在Haskell中处理异常

Question

I write the following code:

give :: [b] -> Int -> b
give list index = list !! index

Now I want to add, if the there is no item at point index, it should show: "No item at that position!"

[1..10] `give` 10
No item at that position!

How can I add this in haskell

Answer 1

You can't really. You can throw exceptions from pure code but you can only catch exceptions in IO .

You could either reimplement !! in give to throw an exception more to your liking or just opt for saner error handling, like Either or Maybe .

An example of handling errors with Either might be

data OutOfRange = OutOfRange Int

give :: [a] -> Int -> Either OutOfRange a
give xs i | length xs > i = Right $ xs !! i
          | otherwise     = Left (OutOfRange i)

Answer 2

I would wrap the return type in an Either monad like this:

give :: [b] -> Int -> Either String b
give [] _ = Left "No item at that position!"
give (x:xs) index | index == 0 = Right x
                  | otherwise  = give xs $ index - 1

Either is a data constructor whose declaration could theoretically look like this:

data Either a b = Left a | Right b

Right and Left are value constructors for the different types that may be used, in our case, b and String .

This function returns Either a String or a b depending on whether or not the indexing succeeded. It uses pattern matching to fail when indexing into an empty list and uses recursion to eventually emulate !! . Note that due to the strictness of Haskell's type system, you cannot use this result as it were simply of type b : you must explicitly handle the possibility of it being a String. Here is an example:

case (give [1..10] 10) of
    (Left s)  -> putStrLn $ "Error" ++ s                                 --String case
    (Right i) -> putStrLn $ "The value you requested is " ++ show $ i    --Int case

Using Either and Maybe is generally a better idea than signaling errors because it allows your code to gracefully handle error conditions within the Haskell type system with simple pattern matching.