Error Handling Without Exceptions

A quick jaunt through functional error handling patterns using Ruby

Transcript

1. Error Handling Without Exceptions
2. A Guiding Example ['1','2','3'].map{ |x| Integer(x) }.reduce(:+)
3. A Guiding Example ['1','2','3'].map{ |x| Integer(x) }.reduce(:+) => 6
4. A Guiding Example ['1','2','3'].map{ |x| Integer(x) }.reduce(:+) => 6 ['1','2','3','zero'].map{ |x| Integer(x) }.reduce(:+)
5. A Guiding Example ['1','2','3'].map{ |x| Integer(x) }.reduce(:+) => 6 ['1','2','3','zero'].map{ |x| Integer(x) }.reduce(:+) ArgumentError: invalid value for Integer(): "zero"
6. “ Errors aren’t exceptions because they’re not exceptional - they’re real possible outcomes with their own information - treat them as such - Some Guy I Know
7. Can We Do Better?
8. Can We Do Better? def safe_to_i(value) Integer(value) rescue nil end
9. Can We Do Better? def safe_to_i(value) Integer(value) rescue nil end ['1','2','3','zero'].map{ |x| safe_to_i(x) }
10. Can We Do Better? def safe_to_i(value) Integer(value) rescue nil end ['1','2','3','zero'].map{ |x| safe_to_i(x) } => [1, 2, 3, nil]
11. Can We Do Better? def safe_to_i(value) Integer(value) rescue nil end ['1','2','3','zero'].map{ |x| safe_to_i(x) } => [1, 2, 3, nil] ['1','2','3','zero'].map{ |x| safe_to_i(x) }.reduce(:+)
12. Can We Do Better? def safe_to_i(value) Integer(value) rescue nil end ['1','2','3','zero'].map{ |x| safe_to_i(x) } => [1, 2, 3, nil] ['1','2','3','zero'].map{ |x| safe_to_i(x) }.reduce(:+) TypeError: nil can't be coerced into Fixnum
13. Take Three
14. Take Three def safer_to_i(value) [Integer(value)] rescue [] end
15. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) }
16. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []]
17. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten
18. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten => [1, 2, 3]
19. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten => [1, 2, 3] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten.reduce(:+)
20. Take Three def safer_to_i(value) [Integer(value)] rescue [] end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten => [1, 2, 3] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.flatten.reduce(:+) => 6
21. But I Wanted To Fail!
22. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end
23. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3'].map{ |x| safer_to_i(x) } => [[1], [2], [3]]
24. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3'].map{ |x| safer_to_i(x) } => [[1], [2], [3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence
25. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3'].map{ |x| safer_to_i(x) } => [[1], [2], [3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]]
26. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []]
27. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence
28. But I Wanted To Fail! class Array def sequence self.reduce([[]]){ |a,b| a.flat_map{ |aa| b.map{ |bb| aa << bb }}} end end ['1','2','3','zero'].map{ |x| safer_to_i(x) } => [[1], [2], [3], []] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence => []
29. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]]
30. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) }
31. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) } => [6]
32. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) } => [6] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence
33. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) } => [6] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence => []
34. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) } => [6] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence => [] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence .map{ |x| x.reduce(:+) }
35. But I Wanted To Fail! ['1','2','3'].map{ |x| safer_to_i(x) }.sequence => [[1, 2, 3]] ['1','2','3'].map{ |x| safer_to_i(x) }.sequence.map{ |x| x.reduce(:+) } => [6] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence => [] ['1','2','3','zero'].map{ |x| safer_to_i(x) }.sequence .map{ |x| x.reduce(:+) } => []
36. Where To Next?
37. Where To Next? def saferToInt(str: String): Option[Int] = try { Some(str.toInt) } catch { case e: Exception => None } class ListOps[A](list: List[Option[A]]) { def sequence[A](a: List[Option[A]]): Option[List[A]] = a.foldRight[Option[List[A]]](Some(Nil))((a,b)=>a.flatMap(aa=>b.map(bb=>aa::bb))) def sequence: Option[List[A]] = sequence(list) } implicit def listToListOps[A](list: List[Option[A]]) = new ListOps(list)
38. Where To Next? List("1","2","3").map(saferToInt) res12: List[Option[Int]] = List(Some(1), Some(2), Some(3)) List("1","2","3").map(saferToInt).sequence res13: Option[List[Int]] = Some(List(1, 2, 3)) List("1","2","3").map(saferToInt).sequence.map(_.reduce(_+_)) res14: Option[Int] = Some(6)
39. Where To Next? List("1","2","3","zero").map(saferToInt) res16: List[Option[Int]] = List(Some(1), Some(2), Some(3), None) List("1","2","3","zero").map(saferToInt).sequence res17: Option[List[Int]] = None List("1","2","3","zero").map(saferToInt).sequence.map(_.reduce(_+_)) res18: Option[Int] = None
40. Where To Next? This all works because List and Option are Monads