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For example, digit parses a digit, and string parses a specific string (like "hello"). Parser combinator libraries like Parsec provide utility functions to run the parsers on real values. A parser to recognize a single digit from a string can be split into two functions: one to create the parser, and a main function that calls one of these ...
The creations of these functions can be automated by Haskell's data record syntax. This OCaml example which defines a red–black tree and a function to re-balance it after element insertion shows how to match on a more complex structure generated by a recursive data type. The compiler verifies at compile-time that the list of cases is ...
Folds can be regarded as consistently replacing the structural components of a data structure with functions and values. Lists, for example, are built up in many functional languages from two primitives: any list is either an empty list, commonly called nil ([]), or is constructed by prefixing an element in front of another list, creating what is called a cons node ( Cons(X1,Cons(X2,Cons ...
List comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical set-builder notation (set comprehension) as distinct from the use of map and filter functions.
In Haskell, the code example filter even [ 1 .. 10 ] evaluates to the list 2, 4, …, 10 by applying the predicate even to every element of the list of integers 1, 2, …, 10 in that order and creating a new list of those elements for which the predicate returns the Boolean value true, thereby giving a list containing only the even members of ...
A simple example (in Haskell) is a program which computes the sum of a list of numbers. We can define the sum function recursively using a pointed style (cf. value -level programming ) as: sum ( x : xs ) = x + sum xs sum [] = 0
Glasgow Haskell, Idris, and F# offer language features designed to ease programming with applicative functors. In Haskell, applicative functors are implemented in the Applicative type class. While in languages like Haskell monads are applicative functors, it is not always so in general settings of Category Theory - examples of monads that are ...
Much of Haskell code is similar to standard mathematical notation in facility and syntax. The first line of the factorial function describes the type of this function; while it is optional, it is considered to be good style [1] to include it. It can be read as the function factorial (factorial) has type (::) from integer to integer (Integer ...