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 I check my email at 9AM and 4PM everyday If you have an EMERGENCY, contact me at +447938674419(UK) or +60125056192(Malaysia) _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners |
Indeed "Int b" is not a valid constraint: the kind of "Int" is Type (or "*") as GHC reports: > Expecting one fewer arguments to ‘Int’ 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 #-}
Cheers On 28/03/2018 20:19, Justin Thong
wrote:
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