foldl' is:exact -package:containers -package:base package:protolude

Left-associative fold of a structure but with strict application of the operator. This ensures that each step of the fold is forced to weak head normal form before being applied, avoiding the collection of thunks that would otherwise occur. This is often what you want to strictly reduce a finite list to a single, monolithic result (e.g. length). For a general Foldable structure this should be semantically identical to,
foldl' f z = foldl' f z . toList