Clojure Koans Walkthrough

1. Introduction
2. Lists
3. Vectors
4. Sets
5. Maps
6. Functions
7. Conditionals
8. Higher Order Functions
9. Runtime Polymorphism
10. Lazy Sequences
11. Sequence Comprehensions
12. Creating Functions
13. Recursion
14. Destructuring
15. Refs
16. Atoms
17. Macros
18. Datatypes
19. Java Interop
20. Partition

Recommended Books

• ← Lazy Sequences
• Creating Functions →

11. Sequence Comprehensions - Koans Walkthrough

This video covers sequence comprehensions, also known as list-comprehensions in most functional languages. Clojure has a "for" macro that provides a powerful way to create sequences from other sequences.

Here is the 11_sequence_comprehensions.clj source code:

(meditations
  "Sequence comprehensions can bind each element in turn to a symbol"
  (= __
     (for [index (range 6)]
       index))

  "They can easily emulate mapping"
  (= '(0 1 4 9 16 25)
     (map (fn [index] (* index index))
          (range 6))
     (for [index (range 6)]
       __))

  "And also filtering"
  (= '(1 3 5 7 9)
     (filter odd? (range 10))
     (for [index __ :when (odd? index)]
       index))

  "And they trivially allow combinations of the two transformations"
  (= '(1 9 25 49 81)
     (map (fn [index] (* index index))
          (filter odd? (range 10)))
     (for [index (range 10) :when __]
       __))

  "More complex transformations can be formed with multiple binding forms"
  (= [[:top :left] [:top :middle] [:top :right]
      [:middle :left] [:middle :middle] [:middle :right]
      [:bottom :left] [:bottom :middle] [:bottom :right]]
       (for [row [:top :middle :bottom] column [:left :middle :right]]
         __)))