PureScript Lunch-n-Learn Lesson 6

Slides for the sixth lesson in my PureScript lunch-n-learn series. This lesson takes a closer look at pattern matching, starting at the top of chapter 5 of "PureScript by Example."

Transcript

1. PureScript Lunch & Learn 6: Pattern Matching (pt. 1) github.com/jimf/purescript-lunchnlearn
2. Last lesson’s “homework” Rewrite count in tail-recursive form using an accumulator parameter.
3. Last lesson’s “homework” count :: forall a. (a -> Boolean) -> Array a -> Int count = count' 0 where count' acc _ [] = acc count' acc p xs = count' (acc + (if p (unsafePartial head xs) then 1 else 0)) p (unsafePartial tail xs)
4. • Focus on pattern matching, covering simple cases, guards, arrays, and records • Solve related problems in an interactive, “dojo” style Goals
5. This lesson is built upon Ch. 5 of PureScript by Example. Be sure to check it out: https://leanpub.com/purescript/read#leanpub-auto-pattern-matching Note
6. • Commonly used in functional programming • Allows for functions to be defined in terms of declarative cases of input • Maps well to mathematical definitions • Cases are tried in order Pattern Matching
7. Pattern Matching gcd :: Int -> Int -> Int gcd n 0 = n gcd 0 m = m gcd n m = if n > m then gcd (n - m) m else gcd n (m - n)
8. Pattern Matching gcd :: Int -> Int -> Int gcd n 0 = n gcd 0 m = m gcd n m = if n > m then gcd (n - m) m else gcd n (m - n) Cases are matched in order
9. Pattern Matching gcd :: Int -> Int -> Int gcd n 0 = n gcd 0 m = m gcd n m = if n > m then gcd (n - m) m else gcd n (m - n) Cases are matched in order Cases can bind names to values
10. Pattern Matching: Literal cases stringToBool :: String -> Boolean stringToBool "true" = true stringToBool _ = false boolToString :: Boolean -> String boolToString true = "true" boolToString false = "false"
11. Pattern Matching: Literal cases stringToBool :: String -> Boolean stringToBool "true" = true stringToBool _ = false boolToString :: Boolean -> String boolToString true = "true" boolToString false = "false" Can match on literal values
12. Pattern Matching: Literal Cases stringToBool :: String -> Boolean stringToBool "true" = true stringToBool _ = false boolToString :: Boolean -> String boolToString true = "true" boolToString false = "false" Can match on literal values _ is a special wildcard to match any value without binding a name
13. Pattern Matching: guards -- gcd rewritten to use guards gcd' :: Int -> Int -> Int gcd' n 0 = n gcd' 0 m = m gcd' n m | n > m = gcd' (n - m) m | otherwise = gcd' n (m - n)
14. Pattern Matching: guards -- gcd rewritten to use guards gcd' :: Int -> Int -> Int gcd' n 0 = n gcd' 0 m = m gcd' n m | n > m = gcd' (n - m) m | otherwise = gcd' n (m - n) Guards use `|` to place additional constraints on the case
15. Pattern Matching: guards -- gcd rewritten to use guards gcd' :: Int -> Int -> Int gcd' n 0 = n gcd' 0 m = m gcd' n m | n > m = gcd' (n - m) m | otherwise = gcd' n (m - n) Guards use `|` to place additional constraints on the case Special keyword to match anything else
16. Pattern Matching: arrays isEmpty :: Array a -> Boolean isEmpty [] = true isEmpty _ = false
17. Pattern Matching: arrays isEmpty :: Array a -> Boolean isEmpty [] = true isEmpty _ = false Matches arrays with no elements
18. Pattern Matching: arrays isEmpty :: Array a -> Boolean isEmpty [] = true isEmpty _ = false Matches arrays with no elements Matches any other array
19. Pattern Matching: arrays > let takeFive [0, 1, a, b, _] = a * b takeFive _ = 0
20. Pattern Matching: arrays > let takeFive [0, 1, a, b, _] = a * b takeFive _ = 0 First case matches all arrays of exactly 5 values, where the first 2 values are 0, 1
21. Pattern Matching: arrays > let takeFive [0, 1, a, b, _] = a * b takeFive _ = 0 > takeFive [0, 1, 2, 3, 4] 6 First case matches all arrays of exactly 5 values, where the first 2 values are 0, 1
22. Pattern Matching: arrays > let takeFive [0, 1, a, b, _] = a * b takeFive _ = 0 > takeFive [0, 1, 2, 3, 4] 6 > takeFive [1, 2, 3, 4, 5] 0 First case matches all arrays of exactly 5 values, where the first 2 values are 0, 1
23. Pattern Matching: arrays > let takeFive [0, 1, a, b, _] = a * b takeFive _ = 0 > takeFive [0, 1, 2, 3, 4] 6 > takeFive [1, 2, 3, 4, 5] 0 > takeFive [] 0 First case matches all arrays of exactly 5 values, where the first 2 values are 0, 1
24. • Patterns can only match arrays of fixed length • Use Data.List to match based on head/tail and other splits Pattern Matching: arrays
25. Pattern Matching: records showPerson :: { first :: String, last :: String } -> String showPerson { first: x, last: y } = y <> ", " <> x
26. Pattern Matching: records showPerson :: { first :: String, last :: String } -> String showPerson { first: x, last: y } = y <> ", " <> x Matches records that, at a minimum, contain first and last properties
27. Pattern Matching: records showPerson :: { first :: String, last :: String } -> String showPerson { first: x, last: y } = y <> ", " <> x Matches records that, at a minimum, contain first and last properties Binds those properties to names
28. Pattern Matching: nesting type Address = { street :: String, city :: String } type Person = { name :: String, address :: Address } livesInLA :: Person -> Boolean livesInLA { address: { city: "Los Angeles" } } = true livesInLA _ = false
29. Pattern Matching: nesting type Address = { street :: String, city :: String } type Person = { name :: String, address :: Address } livesInLA :: Person -> Boolean livesInLA { address: { city: "Los Angeles" } } = true livesInLA _ = false Patterns may be arbitrarily nested
30. Pattern Matching: named sortPair :: Array Int -> Array Int sortPair arr@[x, y] | x <= y = arr | otherwise = [y, x] sortPair arr = arr
31. Pattern Matching: named sortPair :: Array Int -> Array Int sortPair arr@[x, y] | x <= y = arr | otherwise = [y, x] sortPair arr = arr Any pattern may be “named” by using the @ symbol
32. Pattern Matching: named sortPair :: Array Int -> Array Int sortPair arr@[x, y] | x <= y = arr | otherwise = [y, x] sortPair arr = arr Any pattern may be “named” by using the @ symbol Matches arrays with two elements
33. Pattern Matching: named sortPair :: Array Int -> Array Int sortPair arr@[x, y] | x <= y = arr | otherwise = [y, x] sortPair arr = arr Any pattern may be “named” by using the @ symbol Matches arrays with two elements Because of the named pattern, only this case allocates a new array in memory