foldl' is like foldl, but strict in the accumulator.

Left fold with strict accumulator

Left fold with a strict accumulator

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

Strict left fold of the metadatas

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

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;")

Synonym for ofoldl'

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.