I am trying to write a function that retrieves the nth element in a list.
Here's what I have so far:
main :: IO ()
main = do print (nth 3 [1,2,10])
nth _ [] = []
nth a (x:xs)
| (a == 1) = x
| otherwise = nth (a-1) xs
This is the error I get:
Error occurred
ERROR line 2 - Cannot justify constraints in explicitly typed binding
*** Expression : main
*** Type : IO ()
*** Given context : ()
*** Constraints : (Show a, Num [a])
Why Num [a] constraint? Because in the [] clause you have your function returning a list, [], and in the other clause you have it return a list's element, x. Which means that it is only defined for lists whose elements are lists themselves. But you call it with a list of [1,2,10]. So Haskell sees a numeric literal 1 and wants to interpret it as a list that is also a number. It also needs it to be an instance of Show for print to be able to show it:
Prelude> :i print
print :: (Show a) => a -> IO () -- Defined in System.IO
Prelude> let {nth _ [] = [];nth a (x:xs) = x} -- Prelude> :i nth -- nth :: t -> [[a]] -> [a].You are returning list in clause
nth _ [] = []
and a list element in clause
nth a (x:xs)
| (a == 1) = x
| otherwise = nth (a-1) xs
so the type of your function is
*Main> :t nth
nth :: (Eq a1, Num a1) => a1 -> [[a]] -> [a]
you can check this out. Your function can not return element of any index for any list, so you must use Maybe to return "element not found"
nth _ [] = Nothing
nth a (x:xs)
| (a == 1) = Just x
| otherwise = nth (a-1) xs
*Main> :t nth
nth :: (Eq a1, Num a1) => a1 -> [a] -> Maybe a
Here we go:
nth a (x:xs)
| (a == 1) = x
| otherwise = nth (a-1) xs
nth(this is a good practice anyway). That should make the problem here more apparent.nthto return an element of the list, you surely want it to have typeInt -> [a] -> a, right? Well, your first definition,nth _ [] = [], is a list (or "returns a list", if you prefer), so it's a definition of a function of typeInt -> [a] -> [a]. So what shouldnthdo with an empty list? That's a good question! One way is to simply have it fail, another is to give it return typeMaybe a, withNothingreturned in case the list is too short.nthis supposed to be, the compiler figures out some amazing (and often rather ridiculous) signature and only notices a the call site (here,main) that it's bogus. That's why you get an error message demandigNum [a], which is obviously weird and one of the typical signs that you've made an error somewhere else and didn't build a safety-net in form of an explicit type signature there.