Error found:
in module Main
at tests/purs/failing/ConstraintFailure.purs:12:8 - 12:12 (line 12, column 8 - line 12, column 12)

  No type class instance was found for
                      
    Data.Show.Show Foo
                      

while checking that type forall (a :: Type). Show a => a -> String
  is at least as general as type t0 t1 t2
while checking that expression show
  has type t0 t1 t2
in value declaration main

where t0 is an unknown type
      t1 is an unknown type
      t2 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.