CODEWITHSUNDEEP
haskell
user3369427
user3369427

Reputation: 435

A little confused on the '$' character in Haskell

I'm very new to Haskell and I'm trying to understand these basic lines of code. I have a main module that's very simple:

main = do
       words <- readFile "test.txt"
       putStrLn $ reverseCharacters words

where reverseCharacters is defined in another module that I have:

reverseCharacters :: String -> String
reverseCharacters x = reverse x

What I am having trouble understanding is why the $ needs to be there. I've read previous posts and looked it up and I'm still having difficulty understanding this. Any help would be greatly appreciated.

Upvotes: 1

Views: 116

Answers (1)

Ben
Ben

Reputation: 71495

$ is an operator, just like +. What it does is treat its first argument (the expression on the left) as a function, and apply it to its second argument (the expression on the right).

So in this case putStrLn $ reverseCharacters words is equivalent to putStrLn (reverseCharacters words). It needs to be there because function application is left associative, so using no $ or parentheses like putStrLn reverseCharacters words would be equivalent to parenthesising this way (putStrLn reverseCharacters) words, which doesn't work (we can't apply putStrLn to reverseCharacters [something of type String -> String], and even if we could we can't apply the result of putStrLn to words [something of type String]).

The $ operator is just another way of explicitly "grouping" the words than using parentheses; because it's an infix operator it forces a "split" in the expression (and because it's a very low precedence infix operator, it works even if the things on the left or right are using other infix operators).

Upvotes: 10

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