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destruct_exn.md

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Its essential is

external destruct : 'b -> (exn -> 'a)

However it does not prevent things like

destruct v begin fun exn -> 
   Js.log exn ; 
   match exn with 
   | .. 
   | ..

Here it forces us to answer whether v is exception or not,

while such syntax below does not need us answer v is exception or not, it just asks us to answer it matches a branch of exception or not which can be done in a sound way.

match%exn v with 
| .. 
| ..

However, we need make sure such cases not happen

match%exn v with 
| e -> ...

Or any vagous pattern which needs us to answer if it is an exception or not

Another proposal is

match%exn computation with 
| A ..
| B .. 
| Js.NonCamlOpenVariant .. 
| v ->  ..

Here we pack the data v

==>


match (Js_enx.internalTOOCamlException compuation) with
| A .. 
| B 
| exception .. )

The same problem is

match (Js_enx.internalTOOCamlException compuation) with
| _ -> ..

What will happen if JS side raises an OCaml extensible variant, we view it as OCaml exception.. It is slightly differnt in OCaml, since it always start from catch(id).. where id is defined by the compiler

Another very similar proposal would be

fun[@bs:exn] e -> 
  match e with 
  | Js.Exn.Error ..
  | ..