| // Copyright (c) 2012, the Dart project authors. Please see the AUTHORS file |
| // for details. All rights reserved. Use of this source code is governed by a |
| // BSD-style license that can be found in the LICENSE file. |
| |
| // @dart = 2.9 |
| |
| // Interpolation calls `toString`. |
| // The evaluation of the interpolation fails if `toString` throws or returns |
| // null. In Dart 2, any method overriding `Object.toString` must return a |
| // `String` or `null`. In particular, if `object.toString()` returns null, then |
| // `"$object"` must not evaluate to the string `"null"`. |
| // |
| // The specification states that the expression of an interpolation is |
| // evaluated as follows: |
| // |
| // 1. Evaluate $e_i$ to an object $o_i$. |
| // 2. Invoke the `toString` method on *o<sub>i</sub>* with no arguments, |
| // and let *r<sub>i</sub>*$ be the returned value. |
| // 3. If *r<sub>i</sub>* is not an instance of the built-in type `String`, |
| // throw an `Error`. |
| // |
| // (Then the resulting strings are concatenated with the literal string parts). |
| // |
| // |
| // Adding an object to a `StringBuffer` behaves the same as evaluating |
| // an expression in an interpolation. It must immediately fail if the |
| // object's toString throws or returns `null`. |
| // |
| // This ensures that implementing interpolation via a `StringBuffer`is |
| // a valid implementation choice. |
| |
| import "package:expect/expect.dart"; |
| |
| class ToStringString { |
| String toString() => "String"; |
| } |
| |
| class ToStringNull { |
| String toString() => null; |
| } |
| |
| class ToStringThrows { |
| String toString() => throw "Throw"; |
| } |
| |
| void main() { |
| var s = ToStringString(); |
| var n = ToStringNull(); |
| var t = ToStringThrows(); |
| |
| Expect.equals("$s$s", "StringString"); |
| // Throws immediately when evaluating the first interpolated expression. |
| Expect.throws<String>(() => "$t${throw "unreachable"}", (e) => e == "Throw"); |
| Expect.throws<Error>(() => "$n${throw "unreachable"}"); |
| |
| // Throws immediately when adding object that doesn't return a String. |
| Expect.equals( |
| (StringBuffer()..write(s)..write(s)).toString(), "StringString"); |
| Expect.throws<String>( |
| () => StringBuffer()..write(t)..write(throw "unreachable"), |
| (e) => e == "Throw"); |
| Expect.throws<Error>( |
| () => StringBuffer()..write(n)..write(throw "unreachable")); |
| |
| // Same behavior for constructor argument as if adding it to buffer later. |
| Expect.equals((StringBuffer(s)..write(s)).toString(), "StringString"); |
| Expect.throws<String>( |
| () => StringBuffer(t)..write(throw "unreachable"), (e) => e == "Throw"); |
| Expect.throws<Error>(() => StringBuffer(n)..write(throw "unreachable")); |
| } |