在 Maybe 中使用 isNothing function 时出现 Haskell 错误

Question

I am trying to use isNothing in my code in Haskell, and it is giving me the error

<interactive>:97:23: error:
• Variable not in scope: isNothing :: Maybe t -> Bool
• Perhaps you meant data constructor ‘Nothing’ (imported from Prelude)

The code line I have is as follows -

 maybeMap f value = if isNothing (value) then value else Just (f (check value))

this works fine if I replace isNothing value with value == Nothing , so I am confused why the previous is not working.

Answer 1

First thing's first, the key phrase in the error message is:

Variable not in scope: isNothing

Which means that the compiler just isn't aware of anything named isNothing .

That immediately tells you that the code around your your use of isNothing doesn't matter . It's not a problem with types, or anything to do with the actual isNothing function you're trying to call, and there's no way you can change the code around isNothing to get this to work.

Variable not in scope almost always means one of three things:

1. You haven't imported the name you're trying to use
2. You have accidentally misspelled the name you're trying to use
3. You intended to define something with that name, but haven't done so yet

Changing any of the code surrounding your use of isNothing isn't going to change any of those 3 problems no matter which it is. Even looking at that code isn't going to tell you anything relevant; just closely look at the spelling of the name in the error message to confirm you haven't just made a typo, and if not you know you need to look elsewhere.

In this case it's #1. There are a bunch of useful functions that are in the Haskell Prelude which are automatically imported for you, so you're probably used to just using functions without importing them, but the "normal" case is that to use anything that's already defined you have to import it. isNothing isn't in the Prelude , so that means to use it you have to find out which module it is in and add an import declaration to make it available. (If that module is in a package that isn't already installed, you will also have to obtain the package; that's a question I'm not going to address here)

isNothing comes from the Data.Maybe module (in the base package, which is always installed as part of installing GHC, so no worries there). So you need to use:

import Data.Maybe

If you're working in a file you need to add that to the top of the file (just after the module header, but before you define any names yourself; all imports must come before any of your own code). If you're using the interpreter you can just enter the import as a command.

That will bring all of the names defined in Data.Maybe into scope. If you want more control, you can explicitly import only some of the names, like this:

import Data.Maybe ( isNothing, isJust, listToMaybe )

Answer 2

The function isNothing is not part of the standard prelude . Rather, it's distributed as part of the Data.Maybe module. To use isNothing , you'll need to explicitly import that module:

import Data.Maybe