Best Practices for Writing Efficient Haskell Code

Are you tired of your Haskell code running too slow? Do you want to improve your application's performance to make it faster and more efficient? Look no further! In this article, we will explore the best practices for writing efficient Haskell code.

1. Avoid Lazy Evaluation When Possible

Lazy evaluation is one of Haskell's key features. It allows the language to evaluate expressions only when it's needed. However, this feature can also slow down your program.

In some cases, strict evaluation can be more efficient. Consider using strict data types when possible, like Data.Int and Data.Word. Also, use strict evaluation when dealing with small data sets or when you know the exact shape of your data.

2. Use Proper Data Structures

Choosing the right data structure can make a significant difference in terms of performance. Haskell supports several data structures, including lists, arrays, and maps.

When you need to access elements by their index, use a vector or an array instead of a list. This can significantly reduce the time complexity of your code. For example, Data.Vector and Data.Array modules provide efficient implementations of arrays.

For mappings and key-value stores, use Data.Map, which provides a log-time access to its elements.

3. Use Strictness Annotations

Strictness annotations can help you to control the evaluation strategy of your Haskell code at a more granular level. By using the ! operator, you can force the evaluation of an expression.

For example, consider the following code:

let x = 2
let y = 3
let z = x + y

In this code, x and y are not evaluated until z is needed. However, by using strictness annotations, we can force the evaluation of x and y:

let !x = 2
let !y = 3
let z = x + y

4. Memoize Functions

If a function's output does not change given a set of inputs, consider memoizing the function. Memoization can improve performance by caching previous results and returning them if the same input is provided again.

Haskell provides a simple way to memoize functions using the memoize function from the Data.Function.Memoize module. For example:

import Data.Function.Memoize

fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib' (n-1) + fib' (n-2)
    where fib' = memoize fib

In this code, fib' is memoized, which reduces the number of calls to the fib function and makes the code more efficient.

5. Use Streaming I/O

Haskell's lazy evaluation is particularly useful in I/O operations. It allows you to process large data streams efficiently without loading them all into memory at once.

Consider using the conduit or pipes libraries to process data streams. These libraries provide a way to create complex data stream pipelines very efficiently.

Here's an example of using conduit to filter a large file and write the result to a new file:

import Conduit

main :: IO ()
main = runConduitRes $ sourceFile "source.txt" .| filterC (\c -> c /= '\n') .| sinkFile "output.txt"

In this code, we read a file source.txt, filter all line breaks, and write the result to a new file output.txt. The entire process is done in a streaming fashion, making the code very efficient.

6. Use Strict Foldl'

When processing lists in Haskell, it's common to use the foldl or foldr functions. However, using foldl' instead of foldl can make a big difference in terms of performance.

The foldl' function is strict in its accumulator, which means that the intermediate results are evaluated as the algorithm proceeds. In contrast, foldl is lazy, which can cause space leaks and slow down the program.

import qualified Data.List as L

sum' :: [Int] -> Int
sum' = L.foldl' (+) 0

7. Use Parallelism

Haskell has excellent support for parallelism. Leveraging parallelism can speed up your program by taking advantage of multiple CPU cores.

The par and pseq operations can be used to divide computations across multiple threads. However, using parallelism requires careful consideration of the trade-offs and potential pitfalls.

Ensure that the parts of your program that are run in parallel have no dependencies on each other. Use profiling tools like ThreadScope to diagnose performance issues and improve your code.


Haskell is a powerful language that provides a lot of tools for writing efficient code. By following these best practices, you can ensure that your code runs fast and efficiently.

By avoiding lazy evaluation when possible, using proper data structures, and using strictness annotations, you can make your code more efficient. Additionally, memoization can help you cache results, streaming I/O can enable you to handle large data streams, and parallelism can take advantage of multiple CPU cores to speed up computations.

Have you used any of these best practices before? What other tips do you have for writing efficient Haskell code? Let us know in the comments!

Editor Recommended Sites

AI and Tech News
Best Online AI Courses
Classic Writing Analysis
Tears of the Kingdom Roleplay
Cloud Architect Certification - AWS Cloud Architect & GCP Cloud Architect: Prepare for the AWS, Azure, GCI Architect Cert & Courses for Cloud Architects
Tech Debt - Steps to avoiding tech debt & tech debt reduction best practice: Learn about technical debt and best practice to avoid it
Kubernetes Tools: Tools for k8s clusters, third party high rated github software. Little known kubernetes tools
Learn Prompt Engineering: Prompt Engineering using large language models, chatGPT, GPT-4, tutorials and guides
Developer Key Takeaways: Key takeaways from the best books, lectures, youtube videos and deep dives