CODEWITHSUNDEEP
haskell
xdl
xdl

Reputation: 1110

Haskell function application replacing parenthesis

Haskell newbie here. I'm playing around with this expression:

"The sum of 3 and 4 is " ++ (show (3 + 4))

Which evaluates fine. This is also fine:

"The sum of 3 and 4 is " ++ (show $ 3 + 4)

But I get a 'parse error on input $' when I replace the last pair of parenthesis with $:

"The sum of 3 and 4 is " ++ $ show $ 3 + 4

which I'm not really sure why. I'm following the LearnYouAHaskell series which says:

'$ being sort of the equivalent of writing an opening parentheses and then writing a closing one on the far right side of the expression.'

What have I missed?

Upvotes: 2

Views: 208

Answers (1)

AJF
AJF

Reputation: 11924

Partially applied infix operators need to be enclosed in parentheses. So:

"The sum of 3 and 4 is " ++ $ show $ 3 + 4

fails, but...

("The sum of 3 and 4 is " ++) $ show $ 3 + 4

works just fine. Note that you could also use the (.) function to compose the functions, like so:

("The sum of 3 and 4 is " ++) . show $ 3 + 4

... but that comes later in learn-you-a-haskell.

Also, as @Shanthakumar posted, it might be desirable to do it this way:

(++) "The sum of 3 and 4 is " $ show $ 3 + 4

Upvotes: 3

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