dataviewtype

Dataviewtypes are linear analogues of datatypes. As such, their memory must be managed explicitly, which typically entails creating a free function for each dataviewtype introduced. Due to potential complexity in type checking and proofs (as well as it being a concrete type) it is advised to make use of an [absviewtype] provide an interface for dataviewtypes.

Folding

An important notion in dealing with dataviewtypes is being able to access the arguments of constructors. The implications differ if we want to use the arguments as l-values or just as r-values. For instance, take the following example:

datavtype dvt =
  | dvt0 of ()
  | dvt2 of (dvt,dvt)

fun foo (x: !dvt): void =
  case+ x of
  | dvt0 () => ()
  | dvt2 (x1, x2) => ()        // x1 and x2 are values
  | @dvt2 (x1, x2) => fold@(x) // x1 and x2 are l-values

Basically, you need to put @ in front of a linear constructor if you want to
use the arguments of the constructor as l-values. Why l-values? Because
an l-value allows its content to be modified. Note that explicit folding (fold@)
is needed if @ is used.

A more complex example

This example is primarily used to build on the above while introducing a higher degree of nesting, and is based on an example from the ATS1 wiki (warning, external link). Also note that this example doesn't explicitly mention the difference
between the l-values and r-values in the comments, since the difference wasn't accessible in ATS1:

dataviewtype dvt =
    | dvt_0 of ()
    | dvt_1 of (dvt)
    | dvt_2 of (dvt,dvt)

//                                                     (a)ccessible/usable
//                                                     (u)nfolded
//                                                     (f)olded
//                                                     (n)o view available
//                                                     (-) not in scope
//                                                        d0   d1   d11  d12 d121
//                                                        -----------------------
fun use_dvt(d0: !dvt):void = case+ d0 of               // a    -    -    -   -
  | @dvt_1 (d1 as dvt_2(d11,d12) ) => let              // u    a    a    a   -
    val () = case+ (d11,d12) of                        //
      | (@dvt_0 (), @dvt_1 (d121) ) => let             // u    a    u    u   a
        prval () = fold@ d11 and () = fold@ d12        // u    a    af   af  n
        in () end                                      //
      | (_,_) => ()                                    //
    prval () = fold@ d0                                // af   n    n    n   -
    in () end                                          // d0   d1   d2  d11  d12
                                                       //------------------------
  | @dvt_2 (d1,d2) =>                                  // u    a    a
    (case+ d1 of                                       //
      | @dvt_2 (d11,d12) => let                        // u    u    a   a    a
        prval () = fold@ d1                            // u    af   a   n    n
        prval () = fold@ d0                            // af   n    n   n    n
        in () end
      | _ => fold@ d0
    )
  | _ =>  ()