Enums and Pattern Matching

Where Patterns

Pattern cases can add conditions with where.

Match and filter

value
where_patterns.swift
Replay: real traced execution (multi-file project)
enum Score {
    case points(Int)
}

let value = 82
let score = Score.points(value)
var band = ""

switch score {
case Score.points(let amount) where amount >= 90:
    band = "excellent"
case Score.points(let amount) where amount >= 70:
    band = "steady"
case Score.points:
    band = "practice"
}

print("value=\(value)")
print("band=\(band)")

enum Score {
    case points(Int)
}

let value = 45
let score = Score.points(value)
var band = ""

switch score {
case Score.points(let amount) where amount >= 90:
    band = "excellent"
case Score.points(let amount) where amount >= 70:
    band = "steady"
case Score.points:
    band = "practice"
}

print("value=\(value)")
print("band=\(band)")

enum Score {
    case points(Int)
}

let value = 95
let score = Score.points(value)
var band = ""

switch score {
case Score.points(let amount) where amount >= 90:
    band = "excellent"
case Score.points(let amount) where amount >= 70:
    band = "steady"
case Score.points:
    band = "practice"
}

print("value=\(value)")
print("band=\(band)")

  1. value ← 82, score ← points(82), band ← (empty)

    5let value→ 82 = 82  //@value=45, 956let score→ points(82) = Score.points(value82)7var band→ (empty) = ""

  2. switch score

    9switch scorepoints(82) {10case Score.points(let amount) where amount >= 90:11    band = "excellent"

  3. band ← steady

    11    band = "excellent"12case Score.points(let amount82) where amount >= 70:13    band→ steady = "steady"14case Score.points:

  4. print("value=\(value)")

    18print("value=\(value82)")19print("band=\(bandsteady)")
    outputvalue=82
band=steady

  1. value ← 45, score ← points(45), band ← (empty)

    5let value→ 45 = 456let score→ points(45) = Score.points(value45)7var band→ (empty) = ""

  2. switch score

    9switch scorepoints(45) {10case Score.points(let amount) where amount >= 90:11    band = "excellent"

  3. band ← practice

    13    band = "steady"14case Score.points:15    band→ practice = "practice"16}

  4. print("value=\(value)")

    18print("value=\(value45)")19print("band=\(bandpractice)")
    outputvalue=45
band=practice

  1. value ← 95, score ← points(95), band ← (empty)

    5let value→ 95 = 956let score→ points(95) = Score.points(value95)7var band→ (empty) = ""

  2. switch score

    9switch scorepoints(95) {10case Score.points(let amount) where amount >= 90:11    band = "excellent"

  3. band ← excellent

    9switch score {10case Score.points(let amount95) where amount >= 90:11    band→ excellent = "excellent"12case Score.points(let amount) where amount >= 70:

  4. print("value=\(value)")

    18print("value=\(value95)")19print("band=\(bandexcellent)")
    outputvalue=95
band=excellent
where A `where` clause refines a pattern after it has matched the shape of the value.