Can someone please explain how I can fix my program. Very new to Haskell, been trying to create a length
function that calculates the length of a list of any type.
I am aiming to do this using data
as I want to create a brand new type to do so (this is the area of Haskell that I'm currently learning, which is why it might not be the most efficient implementation of this function)
data List a = Nil | Cons a (List a)
len :: List a -> Int
len Nil = 0
len (Cons _ xs) = 1 + len xs
If I run it on len [1,2,3]
I get the error:
• Couldn't match expected type ‘List a0’
with actual type ‘[Integer]’
• In the first argument of ‘len’, namely ‘[1, 2, 3]’
In the expression: len [1, 2, 3]
In an equation for ‘it’: it = len [1, 2, 3]
The function definition is correct, but [1,2,3]
is not a List a
object, it is a [a]
(or more canonical [] a
). A list like [1,2,3]
as List Int
is:
len
Alternatively, you can make List a
an instance of the IsList
type class, and then use the -XOverloadedLists
extension:
{-# LANGUAGE #-}
import GHC.Exts(IsList(Item, fromList, toList))
instance where
type Item (List a) = a
fromList = foldr Cons Nil
toList Nil = []
toList (Cons x xs) = x : toList xs
Then we can use the OverloadedLists
extension:
$ ghci -XOverloadedLists -XTypeFamilies
GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/kommusoft/.ghci
Prelude> data List a = Nil | Cons a (List a)
Prelude> import GHC.Exts(IsList(Item, fromList, toList))
Prelude GHC.Exts> :{
Prelude GHC.Exts| instance IsList (List a) where
Prelude GHC.Exts| type Item (List a) = a
Prelude GHC.Exts| fromList = foldr Cons Nil
Prelude GHC.Exts| toList Nil = []
Prelude GHC.Exts| toList (Cons x xs) = x : toList xs
Prelude GHC.Exts| :}
Prelude GHC.Exts> :{
Prelude GHC.Exts| len :: List a -> Int
Prelude GHC.Exts| len Nil = 0
Prelude GHC.Exts| len (Cons _ xs) = 1 + len xs
Prelude GHC.Exts| :}
Prelude GHC.Exts> len [1,2,3]
3
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