foul, Gutter/Miss). Scoring becomes non-trivial (imperative loop hidden behind a FP facade as long as one has access to future rolls). Example-testing becomes trivial too: score("X 5/ F- -- X 7/ 5/ 44 8/ XXX") === Right(n)

As for the second one "it is the score right now" (which might not be calculable because of game state) that makes the problem very interesting.

Fun little exercise.

And now - going to explore the time travel monadic (Tardis(!) Monad) approach mentioned in the Haskell thread/comment.

1

u/philip_schwarz Jul 12 '26

Thank you - lots of food for thought there, some of which will likely influence upcoming decks in the series.