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# How to Call A Function In Haskell?

In Haskell, calling a function involves providing the function name followed by the required arguments. The basic syntax to call a function is as follows:

 ```1 ``` ```functionName argument1 argument2 ... ```

Here, `functionName` is the name of the function you want to call, and `argument1`, `argument2`, etc. are the arguments or inputs required by the function. You can provide as many arguments as required by the function's definition.

For example, consider a simple function `add` that takes two numbers as arguments and returns their sum:

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

To call this `add` function and pass the numbers 2 and 3 as arguments, you can do:

 ```1 ``` ```result = add 2 3 ```

In this case, `result` will hold the value 5, which is the sum of 2 and 3.

If a function takes multiple arguments, you can also partially apply it by providing fewer arguments. This results in a new function that takes the remaining arguments. For example, consider a function `multiply` that multiplies three numbers:

 ```1 2 ``` ```multiply :: Int -> Int -> Int -> Int multiply x y z = x * y * z ```

To partially apply this function and get a new function that multiplies by 2 and 3, you can do:

 ```1 ``` ```multiplier = multiply 2 3 ```

Now, `multiplier` will be a new function that takes a single argument and returns the result of multiplying it by 2 and 3.

In Haskell, functions are typically called using currying, which means that a function that takes multiple arguments is actually a chain of functions, each taking one argument and returning a new function. This allows for easy partial application and function composition.

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Effective Haskell: Solving Real-World Problems with Strongly Typed Functional Programming

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Effective Haskell: Solving Real-World Problems with Strongly Typed Functional Programming

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Practical Haskell: A Real-World Guide to Functional Programming

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## What happens if you call a function that is not defined in Haskell?

If you call a function in Haskell that is not defined, you will get a compilation error. Haskell uses static typing, so all functions need to be declared and defined before they can be used. If you try to call a function that has not been defined, the compiler will identify this as an error and provide a corresponding error message. This compilation error occurs because Haskell requires all functions to be defined prior to the execution of the program to ensure its correctness and consistency.

## How do you pass a function as an argument in Haskell?

In Haskell, you can pass a function as an argument by specifying the function's type in the function signature and then passing the function as a parameter when calling the function.

Here is an example:

 ``` 1 2 3 4 5 6 7 8 9 10 ``` ```applyTwice :: (a -> a) -> a -> a applyTwice f x = f (f x) addOne :: Int -> Int addOne x = x + 1 main :: IO () main = do let result = applyTwice addOne 5 putStrLn \$ "Result: " ++ show result ```

In the above example, the function `applyTwice` takes a function `f` as its first argument, followed by a value `x`. The function type `(a -> a)` indicates that `f` takes an argument of type `a` and returns a value of the same type. The return type of `applyTwice` is also `a`.

In the `main` function, we define a function `addOne` that increments an integer by 1. We then call `applyTwice` with `addOne` as the first argument and `5` as the second argument. The result of `applyTwice addOne 5` is `7`.

Note that the function being passed as an argument must have a compatible type with the expected function type in the function signature.

## Can a function in Haskell return a value?

Yes, a function in Haskell can return a value. Haskell is a pure functional language, which means that every function in Haskell must return a value. In fact, every expression in Haskell is evaluated to produce a value.

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