Indeed "Int b" is not a valid constraint: the kind of "Int" is Type (or "*") as GHC reports:

> Expecting one fewer arguments to ‘Int’
> Expected kind ‘* -> Constraint’, but ‘Int’ has kind ‘*’

A valid constraint would be "Int ~ b" as in the following example. But I don't see why you would do this in this case, especially if you are beginning with Haskell. It complicates the code for no gain.

{-# LANGUAGE TypeFamilies #-}

elementAt'''' ::(Int ~ b)=>[a]-> b ->a -- #2
elementAt'''' [] _= error "list is empty"
elementAt'''' list index
   | (index < 1) = error "index has to be positive number"
   | otherwise= list !! (index-1)

Cheers
Sylvain

What is the difference between these two type declarations? The second one is wrong but I can't convince myself why it should be wrong. Is it because Int not a constraint class and it is only an instance of one? My curiousity is why #1 can't be written in the form of #2. I apologise if I am using wrong terminology as type, class and constraint class are used with not much distinction. To add context, this problem is to find a function that will find an element by passing in a list and an index argument.

elementAt''' ::[a]-> Int ->a -- #1

elementAt''' [] _= error "list is empty"

elementAt''' list index 

| (index < 1) = error "index has to be positive number"

| otherwise= list !! (index-1)

elementAt'''' ::(Int b)=>[a]-> b ->a -- #2

elementAt'''' [] _= error "list is empty"

elementAt'''' list index 

| (index < 1) = error "index has to be positive number"

| otherwise= list !! (index-1)

Thank you. I just began learning Haskell. 

Yours sincerely,

Justin

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