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# How to Re-Use A Function to Effectively 'Loop' In Haskell?

In Haskell, you can create a recursive function to effectively loop through a specific action multiple times. By designing the function to call itself within its own definition, you can achieve a looping effect without having to explicitly use traditional looping constructs like for or while loops. This can be done by specifying a base case where the function stops calling itself and returns a final result, ensuring that the loop terminates at a certain condition. This recursive approach allows for concise and elegant code that takes advantage of Haskell's functional programming capabilities.

## Top Rated Haskell Books of June 2024

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• Cambridge University Press
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Practical Haskell: A Real World Guide to Programming

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• O Reilly Media
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Learn You a Haskell for Great Good!: A Beginner's Guide

• No Starch Press
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Parallel and Concurrent Programming in Haskell: Techniques for Multicore and Multithreaded Programming

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Haskell: The Craft of Functional Programming (International Computer Science Series)

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Haskell Design Patterns: Take your Haskell and functional programming skills to the next level by exploring new idioms and design patterns

## How to pass functions as parameters in Haskell?

To pass a function as a parameter in Haskell, you can simply define the parameter as a function type. Here is an example to demonstrate this:

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ``` ```-- Define a function that takes another function as a parameter applyFunc :: (Int -> Int) -> Int -> Int applyFunc f x = f x -- Define the function that will be passed as a parameter double :: Int -> Int double x = x * 2 -- Call applyFunc with the function double as a parameter result :: Int result = applyFunc double 5 -- Output the result main :: IO () main = print result ```

In this example, the `applyFunc` function takes a function of type `(Int -> Int)` as its first parameter, and an `Int` as its second parameter. The `double` function is defined as a function that doubles its input. The `applyFunc` function is then called with the `double` function and the input `5`, resulting in the output `10`.

This is how you can pass functions as parameters in Haskell.

## How to define a function in Haskell?

In Haskell, functions are defined using the following syntax:

 ```1 2 ``` ```functionName :: Type1 -> Type2 -> ... -> ReturnType functionName arg1 arg2 ... = ```

Here, `functionName` is the name of the function, `Type1`, `Type2`, etc. are the types of the input arguments, and `ReturnType` is the type of the return value. The `->` symbol is used to indicate the function type.

For example, let's define a simple function in Haskell that adds two numbers:

 ```1 2 ``` ```add :: Int -> Int -> Int add x y = x + y ```

In this example, `add` is the function name, `Int -> Int -> Int` specifies that the function takes two `Int` arguments and returns an `Int`, and `x + y` is the body of the function that adds the two arguments.

## How to write lambda functions in Haskell?

A lambda function in Haskell is written using the backslash symbol "" followed by the input parameters and then an arrow "->" before the body of the function. Here is the general syntax:

\input -> body

For example, a lambda function that adds two numbers together would be written as:

 ```1 ``` ```(\x y -> x + y) ```

Lambda functions are commonly used in Haskell to create anonymous functions that can be passed as arguments to higher-order functions. Here is a more complex example that uses a lambda function with the `map` function to double each element in a list:

 ```1 ``` ```map (\x -> x * 2) [1, 2, 3, 4, 5] ```

This will output `[2, 4, 6, 8, 10]`, as each element in the list is doubled using the lambda function `(\x -> x * 2)`.

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