What wrong in my code? I need a help

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#1

I am studying haskell language , doing some practices. Now I am attempting to implement my own ‘index’ function , meaning it can return the index of the given element within a list, but now my version fails to run , as the following

index' n (s:x) 
    | s/=n  =  1+index' x
    | otherwise  =  0
main = print(index' 3 [1,2,3,5,6,7,9])

However , if simplifying the argument into one and set the target number in the guard , as the following , it run.

index' (s:x) 
    | s/=3  =  1+index' x
    | otherwise  =  0
main = print(index' [1,2,3,5,6,7,9])

Comparing the two program , I can find what make the former fail . So I think there may be a elemental mistake in my understanding , and I need a help , please !

#2

Hi,

your first version does not compile (I guess that implies “not running” but please include this info the next time :wink: ).

The problem is that you need to pass n to index' in the recursive call:

index' n (s : x)
  | s /= n = 1 + index' n x
  | otherwise = 0

this should compile, run and work with your example.

Please note that both versions are partial (for example index' 9 [1,2,3] will run into an non-exhaustive pattern error at runtime) but I guess you’ll expect that.

#3 
| s/=n  =  1+index' x

index' needs two inputs, but you’re only giving it one.

#4

thank you ! your suggestions are illuminating for me . I am a beginner but find the language is amazing . I will do more practices to understand more details .

#5

thank you ! yes , I make a low mistake , thank you for your comment !

#6

The other replies already answered your question, but I have some advice on debugging.

I suggest adding a type signature so that the compiler can give you better advice. If it’s difficult to work out the type, you can look at the error message for a hint. In your example, you can add the following line to your code:

index' :: a-> [a] -> a3

But with this type signature, the error you get is still quite confusing. If you make the type more specific, it can make the error message more helpful. So a better choice of type signature for debugging would be:

index' :: Int -> [Int] -> Int
#7

thank you ! a helpful advice !