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 On 28/03/2018 20:19, Justin Thong wrote:
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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