Pragmatic Purity

Purely functional code is easier to reason about ... is the functional programmer's guiding mantra. But to what extent is it really easier to reason about? This short talk will look at and challenge this claim, and propose a pragmatic attitude towards purity.

Transcript

1. Pragmatic Purity Bill Venners, Artima, Inc. - NEScala, Boston, March 19, 2018
2. Challenge this: Pure functions are easier to reason about than impure functions. Pragmatic Purity
3. def puzzle(size: Int): List[Int] = { val array = new Array[Int](size) var i = 0 while (i < size) { array(i) = i + 1 i += 1 } array.toList } puzzle(3) Pragmatic Purity
4. def puzzle(size: Int): List[Int] = { val array = new Array[Int](size) var i = 0 while (i < size) { array(i) = i + 1 i += 1 } array.toList } puzzle(3) 1. Go into an infinite loop Pragmatic Purity
5. def puzzle(size: Int): List[Int] = { val array = new Array[Int](size) var i = 0 while (i < size) { array(i) = i + 1 i += 1 } array.toList } puzzle(3) 1. Go into an infinite loop 2. Return List(1, 2, 3) Pragmatic Purity
6. def puzzle(size: Int): List[Int] = { val array = new Array[Int](size) var i = 0 while (i < size) { array(i) = i + 1 i += 1 } array.toList } puzzle(3) 1. Go into an infinite loop 2. Return List(1, 2, 3) 3. Return List(1, 2, 3, 4) Pragmatic Purity
7. def puzzle(size: Int): List[Int] = { val array = new Array[Int](size) var i = 0 while (i < size) { array(i) = i + 1 i += 1 } array.toList } puzzle(3) 1. Go into an infinite loop 2. Return List(1, 2, 3) 3. Return List(1, 2, 3, 4) 4. Throw ArrayIndexOutOfBoundsException Pragmatic Purity
8. My Observation Imperative code can be reasoned about if it’s simple. Pragmatic Purity
9. Java was a well orchestrated fraud. - Who said this? Pragmatic Purity
10. Java was a well orchestrated fraud. - James Gosling Pragmatic Purity
11. Java was a well orchestrated fraud. - James Gosling Because: malloc() and free() Pragmatic Purity
12. Why was malloc() & free() hard? Pragmatic Purity
13. Several boiled frogs later… - Who said this? Pragmatic Purity
14. Several boiled frogs later… - Who said this? Because: shared memory and locks Pragmatic Purity
15. Several boiled frogs later… - Brian Goetz Because: shared memory and locks Pragmatic Purity
16. Pragmatic Purity
17. Pragmatic Purity Pure code and… …impure code that doesn’t force you to keep track of too many things at once. Pragmatic Purity
18. • object instantiation, initialization, relocation, and reclamation • lazy initialization • caching • referentially transparent on the outside, “minimally mutating” on the inside • tail call optimization • logging Pragmatic Purity
19. We are: • hiring (careers@artima.com) • available (info@artima.com) • listening Pragmatic Purity
20. http://www.artima.com Can one formally define the rules imperative code must follow so that it can be used “on the inside” while not violating referential transparency “on the outside?”