Best Practices for Writing Clean and Efficient Haskell Code

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That means no messy mutations or side effects allowed!
Here’s an example of what we mean by “pure” code: let’s say you want to add two numbers together. In Haskell, instead of using the traditional `+=` operator like in other languages (which can have unintended consequences), you would write it as a separate function that takes both numbers and returns their sum:

-- This is a function that takes two Integers and returns an Integer
addNumbers :: Int -> Int -> Int
-- The function is named "addNumbers" and takes two parameters, both of type Integer
addNumbers x y = x + y
-- The function body, which adds the two parameters together and returns the result

This code is “pure” because it doesn’t modify any variables or have any side effects it just calculates the sum of two numbers and returns that value. And since Haskell is a functional language, you can use this function over and over again without worrying about any unexpected changes to your data!
Now “strictness” another important concept in Haskell. By default, Haskell is lazy when it comes to evaluating expressions (which means it won’t actually calculate anything until you need the result). But sometimes that can lead to performance issues or unexpected behavior. To avoid this, we can use strictness annotations like `seq` and `!` to force evaluation at runtime:

-- This code is lazy because it doesn't evaluate x + y immediately
-- The variables x and y are defined but not evaluated
let x = 10
    y = 5
    result = (x * y) + 2 -- The result is not calculated until it is needed

-- But this code uses strictness annotations to force evaluation at runtime
-- The strictness annotations force immediate evaluation of the variables
let x = 10 ! -- The exclamation mark forces evaluation of x
    y = 5 ! -- The exclamation mark forces evaluation of y
    result = (x * y) + 2 -- The result is now calculated immediately

In the second example, we’re using `seq` and `!` to ensure that `x` and `y` are evaluated before they’re used in the calculation. This can be especially useful when dealing with large data structures or long-running computations.
By using strictness, avoiding excessive allocation, choosing appropriate data structures, using strict types, and profiling your code, you can ensure that your programs are both elegant and performant. And if you ever get stuck or need help, don’t hesitate to reach out to the vibrant Haskell community for support!

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