Error found:
in module FoldableInstance4
at tests/purs/failing/FoldableInstance4.purs:8:1 - 8:27 (line 8, column 1 - line 8, column 27)

  No type class instance was found for
                                        
    Data.Foldable.Foldable (Function t3)
                                        
  The instance head contains unknown type variables. Consider adding a type annotation.

while applying a function foldl
  of type Foldable t0 => (t1 -> t2 -> t1) -> t1 -> t0 t2 -> t1
  to argument $f1
while inferring the type of foldl $f1

where t0 is an unknown type
      t2 is an unknown type
      t1 is an unknown type
      t3 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.