Inherent type of a slice of a list

[MaryamZi]

Description:
The lang.array:slice method is defined as follows.

# Returns a subarray starting from `startIndex` (inclusive) to `endIndex` (exclusive).
#
# + arr - the array
# + startIndex - index of first member to include in the slice
# + endIndex - index of first member not to include in the slice
# + return - array slice within specified range
public function slice(Type[] arr, int startIndex, int endIndex = arr.length()) returns Type[] = external;

The concern is regarding the actual inherent type of the slice created; is it the type representing the exact slice at the point of creation?

public function main() {
    [int, string, boolean] x = [1, "ballerina", true];

    int i = 1;
    int j = x.length();

    (int|string|boolean)[] y = x.slice(i, j);
}

So the static type here would have to be an array of (int|string|boolean), but what should be the inherent type of the slice, which in this case would be ["ballerina", true]?

Should it be

1. [string, boolean]
2. (string|boolean)[]
3. (string|boolean)[2]
4. (int|string|boolean)[]

Similarly, for an array, should a slice result in a fixed length list?

import ballerina/io;

public function main() {
    int[] x = [1, 2, 3];

    int i = 1;
    int j = x.length();

    int[] y = x.slice(i, j);
    io:println(y.toJsonString()); // [2, 3]
    io:println(y is int[2]); //  should this be true?
}

[jclark]

We want slice (and other lang lib functions) to behave for the most part like generic functions. The language doesn't have generic functions yet, but the basic concept is that you instantiate a generic function with a type parameter, which is determined at compile-time, to get a normal function. For slice, the instantiated function should work as if it was written in a normal way in Ballerina. This means it would create a list of type T[], where T is the type used to instantiate the generic function.

So in your case, in order to call array:slice, we need to instantiate array:slice with Type as (int|string|boolean). So the inherent type of the returned array would be (int|string|boolean)[].

(One of the reasons lang.array is called lang.array not lang.list is that it treats its first argument as an array.)

[MaryamZi]

Ack, thank you.