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Haskell / Lists and Higher Order

Foldr

Learn Foldr in Haskell through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

12 min read • Intermediate

Foldr is a focused Haskell lesson inside Lists and Higher Order. The example uses the construct named in the lesson directly rather than reusing a generic placeholder.

Trace the example from its inputs through Foldr and identify the behavior that would change if that construct were removed or replaced. Pay attention to the language-specific rules shown by the syntax, because neighboring lessons intentionally exercise different mechanisms.

Topic-specific example

haskell-0034-96c058eda2.hs
haskell Try It Live 
total :: [Int] -> Int
total = foldl (+) 0
main = print (total [4,7,3])
-- Independent check: zip [1,2] ["a","b"]
Edge case / engineering note

For Foldr, test one realistic boundary: empty data, invalid input, unavailable resources, lifetime/cleanup, type constraints, or concurrency timing where applicable. A successful happy path is not enough to prove the construct is used correctly.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

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