Websocket snippet has a statement that has dollar sign inside closed parens like this,
any ($ fst client)
Since haskellers use $ sign instead of parens, why do we need parens here?
Why is there a $ symbol between parens?
I tried to see if $ is a function by doing
Prelude>:t $
But it threw the error, parse error on input $
In Haskell, operators are just normal functions that have names made up of symbols and used infix by default. You can use them just like a normal identifier by wrapping them in parentheses:
λ> :t (+)
(+) :: Num a => a -> a -> a
$ is just an operator like this itself. It represents function application and is defined as follows:
f $ x = f x
You can get its type just like (+):
λ> :t ($)
($) :: (a -> b) -> a -> b
Haskell operators can also be partially applied like normal functions, by wrapping them in parentheses with arguments to one side. For example, (+ 1) is the same as \ x -> x + 1 and (1 +) is the same as \x -> 1 + x.
This applies to $ too, so ($ fst client) is the same as \ f -> f $ fst client or just \ f -> f (fst client). The code snippet you have checks if any of a list of functions returns true given fst client.
($ fst client) is an operator section (just like (+ 1) or (* 2)) - it partially applies the operator to its right operand. A more verbose way to write it would be (\f -> f $ fst client).
So you're applying any to a function that takes another function and applies that function to the argument fst client.
