$ is an infix operator often seen in Haskell code. It applies the function on its left to the value on its right. At first glance this operator appears redundant, since ordinary application
(f x) means the same as
(f $ x). However,
$ has the lowest, right-associative binding precedence (infixr 0), so it sometimes allows parentheses to be omitted; for example:
f $ g $ h x = f (g (h x))
$ were omitted, the parse would be different:
f g h x = ((f g) h) x
It is also useful in higher-order situations, such as
map ($ 0) xs, or
zipWith ($) fs xs.
$ comes from the Prelude, where it is defined as:
infixr 0 $ ($) :: (a -> b) -> a -> b f $ x = f x
The $ syntax is also used in Template Haskell with an entirely different meaning. If the $ is immediately followed by a letter or a left parenthesis (with no intervening white space), then it is understood by the GHC compiler as a Template Haskell splice and not the infix operator described above. To get the infix operator be sure to separate the $ from the right argument by at least one white space.