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

by

The $ operator is for avoiding parentheses. Anything appearing after it will take precedence over anything that comes before.

For example, let’s say you’ve got a line that reads:

putStrLn (show (1 + 1))

If you want to get rid of those parentheses, any of the following lines would also do the same thing:

putStrLn (show $ 1 + 1)
putStrLn $ show (1 + 1)
putStrLn $ show $ 1 + 1

The primary purpose of the . operator is not to avoid parentheses, but to chain functions. It lets you tie the output of whatever appears on the right to the input of whatever appears on the left. This usually also results in fewer parentheses, but works differently.

Going back to the same example:

putStrLn (show (1 + 1))
  1. (1 + 1) doesn’t have an input, and therefore cannot be used with the . operator.
  2. show can take an Int and return a String.
  3. putStrLn can take a String and return an IO ().

You can chain show to putStrLn like this:

(putStrLn . show) (1 + 1)

If that’s too many parentheses for your liking, get rid of them with the $ operator:

putStrLn . show $ 1 + 1

More Related Contents:

  1. Dot Operator in Haskell: need more explanation
  2. Why shouldn’t I mix tabs and spaces?
  3. runST and function composition
  4. What does (f .) . g mean in Haskell?
  5. What does the “@” symbol mean in reference to lists in Haskell?
  6. Haskell function composition (.) and function application ($) idioms: correct use
  7. What does $ mean/do in Haskell?
  8. What’s the “|” for in a Haskell class definition?
  9. What does the exclamation mark mean in a Haskell declaration?
  10. Haskell function composition operator of type (c→d) → (a→b→c) → (a→b→d)
  11. What does the `forall` keyword in Haskell/GHC do?
  12. What characters are permitted for Haskell operators?
  13. Converting IO Int to Int
  14. Haskell \p -> not takeWhile
  15. Sum of two numbers with function in haskell
  16. Monads with Join() instead of Bind()
  17. How to get normal value from IO action in Haskell
  18. New state of the art in unlimited generation of Hamming sequence
  19. How do you represent a graph in Haskell?
  20. Haskell types frustrating a simple ‘average’ function
  21. Distinction between typeclasses MonadPlus, Alternative, and Monoid?
  22. Function definition by special cases in GHCi
  23. Lambda for type expressions in Haskell?
  24. Composing Monads v. Applicative Functors
  25. Abusing the algebra of algebraic data types – why does this work?
  26. How to return a pure value from a impure method
  27. How to convert Unicode Escape Sequence to Unicode String in Haskell
  28. How does the ST monad work?
  29. An example of a Foldable which is not a Functor (or not Traversable)?
  30. Random number in Haskell [duplicate]

Leave a Comment