How to partially apply functions in Elm
Elm is a functional programming language for creating web applications that offers various features to enable developers to write clean, maintainable, and type-safe code. One such feature is partial function application, also known as currying.
Understanding partial function application
Partial function application (or currying), is a technique in functional programming that allows you to fix a certain number of arguments of a function, generating a new function that accepts the remaining arguments. This approach is particularly useful in cases where you need to reuse a function with some arguments fixed while varying the others.
In Elm, all functions are curried by default. This means that when you define a function with multiple arguments, Elm automatically translates it into a series of single-argument functions. Each of these functions returns a new function that accepts the next argument until all arguments are provided.
Benefits of partial function application
Partial function application offers several advantages:
Code reusability: By fixing some arguments, we can create specialized versions of a generic function without having to rewrite the entire function.
Improved readability: Partially applied functions can make our code more concise and easier to understand since we can create more descriptive names for specialized functions.
Enhanced composition: Partial function application enables better function composition, allowing us to build complex behaviors from simpler functions.
Using partial function application in Elm
Let's look at some examples to understand how to partially apply functions in Elm:
Basic partial application
Suppose we have a simple function that adds two numbers:
add : Int -> Int -> Intadd x y =x + y
We can create a new function called addFive that adds 5 to its argument by partially applying the add function:
addFive : Int -> IntaddFive =add 5
Now, we can use addFive to add 5 to any number:
result : Intresult =addFive 3 -- result will be 8
Partial application with multiple arguments
Consider a function that calculates the volume of a rectangular box:
volume : Float -> Float -> Float -> Floatvolume length width height =length * width * height
We can create a new function called cuboidVolume that calculates the volume of a cuboid with fixed length and width:
cuboidVolume : Float -> FloatcuboidVolume =volume 10 20
Now, we can use cuboidVolume to calculate the volume for various heights:
result1 : Floatresult1 =cuboidVolume 30 -- result1 will be 6000result2 : Floatresult2 =cuboidVolume 40 -- result2 will be 8000
Partial application and function composition
Partial function application can be combined with function composition to create more complex behaviors. Suppose we have a list of integers, and we want to add a constant value to each element and then filter out the even numbers. You can achieve this using partial application and function composition:
add : Int -> Int -> Intadd x y =x + yisEven : Int -> BoolisEven x =x % 2 == 0addConstantAndFilterEven : List Int -> Int -> List IntaddConstantAndFilterEven list constant =List.filter isEven (List.map (add constant) list)
In this example, the addConstantAndFilterEven function takes a list of integers and a constant as input. It uses the List.map function to create a new list by adding the constant to each element of the input list. Then, it filters the even numbers using the List.filter function.
You can now use this function to add a constant value to a list of integers and filter out the even numbers:
inputList : List IntinputList =[ 1, 2, 3, 4, 5 ]constant : Intconstant =3result : List Intresult =addConstantAndFilterEven inputList constant -- result will be [4, 6, 8]
Conclusion
Partial function application, or currying, is a powerful technique in functional programming languages like Elm. It enables developers to fix a certain number of arguments of a function, creating new, specialized functions without duplicating code. By enhancing code reusability, readability, and composition, partial function application plays a crucial role in creating clean and maintainable Elm applications.
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