What is the difference between . (dot) and $ (dollar sign)?

Question

What is the difference between the dot (.) and the dollar sign ($)?

As I understand it, they are both syntactic sugar for not needing to us

Answer 1

They have different types and different definitions:

infixr 9 .
(.) :: (b -> c) -> (a -> b) -> (a -> c)
(f . g) x = f (g x)

infixr 0 $
($) :: (a -> b) -> a -> b
f $ x = f x

($) is intended to replace normal function application but at a different precedence to help avoid parentheses. (.) is for composing two functions together to make a new function.

In some cases they are interchangeable, but this is not true in general. The typical example where they are is:

f $ g $ h $ x

==>

f . g . h $ x

In other words in a chain of $s, all but the final one can be replaced by .