Simple Haskell solution for https://github.com/josevalim/nested-map-reduce-traversal

NestedMapReduceTraversal.hs

module NestedMapReduceTraversal where

data Section = Section String Bool [String]

data NumberedSection = NumberedSection Int String [(Int, String)]
    deriving (Show, Eq)

numberSection :: [Section] -> [NumberedSection]
numberSection = go (1, 1)
  where
    go _ [] = []
    go (sectionPos, lessonPos) (Section title reset lessons : rest) =
        let lessonStart = if reset then 1 else lessonPos
            numberedLessons = zip ([lessonStart ..]) lessons
            numberedSection = NumberedSection sectionPos title numberedLessons
            nextPos = (sectionPos + 1, lessonStart + length numberedLessons)
         in numberedSection : go nextPos rest

example :: [Section]
example =
    [ Section "Getting started" False ["Welcome", "Installation"]
    , Section "Basic operator" False ["Addition / Subtraction", "Multiplication / Division"]
    , Section "Advanced topics" True ["Mutability", "Immutability"]
    ]

expected :: [NumberedSection]
expected =
    [ NumberedSection 1 "Getting started" [(1, "Welcome"), (2, "Installation")]
    , NumberedSection 2 "Basic operator"  [(3, "Addition / Subtraction"), (4, "Multiplication / Division")]
    , NumberedSection 3 "Advanced topics" [(1, "Mutability"), (2, "Immutability")]
    ]

main = do
    let got = numberSection example
    if got /= expected
        then mapM_ print got
        else putStrLn "Ok!"