Warning found:
in module Main
at tests/purs/warning/UnusedTypeVar.purs:4:1 - 4:24 (line 4, column 1 - line 4, column 24)

  Type variable b is ambiguous, since it is unused in the polymorphic type which introduces it.

in type declaration for f

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