Re: what is the difference here ?

Hi Roelof,

[x;y] : 'a list is a list with two elements x : 'a and y : 'a.

x::y : 'a list is a list with at least one element, its head x : 'a, and
some tail y : 'a list about which we know nothing (it can be empty or
not... etc.).

Notice the difference between the two types of y in the two different
expressions.

For instance, you can define:

let l = [1;2];;
let l' = 1::[2];;

Hope this helps,
Chantal.



Le 05/11/2014 15:28, Roelof Wobben [email protected] [ocaml_beginners] a
écrit :
> Hello,
> 
> I found the solution to exercise 2 of the 99 ocaml problems.
> The solution I found was this :
> 
> let rec last_two list =
>     match list with
>       | [] | [_] -> None
>       | [x;y] -> Some (x,y)
>       | _::xs -> last_two xs ;;
> 
> 
> Now I wonder something.
> For me it looks like [x;y]  and _::xs are the same.
> If I understand it right you can write [x;y] as x::y and _::xs looks 
> then the same to me.
> But if I change [x;y] to x::y then I see a message that _:xs is not used.
> 
> Can someone explain in simple words what is the difference between those 
> two ?
> 
> Roelof
> 
> 


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