Error found:
in module Main
at tests/purs/failing/ClassHeadNoVTA1.purs:8:10 - 8:19 (line 8, column 10 - line 8, column 19)

  No type class instance was found for
                  
    Main.Single t0
                  
  The instance head contains unknown type variables.

  Note: The following type class members found in the expression require visible type applications to be unambiguous (e.g. tyClassMember @Int).
    Main.useSingle
      tyNotAppearInBody

while checking that type forall (t12 :: Type) (@tyNotAppearInBody :: t12). Single @t12 tyNotAppearInBody => Int
  is at least as general as type Int
while checking that expression useSingle
  has type Int
in value declaration single

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.