About "default" keyword in switch statement

1116 · December 6, 2016, 3:48am

Many languages have a switch statement and most (maybe all?) of them use "default" keyword for the condition which doesn't match any of those listed in the switch statement.

But sometimes that condition is just an exception, which literally IS NOT a default condition.

So how about using other keywords like "else" instead of "default"?

1116 · December 6, 2016, 4:11am

p.s.

I've read this mail and totally understand that Swift is designed to be familiar with C family languages, which "default" is widely used.

github.com

apple/swift-evolution/blob/main/commonly_proposed.md

# Commonly Rejected Changes 

This is a list of changes to the Swift language that are frequently proposed but that are unlikely to be accepted.  If you're interested in pursuing something in this space, please familiarize yourself with the discussions that we have already had.  In order to bring one of these topics up, you'll be expected to add new information to the discussion, not just to say, "I really want this" or "this exists in some other language and I liked it there".

Additionally, proposals for out-of-scope changes will not be scheduled for review. The [readme file](README.md) identifies a list of priorities for the next major release of Swift, and the [dashboard](https://www.swift.org/swift-evolution/) includes a list of changes that have been rejected after a formal review.

Several of the discussions below refer to "C family" languages. This is intended to mean the extended family of languages that resemble C at a syntactic level, such as C++, C#, Objective-C, Java, and Javascript. Swift embraces its C heritage. Where it deviates from other languages in the family, it does so because the feature was thought actively harmful (such as the pre/post-increment `++`) or to reduce needless clutter (such as `;` or parentheses in `if` statements).

## Basic Syntax and Operators

 * [Replace `{}` brace syntax with Python-style indentation](https://forums.swift.org/t/brace-syntax/678/3): Surely a polarizing issue, but Swift will not change to use indentation for scoping instead of curly braces.

 * [Remove `;` semicolons](https://forums.swift.org/t/proposal-to-remove-semicolons/523/3): Semicolons within a line are an intentional expressivity feature.  Semicolons at the end of the line should be handled by a linter, not by the compiler.

 * [Replace logical operators (`&&`, `||`, `!`, etc.) with words like "and", "or", "not"](https://forums.swift.org/t/change-the-name-of-the-boolean-operators/30/2), and [allow non-punctuation operators](https://forums.swift.org/t/allowing-characters-for-use-as-custom-operators/952) and infix functions: The operator and identifier grammars are intentionally partitioned in Swift, which is a key part of how user-defined overloaded operators are supported.  Requiring the compiler to see the "operator" declaration to know how to parse a file would break the ability to be able to parse a Swift file without parsing all of its imports.  This has a major negative effect on tooling support. While not needing infix support, `not` would need operator or keyword status to omit the parentheses as `!` can, and `not somePredicate()` visually binds too loosely compared to `!somePredicate()`.

 * [Replace `?:` ternary operator](https://forums.swift.org/t/ternary-operator-suggestion/49/148): Definitely magical, but it serves a very important use-case for terse selection of different values.  Proposals for alternatives have been intensely discussed, but none have been "better enough" for it to make sense to diverge from the precedent established by the C family of languages.

## Strings, Characters, and Collection Types

This file has been truncated. show original

But since Swift 3.0 has already make many source breaking changes and supports many features which most C family languages don't have, like labels in functions (except Objective-C, I guess), "inout" keyword in functions, etc, and one of Swift's goals is to make syntax "clear," hope we can re-consider the "default" keyword.

Thanks for your time.

···

2016/12/06 12:48、星野恵瑠 <el.hoshino@me.com> のメール：

Many languages have a switch statement and most (maybe all?) of them use "default" keyword for the condition which doesn't match any of those listed in the switch statement.

But sometimes that condition is just an exception, which literally IS NOT a default condition.

So how about using other keywords like "else" instead of "default"?

codafi · December 6, 2016, 11:48pm

Perhaps you're thinking about this choice of syntax from a different angle than was intended. `default` is not literally meant to be a "default value" or, in most cases, "the default case", rather a "default pattern"; a place for control to flow given no other option. Some other languages, especially those that emphasize patterns, syntaxize words like "otherwise" or simply use "_", but I've not seen languages that mix C-style switch-case keywords with C-style branching keywords before.

~Robert Widmann

2016/12/05 22:48、星野恵瑠 via swift-evolution <swift-evolution@swift.org> のメッセージ:

···

Many languages have a switch statement and most (maybe all?) of them use "default" keyword for the condition which doesn't match any of those listed in the switch statement.

But sometimes that condition is just an exception, which literally IS NOT a default condition.

So how about using other keywords like "else" instead of "default"?
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swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Joe_Groff · December 7, 2016, 1:38am

"Case Else" is common in Pascal-lineage syntaxes. (Shameless plug) We also used 'else' for the default block signifier in Clay, since we adopted block syntax for cases instead of label-style cases as in C, e.g. 'switch (x) case (1) { ... } else { ... }'.

-Joe

···

On Dec 6, 2016, at 3:48 PM, Robert Widmann via swift-evolution <swift-evolution@swift.org> wrote:

Perhaps you're thinking about this choice of syntax from a different angle than was intended. `default` is not literally meant to be a "default value" or, in most cases, "the default case", rather a "default pattern"; a place for control to flow given no other option. Some other languages, especially those that emphasize patterns, syntaxize words like "otherwise" or simply use "_", but I've not seen languages that mix C-style switch-case keywords with C-style branching keywords before.

jeremyp · December 6, 2016, 5:51pm

p.s.

I've read this mail and totally understand that Swift is designed to be familiar with C family languages, which "default" is widely used.
https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md

Did you read the first paragraph of that page and follow its advice to "familiarize yourself with the discussions that we have already had”? They were heated and protracted and ended with the situation unchanged.

Personally, I don’t see the problem with default. I really don’t think the breakage of pretty much everything caused by removing it is worth it even if there are better synonyms.

Plus, I little while ago, I came across a situation where the code would have read marginally better if I had been able to put the default case at the top of the switch (illegal in Swift 3) with the other cases below it (illegal in Swift 3). If that restriction were ever to be lifted, “else” would suddenly be a very poor choice.

···

On 6 Dec 2016, at 04:11, 星野恵瑠 via swift-evolution <swift-evolution@swift.org> wrote:

But since Swift 3.0 has already make many source breaking changes and supports many features which most C family languages don't have, like labels in functions (except Objective-C, I guess), "inout" keyword in functions, etc, and one of Swift's goals is to make syntax "clear," hope we can re-consider the "default" keyword.

Thanks for your time.

2016/12/06 12:48、星野恵瑠 <el.hoshino@me.com> のメール：

Many languages have a switch statement and most (maybe all?) of them use "default" keyword for the condition which doesn't match any of those listed in the switch statement.

But sometimes that condition is just an exception, which literally IS NOT a default condition.

So how about using other keywords like "else" instead of "default"?

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

1116 · December 7, 2016, 3:20am

Hi everyone

Thanks for all of your replies, and also thanks a lot for your time discussing this topic with me.
Also especially thanks to Robert to explain the meaning of "default" in switch statement. Now I can agree keeping using "default" keyword in switch statement.
Sorry to bother you all with this topic.

Actually before I send that mail I talked about this keyword with some of my Swift developer friends, and some of them told me that they have the same trouble in using default in switch statement, while others told me that they've never thought about it because it's just what they did in C/C++ development.
So thanks to your advices, now I can explain it to my friends.

···

2016/12/07 8:48、Robert Widmann <devteam.codafi@gmail.com> のメール：

Perhaps you're thinking about this choice of syntax from a different angle than was intended. `default` is not literally meant to be a "default value" or, in most cases, "the default case", rather a "default pattern"; a place for control to flow given no other option. Some other languages, especially those that emphasize patterns, syntaxize words like "otherwise" or simply use "_", but I've not seen languages that mix C-style switch-case keywords with C-style branching keywords before.

~Robert Widmann

2016/12/05 22:48、星野恵瑠 via swift-evolution <swift-evolution@swift.org> のメッセージ:

Many languages have a switch statement and most (maybe all?) of them use "default" keyword for the condition which doesn't match any of those listed in the switch statement.

But sometimes that condition is just an exception, which literally IS NOT a default condition.

So how about using other keywords like "else" instead of "default"?
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

David_Sweeris · December 6, 2016, 6:53pm

+1

···

On Dec 6, 2016, at 9:51 AM, Jeremy Pereira via swift-evolution <swift-evolution@swift.org> wrote:

Plus, I little while ago, I came across a situation where the code would have read marginally better if I had been able to put the default case at the top of the switch (illegal in Swift 3) with the other cases below it (illegal in Swift 3).