PassByConstruction
From Erights
When objects are transfered to another vat, they arrive as far references by default. To reconstruct the object as a near reference in the receiving vat, Pass By Construction (pbc) can be used.
To mark an object as pbc, it has to be approved by the pbc
auditor:
def foo implements pbc { to __optUncall() { return [1, "add", [2]] } # ... }
__optUncall is a miranda message. When the object is sent out of the current vat, __optUncall/0 is called by the comm system, and the result of the call is used as the structure to represent the object to the destination vat.
The triple returned by __optUncall, a portrayal, has this format:
[<var>recipient</var> :[[near]], <var>verb</var> :[[String]], <var>args</var> :[[List]]]
which together specify a call to perform in the receiving vat; the object is unserialized as the result of this call.
Example
The first part of the example is a maker for a 2D Vector. The created vectors are Pass By Construction, they should pass the guard :pbc. This code should be positioned in a file in the java classpath at the location "tests/makeVector.emaker"
def makeVector implements pbc { to run(var x, var y) { def vector implements pbc { to getX() :any { return x } to getY() :any { return y } to __optUncall() :any { return [makeVector, "run", [x,y]] } to __printOn(out) {out.print(`<Vector $x $y>`)} } return vector } to __optUncall() :any { return [<import>, "get", ["tests.makeVector"]] } }
Example of usage:
# create a new Vector object ? def makeVector := <import:tests.makeVector> # value: <makeVector> ? def vector := makeVector(6.6,7.7) # value: <Vector 6.6 7.7> ? introducer.onTheAir() # value: ["3DES_SDH_M2", "3DES_SDH_M"] # function for creating a new local vat ? def seedVatAuthor := <elang:interp.seedVatAuthor>(<unsafe>).virtualize(introducer) # value: <virtualSeedVat> # Run this source code in the new vat. The method accepts a pbc object and prints it. ? def scriptSource:="def run(thing :pbc) {println(`Thing: $thing`)}" # value: "def run(thing :near) {println(`Thing: $thing`)}" # create the new vat. Return a far reference to the object and a reference to the vat object ? def [scriptObj, vat] := seedVatAuthor(scriptSource) # value: [<Promise>, <Vat newVirtualSeedVat in <runs in newVirtualSeedVat>>] # pass the pbc object into the new vat ? scriptObj <- run(vector) # value: <Remote Promise> ? Thing: <Vector 6.6 7.7>