Update strong mode doc

STATIC_SAFETY.md

+# Strong Mode for Dart
+
+## Overview
+
+The Dart programming language has an optional, unsound type system.  Although it is similar in appearance to languages such as Java, C#, or C++, its type system and static checking are fundamentally different.  It permits erroneous behavior in ways that may be surprising to programmers coming from those and other traditionally typed languages.
+
+In Dart, static type annotations can be often misleading.  Dart code such as:
+
+```dart
+  var list = ["hello", "world"];
+  List<int> listOfInts = list;
+```
+
+produces neither static nor runtime errors.  Actual errors may show up much later on, e.g., with the following code, only at runtime on the invocation of `abs`:
+
+```dart
+  Iterable<int> iterableOfInts = listOfInts.map((i) => i.abs());
+```
+
+Strong mode aims to catch such errors early by validating that variables - e.g., `listOfInts` - actually match their corresponding static type annotations - e.g., `List<int>`.  It constrains the Dart programming language to a subset of programs that type check under a restricted set of rules.  It statically rejects examples such as the above.
+
+To accomplish this, strong mode involves the following:
+
+ - Type inference.  Dart’s standard type rules treat omitted types as `dynamic` and effectively suppresses any static errors or warnings on variables of this type.  Strong mode infers types based upon context.  In the example above, strong mode infers that `list` has type `List`.
+
+ - Strict subtyping.  Dart’s primary sources of unsoundness are due to its subtyping rules on function types and generic classes.  Strong mode restricts these: e.g., `List` may not used as `List<int>` in the example above.
+
+ - Generic methods.  Standard Dart does not yet provide generic methods.  This makes certain polymorphic methods difficult to use soundly.  For example, the `List.map` invocation above is statically typed to return an `Iterable<dynamic>` in standard Dart.  Strong mode allows methods to be annotated as generic. `List.map` is statically typed to return an `Iterable<T>` where `T` is bound to `int` at this use site.  A number of common higher-order methods are annotated and checked as generic in strong mode, and programmers may annotate their own methods as well.
+
+Strong mode is designed to work in conjunction with the Dart Dev Compiler (DDC), which uses static type verification to generate better code.  It also may alone be used alone for stricter error checking.

[Leaf, 2016/01/15 00:01:10]

'may alone be used alone'  -> extra alone

I think we should be slightly more explicit here that "stricter" != sound in the
absence of the runtime checks.  

Somewhere we should mention that we keep covariant generics. 

[vsm, 2016/01/15 20:13:39]

Done.

+
+## Usage
+
+Strong mode is now integrated into the Dart Analyzer.  The analyzer may be invoked in strong mode as follows:
+
+    $ dartanalyzer --strong myapp.dart
+
+Strong mode may also be enabled in IDEs by creating (if necessary) an `.analysis_options` file in your project and appending the following entry to it:
+
+```
+analyzer:
+  strong-mode: true
+```
+
+## Type Inference
+
+An explicit goal of strong mode to provide stronger typing while preserving the terseness of Dart.  Dart syntax and [common style](https://www.dartlang.org/effective-dart/style/)  encourages a modest level of type annotations.  Strong mode should not compel programmers to add more.  Instead, it relies on type inference.
+
+In Dart, per the specification, the static type of a variable `x` declared as:
+
+```dart
+var x = <String, String>{ "hello": "world"};
+```
+
+is `dynamic` as there is no explicit type annotation on the left-hand side. To discourage code bloat, the Dart style guide generally recommends omitting these type annotations in many situations.  In these cases, the benefits of strong mode would be lost.
+
+To avoid this, strong mode uses type inference.  In the case above, the strong mode infers and enforces the type of `x` as `Map<String, String>`.  An important aspect to inference is ordering: when an inferred type may be used to infer other type.  To maximize the impact, we perform the following inference in the following order:

[Leaf, 2016/01/15 00:01:10]

'the strong mode' -> 'strong mode' ?

'other type' -> 'another type' or 'other types'?

[vsm, 2016/01/15 20:13:39]

Done.

+
+TODO(vsm): Order is out-of-date:
+
+- Top-level and static fields
+- Instance fields and methods
+- Local variables
+- Allocation expressions
+- Generic method invocations
+
+Inference may tighten the static type as compared to the Dart specification.  The `dynamic` type, either alone or in the context of a function or generic parameter type, is inferred to a more specific type.  This inference may result in stricter type errors than standard Dart.
+
+In [DDC](TODO), inference may also affect the reified runtime type.
+
+
+In all cases, inference tightens the static type or runtime type as compared to the Dart specification.  The `dynamic` type, either alone or in the context of a function or generic parameter type, is inferred to a more specific type.  The effect of this inference (other than stricter type errors) should not be observable at runtime outside the use of the mirrors API or by explicitly examining Object.runtimeType.  (Note, in the next section, we discuss corresponding restrictions on `is` and `as` type checks.)
+
+### Top-level and Static Fields
+
+Strong mode will infer the static type of any top-level or static field with:
+
+- No static type annotation
+- An initializer expression
+
+The static type of the declared variable is inferred as the static type of the initializer.  For example, consider:
+
+```dart
+var PI = 3.14159;
+var TAU = PI * 2;
+```
+
+Strong mode would infer the static type of `PI` as `double` directly from its initializer.  It would infer the static type of `TAU` as `double`, transitively using `PI`’s inferred type. Standard Dart rules would treat the static type of both `PI` and `TAU` as `dynamic`.  Note that the following later assignment would be allowed in standard Dart, but disallowed (as a static type error) in strong mode:
+```dart
+PI = "\u{03C0}"; // Unicode string for PI symbol
+```
+Strong mode inference avoids circular dependences.  If a variable’s initializer expression refers to another variable whose type would be dependent (directly or transitively) on the first, the static type of that other variable is treated as `dynamic` for the purpose of inference.  In this modified example,
+
+```dart
+var _PI_FIRST = true;
+var PI = _PI_FIRST ? 3.14159 : TAU / 2;
+var TAU = _PI_FIRST ? PI * 2 : 6.28318;
+```
+
+the variables `PI` and `TAU` are circularly dependent on each other.  Strong mode would leave the static type of both as `dynamic`.
+
+### Instance Fields and Methods
+
+Strong mode performs two types of inference on instances fields and methods.
+
+The first uses base types to constrain overrides in subtypes.  Consider the following example:
+
+```dart
+abstract class A {
+   Map get m;
+   int value(int i);
+}
+
+class B extends A {
+   var m;
+   value(i) => m[i];
+   …
+}
+```
+
+In Dart, overridden method, getter, or setter types should be subtypes of the corresponding base class ones (otherwise, static warnings are given).  In standard Dart, the above declaration of `B` is not an error: both `m`’s getter type and `value`’s return type are `dynamic`.
+
+Strong mode - without inference - would disallow this: if `m` in `B` could be assigned an arbitrarily typed value, it would violate the type contract in the declaration of `A`.
+
+However, rather than rejecting the above code, strong mode employs inference to tighten the static types to obtain a valid override.  The corresponding types in B are inferred as if it was:
+
+```dart
+class B extends A {
+  Map m;
+  int value(int i) => m[i];
+  …
+}
+```
+
+Note that tightening the argument type for `i` is not required for soundness; it is done for convenience as it is the typical intent.  The programmer may explicitly type this as `dynamic` or `Object` to avoid inferring the narrower type.
+
+The second form inference is limited to instance fields (not methods) and is similar to that on static fields.  For instance fields where the static type is omitted and an initializer is present, the field’s type is inferred as the initializer’s type.  In this continuation of our example:
+
+```dart
+class C extends A {
+  var y = 42;
+  var m = <int, int>{ 0: 38};
+  ...
+}
+```
+
+the instance field `y` has inferred type `int` based upon its initializer.  Note that override-based inference takes precedence over initializer-based inference.  The instance field `m` has inferred type `Map`, not `Map<int, int>` due to the corresponding declaration in `A`.
+
+### Local Variables
+
+As with fields, local variable types are inferred if the static type is omitted and an initializer expression is present.  In the following example:
+
+```dart
+Object foo(int x) {
+   final y = x + 1;
+   var z = y * 2;
+   return z;
+}
+```
+
+the static types of `y` and `z` are both inferred as `int` in strong mode.  Note that local inference is done in program order: the inferred type of `z` is computed using the inferred type of `y`. Local inference may result in strong mode type errors in otherwise legal Dart code.  In the above, a second assignment to `z` with a string value:    
+```dart
+z = “$z”;
+```
+would trigger a static error in strong mode, but is allowed in standard Dart.  In strong mode, the programmer must use an explicit type annotation to avoid inference.  Explicitly declaring `z` with the type `Object` or `dynamic` would suffice in this case.
+
+### Allocation Expressions
+
+Strong mode also performs contextual inference on allocation expressions.  This inference is rather different from the above: it tightens the runtime type of the corresponding expression using the static type of its context.  Contextual inference is used on expressions that allocate a new object: closure literals, map and list literals, and explicit constructor invocations (i.e., via `new` or `const`).
+
+#### Closure literals
+
+Consider the following example:
+
+```dart
+int apply(int f(int arg), int value) {
+  return f(value);
+}
+
+void main() {  
+  int result =
+    apply((x) { x = x * 9 ~/ 5; return x + 32; }, 41);
+  print(result);
+}
+```
+
+The function `apply` takes another function `f`, typed `(int) -> int`, as its first argument.  It is invoked in `main` with a closure literal.  In standard Dart, the static type of this closure literal would be `(dynamic) -> dynamic`.  In strong mode, this type cannot be safely converted to `(int) -> int` : it may return a `String` for example.   
+
+Dart has a syntactic limitation in this case: it is not possible to statically annotate the return type of a closure literal.
+
+Strong mode sidesteps this difficulty via contextual inference.  It infers the closure type as `(int) -> int`.  Note, this may trigger further inference and type checks in the body of the closure.
+
+#### List and map literals
+
+Similarly, strong mode infers tighter runtime types for list and map literals.  E.g., in
+
+```dart
+List<String> l = [ "hello", "world" ];
+```
+
+the runtime type is inferred as `List<String>` in order to match the context of the left hand side.  In other words, the code above type checks and executes as if it was:
+
+```dart
+List<String> l = <String>[ "hello", "world" ];
+```
+
+Similarly, the following will now trigger a static error in strong mode:
+
+```dart
+List<String> l = [ "hello", 42 ]; // Strong Mode Error: 42 is not a String
+```
+
+Contextual inference may be recursive:
+
+```dart
+Map<List<String>, Map<int, int>> map =
+        { ["hello"]: { 0: 42 }};
+```
+
+In this case, the inner map literal is inferred and allocated as a `Map<int, int>`.  Note, strong mode will statically reject code where the contextually required type is not compatible.  This will trigger a static error:
+
+```dart
+Map<List<String>, Map<int, int>> map =
+  { ["hello"]: { 0: "world" }}; // STATIC ERROR
+```
+
+as "world" is not of type `int`.
+
+#### Constructor invocations
+
+Finally, strong mode performs similar contextual inference on explicit constructor invocations via new or const.  For example:
+
+```dart
+Set<String> string = new Set.from(["hello", "world"]);
+```
+
+is treated as if it was written as:
+
+```dart
+Set<String> string =
+  new Set<String>.from(<String>["hello", "world"]);
+```
+
+Note, as above, context is propagated downward into the expression.
+
+### Generic method inference
+
+TODO(vsm)
+
+## Strict subtyping
+The primary sources of unsoundness in Dart are generics and functions.  Both introduce circularity in the Dart subtyping relationship.
+
+### Generics
+
+Generics in Dart are covariant, with the added rule that the `dynamic` type may serve as both ⊤ (top) and ⊥ (bottom) of the type hierarchy in certain situations.  For example, let *<:<sub>D</sub>*  represent the standard Dart subtyping rule.  Then, for all types `S` and `T`:
+
+`List<S>` <:<sub>D</sub> `List<dynamic>` <:<sub>D</sub> `List<T>`
+
+where `List` is equivalent to `List<dynamic>`.  This introduces circularity - e.g.:
+
+`List<int>` <:<sub>D</sub> `List` <:<sub>D</sub> `List<String>`<:<sub>D</sub> `List` <:<sub>D</sub> `List<int>`
+
+
+From a programmer’s perspective, this means that, at compile-time, values that are statically typed `List<int>` may later be typed `List<String>` and vice versa.  At runtime, a plain `List` can interchangeably act as a `List<int>` or a `List<String>` regardless of its actual values.
+
+Our running example exploits this.  A `MyList` may be passed to the `info` function as it’s a subtype of the expected type:
+
+`MyList` <:<sub>D</sub> `List` <:<sub>D</sub>`List<int>`
+
+In strong mode, we introduce a stricter subtyping rule <:<sub>S</sub> to disallow this.  In this case, in the context of a generic type parameter, dynamic may only serve as ⊤.  This means that this is still true:
+
+`List<int>` <:<sub>S</sub> `List`
+
+but that this is not:
+
+`List<int>` ~~<:<sub>S</sub> `List`~~
+
+
+Our running example fails in strong mode:
+
+`MyList` <:<sub>S</sub> `List` ~~<:<sub>S</sub> `List<int>`~~
+
+
+### Functions
+
+The other primary source of unsoundness in Dart is function subtyping.  An unusual feature of the Dart type system is that function types are bivariant in both the parameter types and the return type (see Section 19.5 of the [Dart specification](http://www.google.com/url?q=http%3A%2F%2Fwww.ecma-international.org%2Fpublications%2Ffiles%2FECMA-ST%2FECMA-408.pdf&sa=D&sntz=1&usg=AFQjCNGoFPzBNx2fgejKQgSgiS2dUBstBw)).  As with generics, this leads to circularity:
+
+`(int) -> int` <:<sub>D</sub> `(Object) -> Object` <:<sub>D</sub> `(int) -> int`
+
+And, as before, this can lead to surprising behavior.  In Dart, an overridden method’s type should be a subtype of the base class method’s type (otherwise, a static warning is given).  In our running example, the (implicit) `MyList.length` getter has type:
+
+`() -> Object`
+
+while the `List.length` getter it overrides has type:
+
+`() -> int`
+
+This is valid in standard Dart as:
+
+`() -> Object` <:<sub>D</sub> `() -> int`
+
+Because of this, a `length` that returns "hello" (a valid `Object`) triggers no static or runtime warnings or errors.
+
+Strong mode enforces the stricter, [traditional function subtyping](https://en.wikipedia.org/wiki/Subtyping#Function_types) rule: subtyping is contravariant in parameter types and covariant in return types.  This permits:
+
+`() -> int` <:<sub>S</sub> `() -> Object`
+
+but disallows:
+
+`() -> Object` <:<sub>S</sub> `() -> int`
+
+With respect to our example, strong mode requires that any subtype of a List have an int-typed length.  It statically rejects the length declaration in MyList.
+
+Formal details of the strong mode type system may be found  [here](https://dart-lang.github.io/dev_compiler/strong-dart.pdf).
+
+## Generic Methods
+
+Strong mode introduces generic methods to allow stricter typing on polymorphic methods.  Such code in standard Dart today often loses static type information.  For example, the `Iterable.map` method is declared as below:
+
+```dart
+abstract class Iterable<E> {
+  ...
+  Iterable map(f(E e));
+}
+```
+
+Regardless of the static type of the function `f`, the `map` always returns an `Iterable<dynamic>`.
+
+TODO(vsm): Link to DEP.
+
+Generic methods have been proposed as an addition to the Dart language.  To enable experimentation, strong mode provides a comment-based form of generic methods based on the existing DEP, but usable on all existing Dart implementations.  The `Iterable.map` method is declared as follows:
+
+```dart
+abstract class Iterable<E> {
+  ...
+  Iterable/*<T>*/ map/*<T>*/(/*=T*/ f(E e));
+}
+```
+
+At a use site, the generic type may be explicitly provided or inferred from context:
+
+```
+  var l = <int>[1, 2, 3];
+  var i1 = l.map((i) => i + 1);
+  var l2 = l.map/*<String>*/((i) { ... });
+```
+
+In the first invocation of `map`, the closure is inferred (from context) as `int -> int`, and the generic type of map is inferred as `int` accordingly.  As a result, `i1` is inferred as `Iterable<int>`.  In the second, the type parameter is explicitly bound to `String`, and the closure is checked against this type.  `i2` is typed as `Iterable<String>`.
+
+
+## Additional Restrictions
+
+In addition to stricter typing rules, strong mode enforces other
+restrictions on Dart programs.
+
+### Warnings as Errors
+
+Strong mode effectively treats all standard Dart static warnings as static errors.  Most of these warnings are required for soundness (e.g., if a concrete class is missing methods required by a declared interface).  A full list of Dart static warnings may found in the [Dart specification](http://www.google.com/url?q=http%3A%2F%2Fwww.ecma-international.org%2Fpublications%2Ffiles%2FECMA-ST%2FECMA-408.pdf&sa=D&sntz=1&usg=AFQjCNGoFPzBNx2fgejKQgSgiS2dUBstBw), or enumerated here:
+
+[https://github.com/dart-lang/sdk/blob/master/pkg/analyzer/lib/src/generated/error.dart#L3772](https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2Fdart-lang%2Fsdk%2Fblob%2Fmaster%2Fpkg%2Fanalyzer%2Flib%2Fsrc%2Fgenerated%2Ferror.dart%23L3772&sa=D&sntz=1&usg=AFQjCNFc4E37M1PshVcw4zk7C9jXgqfGbw)
+
+### Is / As Restrictions
+
+TODO(vsm): Fix this
+
+Dart is and as runtime checks expose the unsoundness of the type system in certain cases.  For example, consider:
+
+```dart
+var list = ["hello", "world"];
+if (list is List<int>) {
+  ...
+} else if (list is List<String>) {
+  ...
+}
+```
+
+Perhaps surprisingly, the first test - `list is List<int>` - evaluates to true here.  Such code is highly likely to be erroneous.
+
+Strong mode statically disallows problematic `is` or `as` checks..  In general, an expression:
+
+```dart
+x is T
+```
+
+or
+
+```dart
+x as T
+```
+
+is only allowed where `T` is a *ground type*:
+
+- A non-generic class type (e.g., `Object`, `String`, `int`, ...).
+- A generic class type where all type parameters are implicitly or explicitly `dynamic` (e.g., `List<dynamic>`, `Map`, …).
+- A function type where the return type and all parameter types are `dynamic` (e.g., (`dynamic`, `dynamic`) -> `dynamic`, ([`dynamic`]) -> `dynamic`).
+
+In all other cases, strong mode reports a static error.
+
+### Super Invocations
+
+In the context of constructor initializer lists, strong mode restricts `super` invocations to the end.  This restriction simplifies generated code with minimal effect on the program.
+
+### For-in loops
+
+In for-in statements of the form:
+
+```dart
+for (var i in e) { … }
+```
+
+ Strong mode requires the expression `e` to be an `Iterable`.  When the loop variable `i` is also statically typed:
+
+```dart
+for (T i in e) { … }
+```
+
+the expression `e` is required to be an `Iterable<T>`.
+
+*Note: we may weaken these.*
+
+### Await Expressions
+
+In an await expression of the form:
+```dart
+await expr
+```
+strong mode requires `expr` to be a subtype of `Future`.  In standard Dart, this is not required although tools may provide a hint.
+
+### Open Items
+
+TODO(vsm): Field overrides are now restricted.
+
+We do not yet implement but are considering the following restrictions as well.
+
+- Disallow overriding fields: this results in complicated generated
+  code where a field definition in a subclass shadows the field
+  definition in a base class but both are generally required to be
+  allocated.  Users should prefer explicit getters and setters in such
+  cases.  See [issue 52](https://github.com/dart-lang/dev_compiler/issues/52).
+
+- `Future<Future<T>>`: the Dart specification automatically flattens
+  this type to `Future<T>` (where `T` is not a `Future`).  This can be
+  issue as far as soundness.  We are considering forbidding this type
+  altogether (with a combination of static and runtime checks).  See
+  [issue 228](https://github.com/dart-lang/dev_compiler/issues/228).
+
+## DDC, Runtime checks, and Soundness
+
+TODO(vsm): Move this to a different doc?
+
+### Runtime Checks
+
+### Type Narrowing