# foldl' is:exact -package:containers -package:ghc -package:filepath

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 structure to a single strict result (e.g. sum). For a general Foldable structure this should be semantically identical to,
```foldl' f z = foldl' f z . toList
```
A strict version of foldl.
foldl' is like foldl, but strict in the accumulator.
foldl' is like foldl, but strict in the accumulator.
foldl' is like foldl, but strict in the accumulator.
O(n) A strict version of foldl.
A strict version of foldl. Properties
```foldl' f z0 . stream = foldl' f z0
```
O(n) Left fold with strict accumulator.
Left fold with strict accumulator
Left fold with a strict accumulator
O(n) Left fold with strict accumulator.
O(n) Pure left fold with strict accumulator.
O(n) Pure left fold with strict accumulator.
O(n) Left fold with strict accumulator.
O(n) Pure left fold with strict accumulator.
O(n) Left fold with strict accumulator.
O(n) Pure left fold with strict accumulator.
O(n) Left fold with strict accumulator.
O(n) Pure left fold with strict accumulator.
Reduce this map by applying a binary operator to all elements, using the given starting value (typically the left-identity of the operator). Each application of the operator is evaluated before using the result in the next application. This function is strict in the starting value.
Reduce this set by applying a binary operator to all elements, using the given starting value (typically the left-identity of the operator). Each application of the operator is evaluated before before using the result in the next application. This function is strict in the starting value.
Strict left fold of the metadatas
Strict left fold over all key-value pairs in the headers map. Example:
```ghci> :set -XOverloadedStrings
ghci> import Data.Monoid
ghci> let hdrs = H.fromList [("Accept", "text/plain"), ("Accept", "text/html")]
ghci> let f (cntr, acc) _ val = (cntr+1, val <> ";" <> acc)
ghci> H.foldl' f (0, "") hdrs
(2,"text/html;text/plain;")
```