What does $ mean/do in Haskell?

Question

When you are writing slightly more complex functions I notice that $ is used a lot but I don't have a clue what it does?

Answer 1

$ is infix "application". It's defined as

($) :: (a -> b) -> (a -> b)
f $ x = f x

-- or 
($) f x = f x
-- or
($) = id

It's useful for avoiding extra parentheses: f (g x) == f $ g x.

A particularly useful location for it is for a "trailing lambda body" like

forM_ [1..10] $ \i -> do
  l <- readLine
  replicateM_ i $ print l

compared to

forM_ [1..10] (\i -> do
  l <- readLine
  replicateM_ i (print l)
)

Or, trickily, it shows up sectioned sometimes when expressing "apply this argument to whatever function"

applyArg :: a -> (a -> b) -> b
applyArg x = ($ x)

>>> map ($ 10) [(+1), (+2), (+3)]
[11, 12, 13]