A Tutorial on Programming Features in ATS:
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Generating a datcontag-function

A datcontag-function is very similar to a datcon-function. Given a datatype, its datcontag-function is the one that takes a value of the datatype and then returns the tag (which is a small integer) assigned to the (outmost) constructor in the construction of the value. We can use the following directive to indicate (to the ATS compiler) that the datcontag-function for the datatype expr needs to be generated: 

#codegen2("datcontag", expr)

By default, the name of the generated function is datcontag_expr_. If a different name is needed, it can be supplied as the third argument of the #codegen2-directive. For instance, the following directive indicates that the generated function is of the given name my_datcontag_expr: 

#codegen2("datcontag", expr, my_datcontag_expr)

The following ATS code is expected to be generated that implements datcontag_expr_: 

(* ****** ****** *)
//
implement
{}(*tmp*)
datcontag_expr_
  (arg0) =
(
case+ arg0 of
| Int _ => 0
| Var _ => 1
| Add _ => 2
| Sub _ => 3
| Mul _ => 4
| Div _ => 5
| Ifgtz _ => 6
| Ifgtez _ => 7
)
//
(* ****** ****** *)

Note that the funtion template datcontag_expr_ is required to be declared somewhere in order for the generated code to be compiled properly: 

fun{}
datcontag_expr_ : (expr) -> intGte(0) // a function template

Please find on-line the entirety of this presented example plus a Makefile (for illustrating the code generation process).

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