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fixes #25: add HostHasSourceTextAvailable hook #26

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Feb 8, 2018
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fixes #25: add HostHasSourceTextAvailable hook #26

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9 changes: 8 additions & 1 deletion proposal.html
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Original file line number Diff line number Diff line change
Expand Up @@ -523,6 +523,13 @@ <h1>(<emu-xref href="#sec-createdynamicfunction">19.2.1.1.1</emu-xref>) Runtime
</emu-clause>


<emu-clause id="proposal-sec-hosthassourcetextavailable">
<h1>HostHasSourceTextAvailable ( _func_ )</h1>
<p>HostHasSourceTextAvailable is an implementation-defined abstract operation that allows host environments to prevent the source text from being provided for a given function.</p>
<p>An implementation of HostHasSourceTextAvailable may complete normally or abruptly. Any abrupt completions will be propagated to its callers. The default implementation of HostHasSourceTextAvailable is to unconditionally return a normal completion with a value of *true*.</p>
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Not sure we need the flexibility to return abruptly, but it doesn't hurt anything.

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I figured since toString can already return abruptly today, it didn't hurt to allow the engine to control that as well.

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Allowing abrupt completions doesn't seem useful to me. Function.prototype.toString is currently spec'ed to either return a string or throw a TypeError. With this change, it either returns a string or throws any value, so the error case is no longer restricted to TypeError.

For example test262 tests will need to be written as:

function f() {}

var errored = false;
var source;
try {
  source = f.toString();
} catch {
  // HostHasSourceTextAvailable is allowed to return abruptly, unconditionally pass in this case.
  errored = true;
}

if (!errored) {
  // NATIVE_FUNCTION_RE is defined in test262/harness/nativeFunctionMatcher.js
  assert(source === "function f() {}" || NATIVE_FUNCTION_RE.test(source));
}

If we don't allow abrupt completions, the test can be written as:

function f() {}

var source = f.toString();
assert(source === "function f() {}" || NATIVE_FUNCTION_RE.test(source));

Nit: The result must be compared to true, so HostHasSourceTextAvailable(func) is true, then return ... instead of HostHasSourceTextAvailable(func), then return .... And we should probably require HostHasSourceTextAvailable to return a boolean value.

And I assume not requiring stable results is intentional? So HostHasSourceTextAvailable is allowed to return different results over time even when the same argument is passed to it?

</emu-clause>


<!-- 19.2.3.5 -->
<emu-clause id="proposal-sec-function.prototype.tostring">
<h1>(<emu-xref href="#sec-function.prototype.tostring">19.2.3.5</emu-xref>) Function.prototype.toString ( )</h1>
Expand All @@ -549,7 +556,7 @@ <h1>(<emu-xref href="#sec-function.prototype.tostring">19.2.3.5</emu-xref>) Func
<ins class="block">
<emu-alg>
1. If _func_ is a <emu-xref href="#sec-bound-function-exotic-objects">Bound Function exotic object</emu-xref> or a <emu-xref href="#sec-ecmascript-standard-built-in-objects">built-in Function object</emu-xref>, then return an implementation-dependent String source code representation of _func_. The representation must have the syntax of a |NativeFunction|. Additionally, if _func_ is a <emu-xref href="#sec-well-known-intrinsic-objects">Well-known Intrinsic Object</emu-xref>, the portion of the returned String that would be matched by |IdentifierName| must be the initial value of the *name* property of _func_.
1. If _func_ has a [[SourceText]] internal slot and Type(_func_.[[SourceText]]) is String, then return _func_.[[SourceText]].
1. If _func_ has a [[SourceText]] internal slot and Type(_func_.[[SourceText]]) is String and ? HostHasSourceTextAvailable(_func_), then return _func_.[[SourceText]].
1. If Type(_func_) is Object and IsCallable(_func_) is *true*, then return an implementation-dependent String source code representation of _func_. The representation must have the syntax of a |NativeFunction|.
1. Throw a *TypeError* exception.
</emu-alg>
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