Error found:
in module Main
at tests/purs/failing/UnusableTypeClassMethod.purs:4:1 - 6:9 (line 4, column 1 - line 6, column 9)

  The declaration c contains arguments that couldn't be determined.
  These arguments are: { a }

in type class declaration for C

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