Operators in Haskell

Question

I wrote the following function

foldList :: (Double -> Double -> Double) -> [Double] -> Double
foldList op (x:t)
 | t == []     = x
 | otherwise   = (op) x (foldList op t)

and it worked perfectly fine. But when I changed the last line to

  | otherwise  = x op (foldList op t)

it didn't compile anymore. I am still rather new to Haskell but I thought when dealing with operators

a op b

is equivalent to

(op) a b

Do I have to treat op as just a normal function? And if so, in what cases is it regarded an operator and why not here?

Answer 1

Operators are functions with symbol names. They're infix by default, and you can use them like other functions by wrapping them in parentheses.

a + b       (+) a b

Functions with identifier names, like your op , can be used as infix by wrapping them with backticks .

op a b      a `op` b