Differences Between `Foldl` and `Foldr`
Alexis King clarifies how foldl, foldr, and their strict variants behave in Haskell's lazy evaluation model.
read it It provides a clear, foundational mental model for laziness and associativity in functional programming that is essential for writing efficient Haskell code.
summary
Alexis King demystifies the misconception that foldl and foldr traverse lists in opposite directions, showing that both move left-to-right but differ in associativity. In lazy languages like Haskell, a naive foldl builds space-leaking thunks and should be replaced by strict foldl', whereas lazy foldr enables work-saving streaming and infinite lists. The post establishes practical rules of thumb for choosing the right fold based on strictness and memory consumption.
key points
- Both foldl and foldr traverse lists from left to right; their actual difference is associativity, not traversal direction.
- Lazy foldl builds large nested thunks in memory, making foldl' essential for constant-space reduction with strict operations.
- Lazy foldr allows partial consumption and infinite list streaming because the accumulation function is lazy in its second argument.
who should read it
Haskell developers and functional programmers looking to master list folds and understand space leaks caused by laziness.
discussion warm
consensus
Commenters appreciate the clear breakdown, though some debate whether complex folds provide better readability than explicit recursion or monoids.
pushback
A few point out that the strict left-to-right traversal characterization depends entirely on the underlying singly-linked list structure.
A neat fact about foldr on lists, unlike foldl, is that it actually passes control flow entirely to the accumulating function on each fold step. That means you can use foldr to implement arbitrary traversals of lists, including foldl' or list traversals that exit early. See https://github.com/quchen/articles/blob/master/useful_techni...
It highlights how foldr can be leveraged as a control flow mechanism for early exits and custom list traversals.
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