some questions about segment default and guide book

[jin]

when i try to use list0_rev (list0_tail (listo_rev lista)) as initlist function, there notice segment default at runtime, so i change function to the picture below, error disappear, i want to know what cause the error, is there any relation between the error and the "make stable" of the second picture, which seems like not nessesary, that's really confuse

[gmhwxi]

I see that you wrote some ATS2 code.

Unfortunately, there is not much written documentation on ATS2 (or ATS in general).

It is often difficult for one to immediately figure out how various features in ATS2 should

be used.

In your code, you have some recursive templates (e.g., your template list0RevHelper is

recursive).

In general, templates should *not* be recursive. In this case, list0RevHelper does not need

to be a template in the first place. Just turn it into an ordinary (recursive) function. Then I believe

your problem will go away.

--Hongwei

[gmhwxi]

For instance, I see the following kind of recursive templates very often:

fun{a:t@ype}

list0_length(xs: list0(a)): int =

case+ xs of

| list0_nil() => 0

| list0_cons(x1, xs) => 1 + list0_length<a>(xs)

This kind of code may be working, but should be avoided. Instead,

one can replace it with the following code:

fun{a:t@ype}

list0_length(xs: list0(a)): int =

length(xs) where

{

fun length(xs: list0(a)): int =

case+ xs of

| list0_nil() => 0

| list0_cons(_, xs) => 1 + length(xs)

}

BTW, recursive templates, by default, are not supported in ATS3.

[jin]

thanks a lot, in my understand, the function under a function can use its father's type and without convert to templates, and can you explain more about why there is a make sort stable commet at the second picture which is the code in the book i download from ats lang web

[gmhwxi]

Stable sorting means the ordering of equals should not be changed after sorting is done.

For instance, if we sort the following list according to parity (0 < 1):

[3,2,5,4,1,6]

we should get: [2,4,6,3,5,1]

In the mergesort implementation you referred to, list_reverse needs to be called to ensure sorting

done is actually stable sorting. Without list_reverse, the ordering among some equals is reversed.

[jin]

so, the reverse is apllied to make sure the result is what user expected? does it can be removed from function like quick sort, which seems like will not effect the result 

[Hongwei Xi]

You can also make list-based quicksort stable by calling list_reverse (twice).

However, list-based quicksort is not very practical; it is mostly used for illustration purpose (that is, teaching).

On the other hand, list-based mergesort is useful in practice. Unlike array-based mergesort, list-based version

does not need extra memory for merging. Anyway, this not particularly related to ATS.

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