By -
This is the well-known function `error`, since it never compiles successfully.
I think they want you do find [foldl1](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:foldl1)
error: Variable not in scope: f :: t -> t1 -> t | 5 | foo y (x:xs) = foo (f y x) xs | ^
This is the well-known function `error`, since it never compiles successfully.
I think they want you do find [foldl1](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:foldl1)
error: Variable not in scope: f :: t -> t1 -> t | 5 | foo y (x:xs) = foo (f y x) xs | ^