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

The most important part about $ is that it has the lowest operator precedence.

If you type info you'll see this:

λ> :info ($)
($) :: (a -> b) -> a -> b
    -- Defined in ‘GHC.Base’
infixr 0 $

This tells us it is an infix operator with right-associativity that has the lowest possible precedence. Normal function application is left-associative and has highest precedence (10). So $ is something of the opposite.

So then we use it where normal function application or using () doesn't work.

So, for example, this works:

λ> head . sort $ "example"
λ> e

but this does not:

λ> head . sort "example"

because . has lower precedence than sort and the type of (sort "example") is [Char]

λ> :type (sort "example")
(sort "example") :: [Char]

But . expects two functions and there isn't a nice short way to do this because of the order of operations of sort and .