I want to make a list of lists like this:
[1,2,3] [4,5,6] -> [[1,2,3], 4, 5, 6]
This what i have by now:
combine :: [a] -> [a] -> [[a]]
combine xs ys = [xs,ys]
But this code gives me: [[1, 2, 3], [4, 5, 6]] and is not what I need.
2 Answers
As m0nhawk writes in the comments, you can't directly have a Haskell List of both lists of integers and integers. There are several alternatives, though.
One alternative is indeed to use a list of lists of integers ([[1, 2, 3], [4], [5], [6]]), like this:
combine:: [Int] -> [Int] -> [[Int]]
combine xs ys = [xs] ++ [[y] | y <- ys]
main = do
putStrLn $ show $ combine [1, 2, 3] [4, 5, 6]
(running this indeed prints [[1, 2, 3], [4], [5], [6]]).
Another alternative is to use algebraic data types:
Prelude> data ScalarOrList = Scalar Int | List [Int] deriving(Show)
Prelude> [List [1, 2, 3], Scalar 4, Scalar 5, Scalar 6]
[List [1,2,3],Scalar 4,Scalar 5,Scalar 6]
2 Comments
leftaroundabout
Incidentally, if you give
ScalarOrList instances of both Num and IsList then you can actually write [[1,2,3],4,5,6]. I wouldn't necessarily consider it a good idea, though.
Ami Tavory
@leftaroundabout Thanks for the useful info!
There are such things as heterogeneous lists in Haskell, but they're not particularly trivial or beginner-friendly:
https://hackage.haskell.org/package/hvect-0.4.0.0/docs/Data-HVect.html
This is also a good read: https://wiki.haskell.org/Heterogenous_collections
You're probably best off trying to see if you can make your own data type that encapsulates both simple values and lists of values:
data IntOrList = AnInt Int | AList [Int]
But then you'll have to unwrap your values, which might be an added layer that you don't want to deal with. At least they'll all be able to share a list though: someList = [AnInt 5, AnInt 7, AList [1, 2, 5, 8], AnInt 2]
[[1,2,3], 4, 5, 6]. Only this:[[1,2,3], [4], [5], [6]]. You can make this a tuple instead.