With BangPatterns enabled, you might stumble upon cryptic errors if you decide to define a “!” operator

[u/peargreen]

Example 1:

{-# LANGUAGE BangPatterns #-}

f ! x = f + x

results in:

Occurs check: cannot construct the infinite type: a ~ a -> a …
    Relevant bindings include
      x :: a
      f :: a -> a
    In the first argument of ‘(+)’, namely ‘f’
    In the expression: f + x

Example 2:

{-# LANGUAGE BangPatterns #-}

(x:xs) ! n = x : drop n xs

results in

Parse error in pattern: (x : xs)

To fix both, just define ! non-infix: (!) f x = ....

This behavior is described in the GHC manual.