Haskell

A functional programming language with parametric polymorphism and lazy evaluation. The code examples here use the Haskell 2010 Language Report specification.

The function zip :: [a] -> [b] -> [(a,b)] pairs corresponding list positions, stopping when either input ends. Pairing a finite list with [0..] attaches its zero-based indices without constructing an infinite result. With the unbounded Integer index type this avoids index overflow for arbitrarily long finite inputs.

The higher-order function takeWhile :: (a -> Bool) -> [a] -> [a] retains the longest initial segment whose elements all satisfy its predicate. It stops at the first failure and does not resume later. Thus it differs from a list filter, which tests later elements as well.

The higher-order function iterate :: (a -> a) -> a -> [a], satisfying iterate f x = x : iterate f (f x). Its result begins with the initial value and continues with successive applications. Lazy evaluation permits finite prefixes of this infinite list to be observed whenever the necessary applications terminate.

The higher-order function with type (a -> [b]) -> [a] -> [b] and value concat (map f xs). It replaces each input element by a list and concatenates those lists in input order. A direct definition is concatMap f = foldr (\x rest -> f x ++ rest) []. This formulation processes an input element's produced list once; it need not repeatedly append an already accumulated output prefix.

A partially applied infix operator: (a op) means \x -> a op x, and (op b) means \x -> x op b, subject to Haskell's syntax rules. In particular, (f .) maps a function g to the function composition f . g. Composing such a section with a curried binary function composes at two argument levels: ((f .) . g) x y = f (g x y) on total inputs.^[ref]

The automatically available standard module of Haskell, supplying basic types, classes and operations such as list folds, arithmetic and list comprehension support. A program can explicitly hide an imported name when providing an alternative definition. The reference definitions specify behavior, rather than requiring a particular implementation strategy.^[ref]