Create your own operator in Haskell

Lib.hs

module Lib where

someFunc :: IO ()
someFunc = putStrLn "someFunc"

data Expr a =
  Number a
  | Plus (Expr a) (Expr a)
  deriving (Show, Read)

-- shorthand for number, type constructor for Expr
n = Number

($++++$) :: Num a => a -> a -> a
a $++++$ b = a*a + b*b

($+$) :: Expr a -> Expr a -> Expr a
a $+$ b = Plus a b
infixl 6 $+$

($++$) :: Expr a -> Expr a -> Expr a
a $++$ b = Plus a b
infixr 6 $++$

operators_repl_usage.hs

-- Run this in a `ghci` or `stack ghci` -- this isn't really a file.

*Main> :load Lib
*Lib> n 10 $+$ n 20 $+$ n 30
Plus (Plus (Number 10) (Number 20)) (Number 30)
*Lib> n 10 $++$ n 20 $++$ n 30
Plus (Number 10) (Plus (Number 20) (Number 30))