Make an arbitrarily large Haskell error message (and make your computer very warm)

spew-error.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -freduction-depth-0 #-}

import GHC.TypeLits

data N = Z | S N

type F :: Nat -> N
type family F n where
  F 0 = Z
  F n = S (F (n - 1))

data P a = P

x :: P Z
x = P @(F 10000)

• spew-error.hs is 311 + the length of the typelit
• 10000 results in a 40202 character error message for a 316 character file. Ratio = 127.2.
• 100000 results in a 400276 character error message for a 317 character file (and took hours to finish). Ratio = 1262.7.
• So we can drive the ratio up as much as we want if we are willing to wait.