safe typescript aseem rastogi university of maryland, college park end of internship talk joint work...
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Safe TypeScriptAseem Rastogi
University of Maryland, College Park
End of Internship TalkJoint work with: Nikhil Swamy (RiSE, Internship mentor),
Cédric Fournet (MSRC), Gavin Bierman (Oracle),
Panagiotis Vekris (UCSD)
TypeScriptGradually typed superset of JavaScript (www.typescriptlang.org)
• "Strong tools for large applications"• "Static checking, symbol-based navigation, statement completion,
code refactoring"
• "TypeScript offers classes, modules, and interfaces to help you build robust components"
• "Compiled into simple JavaScript"
Compared to JavaScript, this is a great leap forward!
But Typing JavaScript is Hard !For all its dynamic idioms
• TypeScript (like Dart and Closure) gives up soundness, intentionally• Types are uniformly erased when compiling to JavaScript• E.g. casts are unchecked
• Unsound type erasure is beneficial• Lightweight codegen (highly readable JavaScript output)• Performance identical to plain JavaScript• Types don’t get in the way of good programmers
• But it also has its disadvantages• TypeScript components are not robust
interface Iterator<A> { next(): A }
var x = { state:[..], index:0, next() { return state[index++]; } };client(x); //client:Iterator<number> => void TypeScript JavaScript
TypeScript compiler
function client(Iterator<number> it) { it["index"] = true;}
Client:
Provider:
var x = { state:[..], index:0, next() { return state[index++]; } };client(x);
function client(it) { it["index"] = true;}
Client:
Provider:
(Un-)Robustness of TypeScript Components
Type safety violation
interface Iterator<A> { next(): A }
var x = { state:[..], index:0, next() { return state[index++]; } };client(x); //client:Iterator<number> => void TypeScript JavaScript
TypeScript compiler
function client(Iterator<number> it) { (<any> it).index = -1;}
Client:
Provider:
var x = { state:[..], index:0, next() { return state[index++]; } };client(x);
function client(it) { it.index = -1;}
Client:
Provider:
Abstraction violation
(Un-)Robustness of TypeScript Components
Safe TypeScript
• Sound and efficient gradual typing is possible for idiomatic TypeScript
• Sound typing is beneficial• Finds type errors early
• Found and fixed 478 type errors in TypeScript compiler,• 1 functional correctness bug in NavierStokes, a heavily tested Octane benchmark
• Provably robust components
• But it also has its cost• A runtime performance penalty
• Ranging from 6% to 3x on 118,000 lines of code in 8 applications (details follow)
• Need to understand subtle corners of JS semantics and our type system
function f(x:any):number { return x.f;}
function f(x):number { return x.f;}
app.ts tsc app.ts
TypeScript type inference
TypeScript parsing
JavaScript emittingfunction f(x) { return x.f;}
app.js
Syntactic errors
Static diagnostic- basic type errors
TypeScript Workflow
function f(x:any):number { return x.f;}
function f(x):number { return x.f;}
app.ts tsc --safe app.ts
TypeScript type inference
TypeScript parsing
Safe TypeScript type checking
& instrumentation
function f(x:any):number { return RT.check(RT.Num, RT.read(x, "f"));}
JavaScript emitting
function f(x) { return RT.check(RT.Num, RT.read(x, "f"));}
app.js
- inconsistent subtyping- implicit downcast from any- variables not in scope- unsafe use of this- projecting methods as fields
Static diagnostics
Syntactic errors
Static diagnostic- basic type errors
Safe TypeScript WorkflowFully integrated into TypeScript v0.9.5 as an optional compiler flag
Highlights of The Type System• Object-oriented, with a mixture of structural and nominal types
• Nominal types provide a sound model of JavaScript's semantics of classes• In contrast: TypeScript is purely structural
• Types are compiled to provide precise run-time type information (RTTI)• Allows the runtime system to enforce invariants with dynamic checks• In contrast: RTTI in TypeScript is only what is available in JavaScript
• Selective type-erasure for performance and robustness• The type system ensures that erasure does not compromise safety• In contrast: TypeScript uniformly erases all types
By example …
Nominal Classes and Structural Interfaces
interface PointI { x:number; y:number}class PointC { constructor(public x:number, public y:number) { }}function f(p:PointC) { assert(p instanceof PointC);}
Nominal Classes and Structural Interfaces
interface PointI { x:number; y:number}class PointC { constructor(public x:number, public y:number) { }}
f({x:0, y:0});
TypeScript output: leads to runtime error in f
function f(p:PointC) { assert(p instanceof PointC);}
Safe TypeScript: Static Type Error
{x:number;y:number} is not a subtype of PointC
Nominal Classes and Structural Interfaces
interface PointI { x:number; y:number}class PointC { constructor(public x:number, public y:number) { }}
f(new PointC(0, 0));
function f(p:PointC) { assert(p instanceof PointC);}
Safe TypeScript: OK
Nominal Classes and Structural Interfaces
f(new PointC(0, 0));
function f(p:PointI) { return p.x + p.y;}
Safe TypeScript: OK
PointC is a subtype of PointI
interface PointI { x:number; y:number}class PointC { constructor(public x:number, public y:number) { }}
Highlights of The Type System• Object-oriented, with a mixture of structural and nominal types
• Nominal types provide a sound model of JavaScript's semantics of classes• In contrast: TypeScript is purely structural
• Types are compiled to provide precise run-time type information (RTTI)• Allows the runtime system to enforce invariants with dynamic checks• In contrast: RTTI in TypeScript is only what is available in JavaScript
• Selective type-erasure for performance and robustness• The type system ensures that erasure does not compromise safety• In contrast: TypeScript uniformly erases all types
Tag Objects with RTTI to Lock Invariantsfunction f(p:any) { p.x = "boom";}
function g(p:PointI) { f(p); assert(typeof p.x === "number");}
TypeScript output: leads to runtime error in g
Tag Objects with RTTI to Lock InvariantsshallowTag for structural objects
function f(p) { … } //coming up !
function g(p) { f(shallowTag(p, PointI)); …}
Safe TypeScript: Adds RTTI to objects to lock their type
shallowTag(x,t) = x.tag := combine(x.tag,t); x
function f(p:any) { p.x = "boom";}
function g(p:PointI) { f(p); assert(typeof p.x === "number");}
Instrumentation of any Codefunction f(p) { write(p, "x", "boom");}
Safe TypeScript: Enforces type invariants in any code
write(o,f,v) = let t = o.rtti; o[f] = check(v, t[f]);
function f(p:any) { p.x = "boom";}
function g(p:PointI) { f(p); assert(typeof p.x === "number");}
function g(p) { f(shallowTag(p, PointI)); …}
// fails
Tag Objects with RTTI to Lock InvariantsNo tagging for class instances
function g(p) { f(p); // no tagging …}
No tagging for class instances
Class instances have primitive RTTI (prototype chain)
function g(p:PointC) { f(p); assert(typeof p.x === "number");}
function f(p:any) { p.x = "boom";}
Runtime Checked Downcastsfunction f(p:PointI) { assert(typeof p.x === "number");}
function g(p:any) { f(<PointI> p);}
g({x:"boom",y:0});
TypeScript output: leads to runtime error in f
Runtime Checked DowncastsCheck fields invariants for structural types
function f(p) { … }
function g(p) { f(check(p, PointI));}…
Safe TypeScript: Checks downcasts at runtime
check(o, PointI) = if typeof o.x === “number” && typeof o.y === “number” then o.rtti := PointI; o else die
function f(p:PointI) { assert(typeof p.x === "number");}
function g(p:any) { f(<PointI> p);}
g({x:"boom",y:0});
Runtime Checked DowncastsSimple instanceof check for class instances
function f(p) { … }
function g(p) { f(check(p, PointC));}…
check(o, PointC) = if o instanceof PointC then o else die
Fast instanceof check for class instances
function f(p:PointC) { … }
function g(p:any) { f(<PointC> p);}
g({x:"boom",y:0});
Highlights of The Type System• Object-oriented, with a mixture of structural and nominal types
• Nominal types provide a sound model of JavaScript's semantics of classes• In contrast: TypeScript is purely structural
• Types are compiled to provide precise run-time type information (RTTI)• Allows the runtime system to enforce invariants with dynamic checks• In contrast: RTTI in TypeScript is only what is available in JavaScript
• Selective type-erasure for performance and robustness• The type system ensures that erasure does not compromise safety• In contrast: TypeScript uniformly erases all types
Safe TypeScript adds RTTI Tags On-demandinterface 3dPointI extends PointI { z:number;}
function f(r:any) { ... }
function g(q:PointI) { f(q);}
function h(p:3dPointI) { g(p); }
function main(p:3dPointI) { h(p);}
shallowTag(x, t) = x.rtti := combine(x.rtti, t); x
Safe TypeScript adds minimum RTTI to ensure safety
function main(p) { h(p); } // no tagging
function h(p) { g(shallowTag(p, {z:number}); // diff tagging}
function g(q) { f(shallowTag(q, PointI));}
function f(r) { … }
Programmer-controlled Type Erasure
A new operator on types: "Erased t"
A value of type Erased t is known to be a t statically, and at runtime it may not have RTTI
Erased types are erased from the JavaScript output
Programmer Controlled Type Erasure
function f(r) { ... }
function g(q) { f(q);}
function h(p) { g(p);}
Cannot pass erased types to any context
static type error
No tagging despite loss in precision
compiles as is
Recall that previously it was:g(shallowTag(p, {z:number}))
Safe TypeScript: Erased types must only be used statically
interface 3dPointI extends PointI { z:number;}
function f(r:any) { ... }
function g(q:PointI) { f(q);}
function h(p:3dPointI) { g(p);}
interface PointI extends Erased { x:number; y:number}
interface Iterator<A> { next(): A }
var x = { state:[..], index:0, next() { return state[index++]; } };client(x); //client:Iterator<number> => void
function client(Iterator<number> it) { it["index"] = true;}
Client:
Provider:
Revisiting Robust Components
Full formalization and proofs in technical report: http://research.microsoft.com/apps/pubs/default.aspx?id=224900
//runtime error
Robustness provided by Safe TypeScripttype soundness theorem
Several useful corollaries:
-- RTTI tags are always consistent-- RTTI tags evolve in subtyping hierarchy
interface Iterator<A> { next(): A }
var x = { state:[..], index:0, next() { return state[index++]; } };client(x); //client:Iterator<number> => void
function client(Iterator<number> it) { (<any> it).index = -1;}
Client:
Provider:
Revisiting Robust ComponentsProvider’s perspective
interface Iterator<A> extends Erased { next(): A }
var x = { state:[..], index:0, next() { return state[index++]; } };client(x); //client:Iterator<number> => void
function client(Iterator<number> it) { (<any> it).index = -1;}
Client:
Provider:
Revisiting Robust ComponentsProvider’s perspective
Stronger abstraction using erased types
Safe TypeScript provides an abstraction theorem for erased types
Full formalization and proofs in technical report: http://research.microsoft.com/apps/pubs/default.aspx?id=224900
//static error
Much more …
• Arrays (with mutability controls)• Dictionaries• Inheritance• Overloading• Generics with bounded polymorphism• Optional fields/arguments/variadic functions• Auto-boxing• Primitive prototype hierarchy• Closed vs. open records• Nominal interfaces• Enums• … All these features allow us to handle practical TypeScript developments …
Experience with SafeTypeScriptBootstrapping Safe TypeScript compiler (implemented in TypeScript v0.9.5)
• 90,000 lines of code (80,000 lines of TypeScript compiler)• Heavily class based, most of the code is carefully type annotated
• Static errors• 478 in total
• 98 uses of bi-variant array subtyping• 130 uses of covariant method argument subtyping• 128 cases of variable scoping issues• 52 cases of projecting a method (leading to potential unsound use of this
parameter)• …
• Runtime errors• 26 failed downcasts• 5 in our own code !
15% runtime overhead of type safety
Experience with SafeTypeScriptCompiling TypeScript v1.1
• 18,000 lines of code• Heavily interface based
• Static errors• 81 in total
• Mainly variable scoping and array subtyping
3x runtime overhead of type safety
High overhead because of more structural types
Have not optimized Safe TypeScript runtime for structural types
Experience with SafeTypeScriptOctane benchmarks
• 10,000 lines of code
• Static errors• Found 1 variable scoping bug in heavily tested NavierStokes
• High runtime overhead when no annotations• 2.4x (Splay) to 72x (Crypto), Average: 22x
• Performance recovers once we add type annotations• Average overhead: 6.5%
Demo
( Examples on Online Playground: http://research.microsoft.com/en-us/um/people/nswamy/Playground/TSSafe/)
Limitations and Work in Progress
• eval and friends• Adversarial typing of unsafe constructs – Swamy et. al. POPL'14
• Implementation limitations:• Does not support external modules• Current implementation is in TypeScript v0.9.5 that has
evolved to v1.1
• Ongoing discussion about integrating Safe TypeScript in TypeScript v1.1
Safe TypeScript
Download:http://research.microsoft.com/en-us/downloads/b250c887-2b79-4413-9d7a-5a5a0c38cc57/
Submitted POPL'15 paper:http://www.cs.umd.edu/~aseem/safets.pdf
Technical report (with full formalization and proofs):http://research.microsoft.com/apps/pubs/default.aspx?id=224900
Online playground:http://research.microsoft.com/en-us/um/people/nswamy/Playground/TSSafe/
Sound and efficient gradual type system for TypeScript
Structural types distinguish fields from methodsHandling this soundly
class Line { constructor(public p1:Point,
public p2:Point){} public moveUp() {
this.p1.y++; this.p2.y++; }}
function g(l:{moveUp:() => void}) {
var f = l.moveUp; f();
}
function h(p:Point) {g(new Line(p, p));
}
Compiles without warnings in TypeScript• Classes are convertible with their structure
window.p1 is undefined, so p1.y crashes
SafeTypeScript• Line does not contain a field called moveUp• Only a method called moveUp
Line <: {moveUp() :void; //method p1:Point; //field p2:Point} //field
class Line { constructor(public p1:Point,
public p2:Point){} public moveUp() {
this.p1.y++; this.p2.y++; }}
function g(l:{moveUp(): void}) {var f = l.moveUp; f();
}
function h(p:Point) {g(new Line(p, p));
}
Compiles without warnings in TypeScript• Classes are convertible with their structure
SafeTypeScript• Cannot project a method
Line <: {moveUp() :void; //method p1:Point; //field p2:Point} //field
Structural types distinguish fields from methodsHandling this soundly
class Line { constructor(public p1:Point,
public p2:Point){} public moveUp() {
this.p1.y++; this.p2.y++; }}
function g(l:{moveUp(): void}) {l.moveUp();
}
function h(p:Point) {g(new Line(p, p));g({moveUp() { this.p1.y++;
},p1:p,p2:p});
}
Compiles without warnings in TypeScript• Classes are convertible with their structure
SafeTypeScript: Ok!
Line <: {moveUp() :void; //method p1:Point; //field p2:Point} //field
SafeTypeScript: Object literal with a methodg({moveUp : () => {this.p1.y++;}, p1:p, p2:p})
SafeTypeScript: A function is not a method
Structural types distinguish fields from methodsHandling this soundly
Field Addition and DeletionDynamically typed unless it becomes static
function f(p:PointI) { p["z"] = 0; delete p.z;}
function f(p: PointI) { write(p, "z", 0); delete(p, "z"); }
SafeTypeScript: Both write and delete succeed at runtime
Field Addition and DeletionDynamically typed unless it becomes static
function g(p:3dPointI) { … }
function f(p:PointI) { p["z"] = 0; g(<3dPointI> p); delete p.z;}
function g(p:3dPointI) { … }
function f(p:PointI) { write(p, "z", 0); g(check(p, 3dPointI)); delete(p, "z");}
SafeTypeScript: write succeeds at runtime
SafeTypeScript: check succeeds at runtime
SafeTypeScript: delete fails at runtime – violates invariant of g