More info here: https://wiki.haskell.org/Bottom As bottom is the value of a non-terminating computation and a computation of any type may fail to terminate, bottom must inhabit all types. The same goes for 'error'. Although in real world Haskell code (using GHC) you can trap error values if you really want to, so 'error' isn't in fact indistinguishable from non-termination.