Haskell新手求助：Prelude.undefined错误排查

Hey there! Let's break down this super common Haskell error you're hitting—it's actually easier to fix than it looks at first glance.

First, let's unpack the error message piece by piece:

*** Exception: Prelude.undefined
CallStack (from HasCallStack):
error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
undefined, called at fileName.hs:35:12 in main:Main

• The core issue here is Prelude.undefined: In Haskell, undefined is a built-in placeholder value. It's basically Haskell's way of saying "this part of the code isn't finished yet"—and when your program tries to run that line, it panics with this exception.
• The CallStack line is your direct guide: undefined, called at fileName.hs:35:12 tells you exactly where to look. Open up your fileName.hs file, jump to line 35, and check the code around the 12th character. That's where the problematic undefined is hiding.

How to Fix It #

• Head straight to that line in your file. You'll almost certainly see something like this:

  myUnfinishedFunction = undefined
  -- Or maybe undefined is nested inside another expression, like:
  calculateTotal items = undefined + sum items
  

• Replace that undefined with actual working code that does what you intended. For example, if it's a function that should return the square of a number, you'd write:

  square x = x * x
  

Common Reasons New Haskell Devs Hit This #

• You used undefined as a placeholder while sketching out code and forgot to fill it in (we've all done this!).
• You accidentally typed undefined instead of a value or function you meant to use.
• You're following a tutorial that used undefined as an exercise prompt, and you need to replace it with your solution.

If you're still stuck after checking line 35, feel free to share the code snippet around that line, and we can help you work out what to put there instead!

内容的提问来源于stack exchange，提问作者Trent Frowiss