Error found:
in module Main
at tests/purs/failing/NonExhaustivePatGuard.purs:4:1 - 4:16 (line 4, column 1 - line 4, column 16)

  A case expression could not be determined to cover all inputs.
  The following additional cases are required to cover all inputs:

    _

  Alternatively, add a Partial constraint to the type of the enclosing value.

while applying a function $__unused
  of type Partial => t0 -> t0
  to argument case x of        
                x | 1 <- x -> x
while checking that expression $__unused (case x of        
                                            x | 1 <- x -> x
                                         )                 
  has type Int
in value declaration f

where t0 is an unknown type

See https://github.com/purescript/documentation/blob/master/errors/NoInstanceFound.md for more information,
or to contribute content related to this error.