pattern matching in Haskell

Main.hs

module Main where

-- data, you can think this is something like {:user, name, age} in Elixir
data User = User String Int

foo (User "John" _) = 100
foo (User _ age) = age

-- tuple
bar (_, _, 3) = 100
bar (_, y, z) = y + z

-- record syntax
data Point =
  Point { x :: Int
        , y :: Int
        , z :: Maybe Int }

makePoint = Point {x=0, y=0, z=Nothing}

baz :: Point -> (Int, Int)
baz Point {x=1, y=y} = (100, 100)
baz Point {x=x, y=y} = (x, y)

main :: IO ()
main = do
  print $ foo (User "John" 20) -- => 100
  print $ foo (User "Bob" 20) -- => 20

  print $ bar (1, 2, 3) -- => 100
  print $ bar (3, 4, 5) -- => 9

  print $ baz makePoint {x=1, y=2}  -- => (100, 100)
  print $ baz makePoint {x=3, y=4, z=Just 5} -- => (3, 4)