CODEWITHSUNDEEP
haskell
netik
netik

Reputation: 1836

Confusion about the $ operator and parentheses

I'm working my way through the first haskell book and struggle with the $ operator:

The following line works:

map (>= 16) . take 5 $ iterate  (\x -> x^2) 2

However, the following doesn't:

map (>= 16) . take 5 (iterate  (\x -> x^2) 2)

Possible cause: `take' is applied to too many arguments

I don't see the problem here. take takes an int and a list. To my understanding, I provided both arguments.

What do I have to do if I want to avoid the $ operator?

Upvotes: 1

Views: 101

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477607

The ($) :: (a -> b) -> a -> b operator is a function that simply has a the lowest priority (infixr 0, only ($!) and seq have the same priority). As a result:

map (>= 16) . take 5 $ iterate  (\x -> x^2) 2

is equivalent to:

(map (>= 16) . take 5) (iterate  (\x -> x^2) 2)

so also with brackets for the left operand as well.

It is actually a nice thing about Haskell that you can use operators as a grouping mechanism: ($) is simply defined as ($) f x = f x, but because of the fact that it is an operator, it can be used as a way to avoid brackets.

Upvotes: 7

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