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1b2454b85e
Change-Id: Ib09f268c8a8edffba007cdd4962550a5e78ff808 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151090 Reviewed-by: Erik Ernst <eernst@google.com> Commit-Queue: Bob Nystrom <rnystrom@google.com> Auto-Submit: Bob Nystrom <rnystrom@google.com>
60 lines
2.2 KiB
Dart
60 lines
2.2 KiB
Dart
// Copyright (c) 2012, the Dart project authors. Please see the AUTHORS file
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// for details. All rights reserved. Use of this source code is governed by a
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// BSD-style license that can be found in the LICENSE file.
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// Interpolation calls `toString`.
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// The evaluation of the interpolation fails if `toString` throws or returns
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// null. In Dart 2, any method overriding `Object.toString` must return a
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// `String` or `null`. In particular, if `object.toString()` returns null, then
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// `"$object"` must not evaluate to the string `"null"`.
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//
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// The specification states that the expression of an interpolation is
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// evaluated as follows:
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//
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// 1. Evaluate $e_i$ to an object $o_i$.
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// 2. Invoke the `toString` method on *o<sub>i</sub>* with no arguments,
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// and let *r<sub>i</sub>*$ be the returned value.
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// 3. If *r<sub>i</sub>* is not an instance of the built-in type `String`,
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// throw an `Error`.
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//
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// (Then the resulting strings are concatenated with the literal string parts).
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//
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//
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// Adding an object to a `StringBuffer` behaves the same as evaluating
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// an expression in an interpolation. It must immediately fail if the
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// object's toString throws or returns `null`.
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//
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// This ensures that implementing interpolation via a `StringBuffer`is
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// a valid implementation choice.
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import "package:expect/expect.dart";
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class ToStringString {
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String toString() => "String";
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}
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class ToStringThrows {
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String toString() => throw "Throw";
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}
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void main() {
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var s = ToStringString();
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var t = ToStringThrows();
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Expect.equals("$s$s", "StringString");
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// Throws immediately when evaluating the first interpolated expression.
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Expect.throws<String>(() => "$t${throw "unreachable"}", (e) => e == "Throw");
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// Throws immediately when adding object that doesn't return a String.
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Expect.equals(
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(StringBuffer()..write(s)..write(s)).toString(), "StringString");
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Expect.throws<String>(
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() => StringBuffer()..write(t)..write(throw "unreachable"),
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(e) => e == "Throw");
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// Same behavior for constructor argument as if adding it to buffer later.
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Expect.equals((StringBuffer(s)..write(s)).toString(), "StringString");
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Expect.throws<String>(
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() => StringBuffer(t)..write(throw "unreachable"), (e) => e == "Throw");
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}
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