Question

I have a list:

let a = [1, 2, 3]

I need to get another list:

[1, 2, 3] ++ [1*2, 2*3, 1*3] ++ [1*2*3]

It is a product of all possible unique coombinations of list’s elements. I have founded permutations in Data.List, but as I see it is something different.

Is there any library functions to get this list or can you give me examles how can I create your own function.

Thanks.

Answer 1

For a library function, you can use subsequences from Data.List:

Prelude Data.List> subsequences [1,2,3]
[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

You can get all of the products using map product $ subsequences [1,2,3].

But that is not in the same order as you specified. So you can sort it,
using sortBy from Data.List and comparing from Data.Ord:

Prelude Data.List Data.Ord> sortBy (comparing length) $ subsequences [1,2,3]
[[],[1],[2],[3],[1,2],[1,3],[2,3],[1,2,3]]

Again, get the products using map product.

Your other idea, to write a function yourself, is
the best idea if you are learning Haskell. Give it a try!