How to resolve the algorithm Extend your language step by step in the Haskell programming language

Published on 7 June 2024 03:52 AM
#Haskell

How to resolve the algorithm Extend your language step by step in the Haskell programming language

Table of Contents

Problem Statement

Some programming languages allow you to extend the language. While this can be done to a certain degree in most languages (e.g. by using macros), other languages go much further. Most notably in the Forth and Lisp families, programming per se is done by extending the language without any formal distinction between built-in and user-defined elements. If your language supports it, show how to introduce a new flow control mechanism. A practical and useful example is a four-way branch: Occasionally, code must be written that depends on two conditions, resulting in up to four branches (depending on whether both, only the first, only the second, or none of the conditions are "true"). In a C-like language this could look like the following: Besides being rather cluttered, the statement(s) for 'condition2isTrue' must be written down twice. If 'condition2isTrue' were a lengthy and involved expression, it would be quite unreadable, and the code generated by the compiler might be unnecessarily large. This can be improved by introducing a new keyword if2. It is similar to if, but takes two conditional statements instead of one, and up to three 'else' statements. One proposal (in pseudo-C syntax) might be: Pick the syntax which suits your language. The keywords 'else1' and 'else2' are just examples. The new conditional expression should look, nest and behave analogously to the language's built-in 'if' statement.

Let's start with the solution:

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

Overview

The provided code snippet is a Haskell function that implements a conditional expression (if2). It takes two boolean predicates (p1 and p2) and four values (e12, e1, e2, and e), and returns one of the four values based on the combination of the predicates' truth values.

Function Definition

if2 :: Bool -> Bool -> a -> a -> a -> a -> a

The if2 function is a curried function. It takes the following arguments:

  • p1: A boolean predicate.
  • p2: A boolean predicate.
  • e12: The value to return if both p1 and p2 are True.
  • e1: The value to return if p1 is True and p2 is False.
  • e2: The value to return if p1 is False and p2 is True.
  • e: The value to return if both p1 and p2 are False.

Implementation

The implementation of the if2 function uses nested if statements to evaluate the predicates and return the appropriate value:

if2 p1 p2 e12 e1 e2 e =
 if p1 then
   if p2 then e12 else e1
   else if p2 then e2 else e
  1. It first checks if p1 is True. If it is, it proceeds to the inner if statement.
  2. In the inner if statement, it checks if p2 is True. If it is, it returns e12. Otherwise, it returns e1.
  3. If p1 is False, it checks if p2 is True. If it is, it returns e2. Otherwise, it returns e.

Main Function

main = print $ if2 True False (error "TT") "TF" (error "FT") (error "FF")

The main function demonstrates the usage of the if2 function. It calls if2 with the following arguments:

  • p1: True
  • p2: False
  • e12: An error value with the message "TT"
  • e1: The string "TF"
  • e2: An error value with the message "FT"
  • e: An error value with the message "FF"

Since p1 is True and p2 is False, the function returns the value e1, which is the string "TF". This string is then printed to the console using the print function.

Source code in the haskell programming language

if2 :: Bool -> Bool -> a -> a -> a -> a -> a
if2 p1 p2 e12 e1 e2 e =
  if p1 then
    if p2 then e12 else e1
    else if p2 then e2 else e

main = print $ if2 True False (error "TT") "TF" (error "FT") (error "FF")

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