Haskell:
Using a debugger
How to:
Let’s take a stroll with GHCi, Haskell’s interactive environment that can act as a basic debugger. You fire it up with your Haskell code and start poking around. Here’s an example:
main :: IO ()
main = do
putStrLn "Hey, what's your name?"
name <- getLine
putStrLn $ "Hello, " ++ name ++ "! Let's debug."
let result = buggyFunction 5
print result
buggyFunction :: Int -> Int
buggyFunction n = n * 2 -- Pretend there's a bug hereTo start debugging with GHCi:
$ ghci YourHaskellFile.hsSet a breakpoint at buggyFunction:
Prelude> :break buggyFunctionRun your program:
Prelude> :main
Hey, what's your name?Your program pauses at buggyFunction. Now you can inspect variables, step through code, and evaluate expressions.
Deep Dive:
Historically, Haskell’s reputation for pure functions and strong typing led to the belief that debugging tools were less critical. The reality is different—complex programs always benefit from good debugging tools. GHCi provides basic debugging commands. However, for a more visual experience or larger-scale applications, you might explore IDEs with integrated debuggers, like Visual Studio Code with Haskell extensions or IntelliJ’s Haskell plugin.
Alternatives to debugger include using print statements, known as “printf debugging,” or leveraging Haskell’s strong type system to make incorrect states unrepresentable. Yet, nothing replaces stepping through the code sometimes.
As for implementation details, Haskell’s debugger works with the runtime system. It can handle breakpoints, step execution, and allow variable inspection. However, since Haskell is lazily evaluated, things can get a bit non-intuitive. Debugging a Haskell program often means keeping an eye on when and how expressions are evaluated.