Multi-level programming with NX/Tcl

stefan.sobernig_at_wu.ac.at

This is a proof-of-concept implementation of a notion of deep characterization (potency) in NX/Tcl as found in the following paper:

Thomas Kühne and Daniel Schreiber (2007). Can programming be liberated from the two-level style: Multi-level programming with DeepJava. In: Proc. OOPLSA'07. ACM, DOI: http://dx.doi.org/10.1145/1297027.1297044

For a background on NX/Tcl, pls. see https://next-scripting.org/.

On the surface similar, but naturally quite different beneath, deep characterization in NX/Tcl is implemented using:

1. instantiation chains between metaclasses (everything-is-a-class, EIAC). In NX/Tcl, classes are objects themselves, and can be made behaving like clabjects (i.e., metaclasses creating further metaclasses).

2. property and method combination along these instantiation chains with explicit depth

Usage

This proof-of-concept is implemented as Tcl module, to you can require it into your Tcl scripts:

package require deep

If you want to run the embedded tests, run:

tclsh deep-0.1.tm

For demonstration purposes, consider the example taken from the OOPSLA'07 paper, Fig. 6: "Deep Instantiation".

The top level L2 can be defined as follows, using the root metaclass Clabject:

 Clabject create ProductType {
  :property -potency 0 -accessor public count; # potency 0 -> object property
  :property -potency 1 -accessor public taxRate:integer; # potency 1 -> property

  :property -potency 2 -accessor public price:double; # potency 1+
  :public -potency 2 method checkPrice {} {
    return [expr {${:price} > 1000}]
  }

}

Any Clabject instance (e.g., ProductType):

• is a Clabject on its own, and capable of producing further Clabject instance downstream.

• allows for annotating property and method with a -potency (positive number: 0, 1, 2, …​)

• a -potency of 0 maps to the per-object scope in NX/Tcl, a -potency of 1 to the instance level.

• any -potency greater or equal than 2 is a deep feature. For example, the property price and the method checkPrice defined at L2 so become available at level L0.

The intermediate level L1 is populated by instantiating the L2 clabject ProductType:

ProductType create Book
ProductType create Dvd

Everything is a clabject (L2, L1):

% ProductType info has type Clabject
1
% Book info has type Clabject
1
% Dvd info has type Clabject
1

The level L0 is then populated by instances from the L1 concepts:

 Dvd create 2001
Book create MobyDick

Again, the resulting L0 concepts are again represented as clabjects:

% 2001 info has type Clabject
1
% MobyDick info has type Clabject
1

We can introspect on the instantiation chain of clabjects:

% MobyDick info clabjects
::deep::test::Book ::deep::test::ProductType ::deep::Clabject

The two deep features defined at level L2 with potency 2 now are available at L0:

% 2001 price set 9.95
9.95
% 2001 checkPrice
0
% MobyDick price set 9.95
9.95
% MobyDick checkPrice
0

and only at level L0, not L1:

% Book info lookup methods price
% Book taxRate set 10
10

There are some goodies in NX/Tcl. We can add new deep features dynamically:

ProductType public -potency 2 method foo args {
  return [string toupper [current method]]
}

They become available to the deeply characterized level right away:

% 2001 info lookup methods foo
foo
% 2001 foo
FOO

In addition, we can add an arbitrary number of further instantiation levels (L-1):

 Book property -potency 2 -accessor public printVolume:integer; # L1!

MobyDick create 1stEdition -printVolume 1000; # L-1!
MobyDick create 2ndEdition -printVolume 2000; # L-1!

Again, all clabjects:

% 1stEdition info has type Clabject
1
% 2ndEdition info has type Clabject
1
% 2ndEdition printVolume get
2000

Implementation

TODO list

• Extend introspection info clabjects to walk both directions

• Test preservation of local-precedence orders in setAncestors

• Add more examples

nx::Class create Clabject -superclasses Class {

  :property -accessor public [list superclasses:class [self]] {
    :public object method value=set {class property value} {
      set value [uplevel 2 [list lmap x $value {$x}]]; # Irgh! This should not be necessary.
      $class setAncestors $value ${:default}
    }
    :public object method value=get {class property} {
      return [::nsf::relation::get $class superclass]
    }
  }
   :private method computePotencyClasses {} {
    set clobjs [:info clabjects]
    set lvlcls [list]
    for {set i 0} {$i < [llength $clobjs]} {incr i} {
      set clobj [lindex $clobjs $i]
      set lvl [expr {$i+2}]
      if {[::nsf::object::exists ${clobj}::$lvl]} {
        lappend lvlcls ${clobj}::$lvl
      }
    }
    return $lvlcls
  }

  :public method setAncestors {local default} {
    set current [nsf::relation::get [self] superclass]
    set lvlcls [: -local computePotencyClasses]
    if {$current eq "::nx::Object"} {
      # the default scenario, value -> Clabject
      set scl [list {*}$lvlcls {*}$local]
    } else {
      set scl [list {*}$local {*}$lvlcls $default]
    }
    ::nsf::relation::set [self] superclass [dict keys [dict create {*}"[join $scl " _ "] _"]]
  }

  :public method "info clabjects" {} {
    set cl [:info class]
    if {$cl eq [current class]} {
      return $cl
    } else {
      return [list $cl {*}[$cl info clabjects]]
    }
  }
  :private method makeDeepFeature {potency args} {
    set potClassName "[self]::${potency}"
    if {![nsf::object::exists $potClassName]} {
      nx::Class create $potClassName
    }
    $potClassName {*}$args
    :updateDescendants
  }
   :public method updateDescendants {} {
    foreach i [:info instances] {
      $i setAncestors [$i cget -superclasses] [current class]
      $i updateDescendants
    }
  }

  :public method property {{-potency 1} args} {
    switch -- $potency {
      0 {:object property {*}$args}
      1 {next [list {*}$args]}
      default {
        : -local makeDeepFeature $potency [current method] {*}$args
      }
    }
  }
   :public method public {{-potency 1} args} {
    switch -- $potency {
      0 {:public object {*}$args}
      1 {next [list {*}$args]}
      default {
        : -local makeDeepFeature $potency [current method] {*}$args
      }
    }
  }
   :public method protected {{-potency 1} args} {
    switch -- $potency {
      0 {:protected object {*}$args}
      1 {next [list {*}$args]}
      default {
        : -local makeDeepFeature $potency [current method] {*}$args
      }
    }
  }
}
 namespace export Clabject