Nested viewtype destructuring ...

[aditya siram]

Hi all,
I'm sure there's an easy answer to this but I'm not having much luck. How do I destructure into a view type that nested in another viewtype? For example, I'd to pattern match on something of type 'Option_vt(Option_vt(list_vt int))'. I'm not having much luck doing it in one fell swoop like: ' | Some_vt(Some_vt(list_vt_cons(x, xs))) => ...' especially when the linear proof of some inner value (like 'x') needs to be preserved.

Thanks!

[Hongwei Xi]

I am so clear as to what you need precisely.

Could you show me how to do it using pattern matching repeatly?

Then I may be able to do it using a nested pattern.

Here is an example that may be useful:

| ~Some_vt(~Some_vt(xs as list_vt_cons _)) => // using xs here

The two Some_vt constructors are freed but list_vt_cons is not.

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[Hongwei Xi]

Oops. I am NOT so clear ...

On Tue, May 15, 2018 at 9:27 PM, Hongwei Xi <gmh...@gmail.com> wrote:

I am so clear as to what you need precisely.

Could you show me how to do it using pattern matching repeatly?

Then I may be able to do it using a nested pattern.

Here is an example that may be useful:

| ~Some_vt(~Some_vt(xs as list_vt_cons _)) => // using xs here

The two Some_vt constructors are freed but list_vt_cons is not.

On Tue, May 15, 2018 at 9:04 PM, aditya siram <aditya...@gmail.com> wrote:

Hi all,
I'm sure there's an easy answer to this but I'm not having much luck. How do I destructure into a view type that nested in another viewtype? For example, I'd to pattern match on something of type 'Option_vt(Option_vt(list_vt int))'. I'm not having much luck doing it in one fell swoop like: ' | Some_vt(Some_vt(list_vt_cons(x, xs))) => ...' especially when the linear proof of some inner value (like 'x') needs to be preserved.

Thanks!

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[aditya siram]

#include "share/atspre_staload.hats"

datavtype nested = nested of (Option_vt(List_vt(int)))

extern fun add_heads(
  a : Option_vt(nested),
  b : Option_vt(nested)
): Option_vt(nested)

implement add_heads(a,b) =
  case (a,b) of
    | (~None_vt(),g) => g
    | (l, ~None_vt()) => l
    | (Some_vt(nested(Some_vt(list_vt_cons(x,_)))),
         Some_vt(nested(Some_vt(list_vt_cons(y,ys))))) =>
       Some_vt(nested(Some_vt(list_vt_cons(x+y,ys))))

implement main0(argc,argv) = ()

The above is a minimal non-compiling example from a larger program. I even tried adding '@' to the pattern match with 'fold@' in the body but it didn't work.

Thanks!

On Tuesday, May 15, 2018 at 8:28:33 PM UTC-5, gmhwxi wrote:

Oops. I am NOT so clear ...

On Tue, May 15, 2018 at 9:27 PM, Hongwei Xi <gmh...@gmail.com> wrote:

I am so clear as to what you need precisely.

Could you show me how to do it using pattern matching repeatly?

Then I may be able to do it using a nested pattern.

Here is an example that may be useful:

| ~Some_vt(~Some_vt(xs as list_vt_cons _)) => // using xs here

The two Some_vt constructors are freed but list_vt_cons is not.

On Tue, May 15, 2018 at 9:04 PM, aditya siram <aditya...@gmail.com> wrote:

Hi all,
I'm sure there's an easy answer to this but I'm not having much luck. How do I destructure into a view type that nested in another viewtype? For example, I'd to pattern match on something of type 'Option_vt(Option_vt(list_vt int))'. I'm not having much luck doing it in one fell swoop like: ' | Some_vt(Some_vt(list_vt_cons(x, xs))) => ...' especially when the linear proof of some inner value (like 'x') needs to be preserved.

Thanks!

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[Hongwei Xi]

Here is some code that typechecks:

implement add_heads(a,b) =
  case (a,b) of
  | (~None_vt(),g) => g
  | (l, ~None_vt()) => l

  | (~Some_vt(~nested(~Some_vt(~list_vt_cons(x,xs)))),
     ~Some_vt(~nested(~Some_vt(~list_vt_cons(y,ys))))) =>
    (free(xs); Some_vt(nested(Some_vt(list_vt_cons(x+y,ys)))))

Some warnings are issued due to non-exhaustiveness of pattern matching involved.

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[aditya siram]

Awesome, thanks! I think my confusion was mostly that the error messages complained about needing to retain a linear variable instead of consuming it so I never tried to free them. Also why did 'xs' have to freed explicitly with 'free(xs)' instead of using the '~' syntax in the pattern match?

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[Hongwei Xi]

The pattern ~list_vt_cons(x, xs) means the list constructor/node (that matches it) is freed;

the head of the list is stored in 'x' and the tail in 'xs'.

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[aditya siram]

I meant why couldn't you still a '~' on the 'xs' like

| (~Some_vt(~nested(~Some_vt(~list_vt_cons(x,~xs)))), ... =>

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[gmhwxi]

I see. A pattern like '~xs' is problematic as not every linear value can be

freed.