mapSecond

MapSecond is a programming utility that applies a function to the second element of each pair in a collection, leaving the first element unchanged. Formally, for a function g: B -> C and a list of pairs [(A,B)], mapSecond g yields [(A,C)]. This pattern is common when data are stored as key-value pairs and only the values need transformation.

In functional programming terms, mapSecond can be seen as mapping over the second component of a bifunctor. In Haskell, with second from Data.Bifunctor, it can be written as map (second g) xs, transforming only the second element of each pair in the list xs. In languages with explicit pair handling, such as JavaScript, mapSecond can be implemented as a small helper: const mapSecond = (arr, f) => arr.map(([a, b]) => [a, f(b)]). In Python, a typical implementation is: def map_second(iterable, f): return [(a, f(b)) for a, b in iterable].

Common use cases include transforming values in a collection of key-value pairs while preserving keys, such as converting a list of (key, value) pairs by applying a function to the values, changing units, or normalizing scores without altering the associated keys. The operation is linear in the number of pairs, requiring O(n) time and producing a new collection of the same size.

Related concepts include mapFirst (transforming the first element), bimap (transforming both components), and functions dedicated to applying transformations to dictionary values or to the second component of a pair in various language libraries.

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