Haskell:
Refactoring

How to:

Let’s say you’ve got a chunk of Haskell code that’s repeating itself more than your favorite song. Here’s a quick look at how you might refactor that using functions.

Before refactoring:

printInvoice :: String -> Float -> String -> IO ()
printInvoice customer total item = do
  putStrLn $ "Customer: " ++ customer
  putStrLn $ "Total: " ++ show total
  putStrLn $ "Item: " ++ item

After a bit of refactoring:

printDetail :: String -> String -> IO ()
printDetail label value = putStrLn $ label ++ ": " ++ value

printInvoice :: String -> Float -> String -> IO ()
printInvoice customer total item = do
  printDetail "Customer" customer
  printDetail "Total" (show total)
  printDetail "Item" item

-- Sample output:
-- Customer: Alice
-- Total: $42.00
-- Item: Haskell Programming Guide

As you can see, by extracting the common pattern into a separate printDetail function, we avoid repetition and make printInvoice clearer and easier to manage.

Deep Dive

When Haskell hit the scene in the late ’80s, it was clear that the functional paradigm could bring some fresh air to coding practices. Fast forward, and refactoring in Haskell is particularly elegant thanks to functions being first-class citizens and its strong static type system. You refactor without fearing that you’d break your app since the compiler’s got your back.

Alternatives to manual refactoring may include using automated tools, though the functional nature and type safety of Haskell can sometimes make this less prevalent compared to other languages. Implementation-wise, it’s important to leverage Haskell’s features such as higher-order functions, purity, and immutability to make refactoring smoother.

Refactorings like “Extract Function”, just showcased, are common, but you can also do “Inline Function”, “Rename Variable”, and “Change Function Signature” with confidence, thanks to the type system. Haskell’s powerful type inference can sometimes catch errors that would slip through in other languages.

See Also

For a deep dive into refactoring in Haskell, hit the books with “Refactoring: Improving the Design of Existing Code” by Martin Fowler, where the concepts are universally applicable. Check out the hlint tool for automated hints on improving your Haskell code. Also, swing by the Haskell wiki (https://wiki.haskell.org/Refactoring) for community insights and further reading.