Haskell generate list of lists

Question

I am preparing for test in haskell and I found a task from last test where was given to generate infinite list lists.

generate :: Num n => n -> [[n]]

where list is n-tuple, n is from parameter.

moreover the lists has to be sorted.

For exaple:

generate 2 = [[0,0],[0,1],[1,0],[1,1],[0,2],[1,2],[2,0],[2,1],[2,2],[0,3],....]

I have to use only basic functions.

I know that it surely is simple, but I dont know how to do it and I also can't find it on the internet.

I will be really glad for any help.

Infinite tuples are not possible in Haskell, that would be something like type tup = (X, tup). GHC rejects such infinite types. [[n]] is the type of a list, which can indeed be infinite (but only at runtime; it's perfectly finite to the type checker). — leftaroundabout
– leftaroundabout, Commented Jun 21, 2016 at 16:24
Not really; you need dependent types to specify that the return value is a list of lists of length 2, etc. Even if you could, Num is an overly broad constraint; you can't have a list of length 1.5 (or a 1.5-tuple). — chepner
– chepner, Commented Jun 21, 2016 at 16:32
I think it is clear from the provided example that the OP wants to generate an infinite list of lists - not tuples. — ErikR
– ErikR, Commented Jun 21, 2016 at 16:38
You want a list-of-lists. There are no tuples in the type n -> [[n]]. Also, Int n => ... is not Haskell, unless Int is a custom class -- was that Num n => ... instead? — chi
– chi, Commented Jun 21, 2016 at 16:38

ErikR · Accepted Answer · 2016-06-22 03:56:58Z

1

Here's a hint for the allTuplesWithMaxElem function...

Suppose you want to generate all lists [a,b,c,d] from the numbers [0..2] with the restriction that 2 must appear as one of the elements.

Consider these questions:

If we set a = 0, what are the possibilities for [b,c,d] ?
If we set a = 1, what are the possibilities for [b,c,d] ?
If we set a = 2, what are the possibilities for [b,c,d] ?

answered Jun 22, 2016 at 3:56

ErikR

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