Multiple Ownership

Nicholas CameronSophia Drossopoulou

James NobleMatthew Smith

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• Background – Ownership Types

• Multiple Ownership• Objects in Boxes• Variance - ?• MOJO

• Effects

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Background

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Ownership Types

• The heap is messy:

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Ownership Types

• Organise it:

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Ownership Types

• Give each object an owner:

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Ownership Types

class Project<o> { Task<this> t1; Task<this> t2; List<this, o> clients;}

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Our Work

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The Objects In Boxes Model

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A Box is a Set of Objects

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Objects in Boxes

• Single owner interpretation:

• An object is in its owner’s box

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Programs are not Trees

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One Owner is not Enough

• 75% of ownership structures require multiple ownership• [34] Mitchell, ECOOP, ’06

• Trees can not describe non-hierarchical structures

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Objects in Boxes

• Multiple Ownership: objects may be in more than one box

• An object with multiple owners is in the intersection of their boxes:

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Objects in Boxes

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A Descriptive System

• Describes the heap• Does not restrict

references etc.

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MOJO

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MOJO

• Class declarations have the usual, formal ownership parameters:

class C<o, d> …

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MOJO

• Class declarations have the usual, formal ownership parameters:

class C<o, d> …• Types may have multiple corresponding

actual owner parameters:

C<b, c>

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MOJO

• Class declarations have the usual, formal ownership parameters:

class C<o, d> …• Types may have multiple corresponding

actual owner parameters:

C<b, c>

C<a & b, c >

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MOJO

• Class declarations have the usual, formal ownership parameters:

class C<o, d> …• Types may have multiple corresponding

actual owner parameters:

C<b, c>

C<a & b, c >

C<a & b & ?, c & a>

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MOJO

class Connection<o> { … }

class Client<o, s> { Connection<this & s> c;}

class Server<o> { Connection<this & ?> c;}

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Variance

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Variance

• Connection<this & ?>• Some owner

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Variance

• Connection<this & ?>• Some box

• May be the intersection of several boxes

• Variance in the number of owners

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Subtyping

• Subtyping with wildcards is variant with respect to owners:

C<a> <: C<?>

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Subtyping

• Subtyping with wildcards is variant with respect to owners:

C<a> <: C<?>

C<a> <: C<a & ?>

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Subtyping

• Subtyping with wildcards is variant with respect to owners:

C<a> <: C<?>

C<a> <: C<a & ?>

C<a & b & ?> <: C<a & ?>

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Subtyping

• Subtyping with wildcards is variant with respect to owners:

C<a> <: C<?>

C<a> <: C<a & ?>

C<a & b & ?> <: C<a & ?>

C<?> </: C<a>

C<a> </: C<a & b>

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Constraints

class C<a, b, c> a intersects b, b disjoint c{ … }

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Constraintsclass D<a, b> a intersects b { D<a, b>

}

class E<a, b> a disjoint b { }

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Constraintsclass D<a, b> a intersects b { D<a, b> }

class E<a, b> a disjoint b { D<a, b> }

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Constraintsclass D<a, b> a intersects b { D<a, b> Object<a & b> }

class E<a, b> a disjoint b { D<a, b> }

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Typing - strict

• Strict method sends and assignments to deal with variance of owners:class C<o> { C<o> f;}

C<a> ca; C<a & ?> c1; C<a & ?> c2;

ca.f = ca;

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Typing - strict

• Strict method sends and assignments to deal with variance of owners:class C<o> { C<o> f;}

C<a> ca; C<a & ?> c1; C<a & ?> c2;

ca.f = ca; c1.f = c2;

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Typing - strict

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Effects

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Effects

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Effects

Task<x> t1;Task<y> t2;…t1.f; // reads x / writes ε

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Effects

Task<x> t1;Task<y> t2;…t1.f; // reads x / writes εt1.f = t2.f; // reads x,y / writes x

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Effects with Multiple Owners

Task<x & ?> t1;Task<y & z> t2;…t1.f; // reads x & ? / writes ε

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Effects with Multiple Owners

Task<x & ?> t1;Task<y & z> t2;…t1.f; // reads x & ? / writes εt1.f = t2.f; // reads x & ?,y & z/ writes x & ?

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Effects with Multiple Ownerst1.f = t2.f; // reads x & ?,y & z/ writes x & ?

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Disjointness

• Two expressions are disjoint if their effects do not overlap

• Complicated by ? – but & = intersection

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Future Work

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Future Work

• Explore variant owners using existential types

class TaskList<o, d> { Task<d & ?> datum; TaskList<o, d> next;}

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Future Work

• Reference and modification control

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Future Work

• Constraints• Topology - inside• Unary - may read, may write• Binary – may point to, may modify

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Summary

• Ownership Types• Multiple Ownership

• Objects in Boxes• Variance• MOJO

• Effects

• Thank you!• Any questions?