Error found:
in module Main
at tests/purs/failing/4158.purs:9:10 - 9:11 (line 9, column 10 - line 9, column 11)

  Could not match type
      
    a1
      
  with type
      
    b0
      

while trying to match type { foo :: Int
                           | a1        
                           }           
  with type { foo :: Int
            | b0        
            }           
while checking that expression r
  has type Maybe         
             { foo :: Int
             | b0        
             }           
in value declaration evil

where a1 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)
      b0 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)

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