Okay so I am a little confused about the cons operator : ::
I know this is acceptable:
> [2]
or this
2::[2,4]
> [2,2,4]
and i know this is forbbiden
[2]::[3]
it won't work.
but I recently found out that :
[2]::[3]::nil
will work :
> [[2],[3]]
or
[2]::[] will do as well.
>[[2]]
My question is why? I was thinking , maybe it's a rule like if i have a list of type a then with operator cons I can only create a new list of type a. So for example when I have
[2] :: [3]
My lists have elements of type int but I am asking to concatenate into an int list a non init value [3] ??
But when I have [2]::[3]::[] , the empty list [] could be an empty list of any type so here I need it to be an empty list, with elements like [3], so it adds to the nil the element [3] and then the element [2] and so on ...
So is that the reason? Is there something else?
In SML, lists are defined as follows:
datatype 'a list = nil | :: of 'a * 'a list
In particular, :: is an infix constructor of type 'a * 'a list -> 'a list.
The syntax [1, 2, 3] is sugar for 1 :: 2 :: 3 :: nil.
You need not only construct lists containing ints (i.e., where 'a = int); for example, you could have list containing int lists, i.e. an int list list. That's what you created:
[2] :: [3] :: nil
(* can be sugared as *)
[[2], [3]]
(Similarly, you could pick 'a = string and create the list "hello" :: "world" :: nil, i.e. ["hello", "world"].)
If you want to append two lists, you can use infix @, which has type 'a list * 'a list -> 'a list. For example:
[1, 2] @ [3]
(* evaluates to [1, 2, 3] *)