How to resolve the algorithm Quine step by step in the Haskell programming language

Published on 7 June 2024 03:52 AM
#Haskell

How to resolve the algorithm Quine step by step in the Haskell programming language

Table of Contents

Problem Statement

A quine is a self-referential program that can, without any external access, output its own source.

A   quine   (named after Willard Van Orman Quine)   is also known as:

It is named after the philosopher and logician who studied self-reference and quoting in natural language, as for example in the paradox "'Yields falsehood when preceded by its quotation' yields falsehood when preceded by its quotation." "Source" has one of two meanings. It can refer to the text-based program source.
For languages in which program source is represented as a data structure, "source" may refer to the data structure: quines in these languages fall into two categories: programs which print a textual representation of themselves, or expressions which evaluate to a data structure which is equivalent to that expression. The usual way to code a quine works similarly to this paradox: The program consists of two identical parts, once as plain code and once quoted in some way (for example, as a character string, or a literal data structure). The plain code then accesses the quoted code and prints it out twice, once unquoted and once with the proper quotation marks added. Often, the plain code and the quoted code have to be nested.

Write a program that outputs its own source code in this way. If the language allows it, you may add a variant that accesses the code directly. You are not allowed to read any external files with the source code. The program should also contain some sort of self-reference, so constant expressions which return their own value which some top-level interpreter will print out. Empty programs producing no output are not allowed. There are several difficulties that one runs into when writing a quine, mostly dealing with quoting:

Next to the Quines presented here, many other versions can be found on the Quine page.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Quine step by step in the Haskell programming language

The provided Haskell code consists of several expressions that perform string manipulation and logging using the putStrLn function, along with the use of lambda expressions and function composition. Let's break down each expression step by step:

  1. let q s = putStrLn (s ++ show s) in q "let q s = putStrLn (s ++ show s) in q ":

    • This is a let binding that defines a function q which takes a string s as input and prints the concatenation of s and its string representation (show s) using putStrLn.
    • The q function is then immediately applied to the string "let q s = putStrLn (s ++ show s) in q ".

  2. (\s -> putStrLn (s ++ show s)) "(\\s -> putStrLn (s ++ show s)) ":

    • This is a lambda expression that defines an anonymous function that takes a string s as input and prints the concatenation of s and its string representation.
    • The lambda expression is applied to the string "(\\s -> putStrLn (s ++ show s)) ".

  3. (putStrLn . \s -> s ++ show s) "(putStrLn . \\s -> s ++ show s) ":

    • This is a function composition using the dot operator (.).
    • It composes the putStrLn function with the lambda expression defined in the previous step, resulting in a new function that takes a string s and prints the concatenation of s and its string representation.
    • The new function is then applied to the string "(putStrLn . \\s -> s ++ show s) ".

  4. (putStrLn . ap (++) show) "(putStrLn . ap (++) show) ":

    • This is a function composition using the ap function from the Control.Monad.Reader module.
    • The ap function applies a function wrapped in the Reader monad to a value.
    • In this case, the ap function applies the putStrLn function to the result of applying the (++) function (which concatenates two strings) to the show function, which converts a value to a string.
    • The result is a function that takes a string s and prints the concatenation of s and its string representation.
    • The new function is then applied to the string "(putStrLn . ap (++) show) ".

  5. ap(++)show"ap(++)show":

    • This is the result of applying the ap function with the (++) and show functions as arguments, resulting in a function that takes a string s and returns the concatenation of s and its string representation.

  6. main = putStrLn $ (++) <*> show $ "main = putStrLn $ (++) <*> show $ ":

    • This is the main function of the program.
    • It applies the putStrLn function to the result of applying the (++) function (using the infix operator <*>) to the show function.
    • The result is the concatenation of the string "main = putStrLn $ (++) <*> show $ " and its string representation, which is then printed using putStrLn.

Source code in the haskell programming language

let q s = putStrLn (s ++ show s) in q "let q s = putStrLn (s ++ show s) in q "


(\s -> putStrLn (s ++ show s)) "(\\s -> putStrLn (s ++ show s)) "


(putStrLn . \s -> s ++ show s) "(putStrLn . \\s -> s ++ show s) "


import Control.Monad.Reader
(putStrLn . ap (++) show) "(putStrLn . ap (++) show) "


ap(++)show"ap(++)show"


main = putStrLn $ (++) <*> show $ "main = putStrLn $ (++) <*> show $ "

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