> This is unmaintainable --
>
> const x = v === 'foo' ? 1 : v === 'bar' ? 3 : v === 'baz' ? 6 : 99;
>
i feel proposed switch-expressions are no more readable/maintainable than
ternary-operators, if you follow jslint's style-guide. i'll like to see more
convincing evidence/use-case that they are better:
```javascript
/*jslint*/
"use strict";
const v = "foo";
const x = (
v === "foo"
? 1
: v === "bar"
? 3
: v === "baz"
? 6
: 99
);
```
here's another example from real-world production-code, where
switch-expressions probably wouldn't help:
```javascript
$ node -e '
/*jslint devel*/
"use strict";
function renderRecent(date) {
/*
* this function will render <date> to "xxx ago"
*/
date = Math.ceil((Date.now() - new Date(date).getTime()) * 0.0001) * 10;
return (
!Number.isFinite(date)
? ""
: date < 60
? date + " sec ago"
: date < 3600
? Math.round(date / 60) + " min ago"
: date < 86400
? Math.round(date / 3600) + " hr ago"
: date < 129600
? "1 day ago"
: Math.round(date / 86400) + " days ago"
);
}
console.log(renderRecent(new Date().toISOString())); // "0 sec ago"
console.log(renderRecent("2019-02-28T05:32:00Z")); // "10 sec ago"
console.log(renderRecent("2019-02-28T05:27:30Z")); // "5 min ago"
console.log(renderRecent("2019-02-28T05:14:00Z")); // "18 min ago"
console.log(renderRecent("2019-02-28T03:27:00Z")); // "2 hr ago"
console.log(renderRecent("2019-02-12T05:27:00Z")); // "16 days ago"
console.log(renderRecent("2018-02-28T05:27:00Z")); // "365 days ago"
'
0 sec ago
10 sec ago
5 min ago
18 min ago
2 hr ago
16 days ago
365 days ago
$
```
> On 27 Feb 2019, at 13:12, David Koblas <da...@koblas.com> wrote:
>
> Just for folks who might be interested, added a babel-plugin to see what was
> involved in making this possible.
>
> Pull request available here -- https://github.com/babel/babel/pull/9604
>
> I'm sure I'm missing a bunch of details, but would be interested in some help
> in making this a bit more real.
>
> Thanks
>
> On 2/26/19 2:40 PM, Isiah Meadows wrote:
>> You're not alone in wanting pattern matching to be expression-based:
>>
>> https://github.com/tc39/proposal-pattern-matching/issues/116
>>
>> -----
>>
>> Isiah Meadows
>> cont...@isiahmeadows.com
>> www.isiahmeadows.com
>>
>> -----
>>
>> Isiah Meadows
>> cont...@isiahmeadows.com
>> www.isiahmeadows.com
>>
>>
>> On Tue, Feb 26, 2019 at 1:34 PM David Koblas <da...@koblas.com> wrote:
>>> Jordan,
>>>
>>> Thanks for taking time to read and provide thoughts.
>>>
>>> I just back and re-read the pattern matching proposal and it still fails on
>>> the basic requirement of being an Expression not a Statement. The problem
>>> that I see and want to address is the need to have something that removes
>>> the need to chain trinary expressions together to have an Expression.
>>>
>>> This is unmaintainable --
>>>
>>> const x = v === 'foo' ? 1 : v === 'bar' ? 3 : v === 'baz' ? 6 : 99;
>>>
>>> This is maintainable, but is less than ideal:
>>>
>>> let x;
>>>
>>> switch (v) {
>>> case "foo":
>>> x = 1;
>>> break;
>>> case "bar":
>>> x = 3;
>>> break;
>>> case "baz":
>>> x = 6;
>>> break;
>>> default:
>>> x = 99;
>>> break;
>>> }
>>>
>>> Pattern matching does shorten the code, but you have a weird default case
>>> and also still end up with a loose variable and since pattern matching is a
>>> statement you still have a initially undefined variable.
>>>
>>> let x;
>>>
>>> case (v) {
>>> when "foo" -> x = 1;
>>> when "bar" -> x = 3;
>>> when "baz" -> x = 6;
>>> when v -> x = 99;
>>> }
>>>
>>> Let's try do expressions, I'll leave people's thoughts to themselves.
>>>
>>> const x = do {
>>> if (v === "foo") { 1; }
>>> else if (v === "bar") { 3; }
>>> else if (v === "baz") { 6; }
>>> else { 99; }
>>> }
>>>
>>> Or as another do expression variant:
>>>
>>> const x = do {
>>> switch (v) {
>>> case "foo": 1; break;
>>> case "bar": 3; break;
>>> case "baz": 6; break;
>>> default: 99; break;
>>> }
>>> }
>>>
>>> And as I'm thinking about switch expressions:
>>>
>>> const x = switch (v) {
>>> case "foo" => 1;
>>> case "bar" => 3;
>>> case "baz" => 6;
>>> default => 99;
>>> }
>>>
>>> What I really like is that it preserves all of the normal JavaScript syntax
>>> with the small change that a switch is allowed in an expression provided
>>> that all of the cases evaluate to expressions hence the use of the '=>' as
>>> an indicator. Fundamentally this is a very basic concept where you have a
>>> state machine and need it switch based on the current state and evaluate to
>>> the new state.
>>>
>>> const nextState = switch (currentState) {
>>> case ... =>
>>> }
>>>
>>>
>>>
>>> On 2/25/19 4:00 PM, Jordan Harband wrote:
>>>
>>> Pattern Matching is still at stage 1; so there's not really any permanent
>>> decisions that have been made - the repo theoretically should contain
>>> rationales for decisions up to this point.
>>>
>>> I can speak for myself (as "not a champion" of that proposal, just a fan)
>>> that any similarity to the reviled and terrible `switch` is something I'll
>>> be pushing back against - I want a replacement that lacks the footguns and
>>> pitfalls of `switch`, and that is easily teachable and googleable as a
>>> different, distinct thing.
>>>
>>> On Mon, Feb 25, 2019 at 12:42 PM David Koblas <da...@koblas.com> wrote:
>>>> Jordan,
>>>>
>>>> One question that I have lingering from pattern matching is why is the
>>>> syntax so different? IMHO it is still a switch statement with a variation
>>>> of the match on the case rather than a whole new construct.
>>>>
>>>> Is there somewhere I can find a bit of discussion about the history of the
>>>> syntax decisions?
>>>>
>>>> --David
>>>>
>>>>
>>>> On Feb 25, 2019, at 12:33 PM, Jordan Harband <ljh...@gmail.com> wrote:
>>>>
>>>> Additionally, https://github.com/tc39/proposal-pattern-matching - switch
>>>> statements are something I hope we'll soon be able to relegate to the
>>>> dustbin of history.
>>>>
>>>> On Mon, Feb 25, 2019 at 6:01 AM David Koblas <da...@koblas.com> wrote:
>>>>> I quite aware that it’s covered in do expressions. Personally I find do
>>>>> expressions non-JavaScript in style and it’s also not necessarily going
>>>>> to make it into the language.
>>>>>
>>>>> Hence why I wanted to put out there the idea of switch expressions.
>>>>>
>>>>> --David
>>>>>
>>>>>
>>>>> On Feb 25, 2019, at 5:28 AM, N. Oxer <bluesh...@gmail.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> This would be covered by do expressions. You could just do:
>>>>>
>>>>> ```js
>>>>> const category = do {
>>>>> switch (...) {
>>>>> ...
>>>>> };
>>>>> };
>>>>> ```
>>>>>
>>>>> On Sun, Feb 24, 2019 at 10:42 AM David Koblas <da...@koblas.com> wrote:
>>>>>> After looking at a bunch of code in our system noted that there are many
>>>>>> cases where our code base has a pattern similar to this:
>>>>>>
>>>>>> let category = data.category;
>>>>>>
>>>>>> if (category === undefined) {
>>>>>> // Even if Tax is not enabled, we have defaults for incomeCode
>>>>>> switch (session.merchant.settings.tax.incomeCode) {
>>>>>> case TaxIncomeCode.RENTS_14:
>>>>>> category = PaymentCategory.RENT;
>>>>>> break;
>>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17:
>>>>>> category = PaymentCategory.SERVICES;
>>>>>> break;
>>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17:
>>>>>> category = PaymentCategory.SERVICES;
>>>>>> break;
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> I also bumped into a block of go code that also implemented similar
>>>>>> patterns, which really demonstrated to me that there while you could go
>>>>>> crazy with triary nesting there should be a better way. Looked at the
>>>>>> pattern matching proposal and while could possibly help looked like it
>>>>>> was overkill for the typical use case that I'm seeing. The most relevant
>>>>>> example I noted was switch expressions from Java. When applied to this
>>>>>> problem really create a simple result:
>>>>>>
>>>>>> const category = data.category || switch (setting.incomeCode) {
>>>>>> case TaxIncomeCode.RENTS_14 => PaymentCategory.RENT;
>>>>>> case TaxIncomeCode.ROYALTIES_COPYRIGHTS_12 =>
>>>>>> PaymentCategory.ROYALTIES;
>>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17 =>
>>>>>> PaymentCategory.SERVICES;
>>>>>> default => PaymentCategory.OTHER;
>>>>>> }
>>>>>>
>>>>>> Note; the instead of using the '->' as Java, continue to use => and with
>>>>>> the understanding that the right hand side is fundamentally function.
>>>>>> So similar things to this are natural, note this proposal should remove
>>>>>> "fall through" breaks and allow for multiple cases as such.
>>>>>>
>>>>>> const quarter = switch (foo) {
>>>>>> case "Jan", "Feb", "Mar" => "Q1";
>>>>>> case "Apr", "May", "Jun" => "Q2";
>>>>>> case "Jul", "Aug", "Sep" => "Q3";
>>>>>> case "Oct", "Nov", "Dec" => { return "Q4" };
>>>>>> default => { throw new Error("Invalid Month") };
>>>>>> }
>>>>>>
>>>>>> Also compared this to the do expression proposal, it also provides a
>>>>>> substantial simplification, but in a way that is more consistent with
>>>>>> the existing language. In one of their examples they provide an example
>>>>>> of the Redux reducer
>>>>>> https://redux.js.org/basics/reducers#splitting-reducers -- this would be
>>>>>> a switch expression implementation.
>>>>>>
>>>>>> function todoApp(state = initialState, action) => switch
>>>>>> (action.type) {
>>>>>> case SET_VISIBILITY_FILTER => { ...state, visibilityFilter:
>>>>>> action.filter };
>>>>>> case ADD_TODO => {
>>>>>> ...state, todos: [
>>>>>> ...state.todos,
>>>>>> {
>>>>>> text: action.text,
>>>>>> completed: false
>>>>>> }
>>>>>> ]
>>>>>> };
>>>>>> case TOGGLE_TODO => {
>>>>>> ...state,
>>>>>> todos: state.todos.map((todo, index) => (index ===
>>>>>> action.index) ? { ...todo, completed: !todo.completed } : todo)
>>>>>> };
>>>>>> default => state;
>>>>>> }
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> es-discuss mailing list
>>>>>> es-discuss@mozilla.org
>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>> _______________________________________________
>>>>> es-discuss mailing list
>>>>> es-discuss@mozilla.org
>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>> On 2/25/19 3:42 PM, David Koblas wrote:
>>>
>>> Jordan,
>>>
>>> One question that I have lingering from pattern matching is why is the
>>> syntax so different? IMHO it is still a switch statement with a variation
>>> of the match on the case rather than a whole new construct.
>>>
>>> Is there somewhere I can find a bit of discussion about the history of the
>>> syntax decisions?
>>>
>>> --David
>>>
>>>
>>> On Feb 25, 2019, at 12:33 PM, Jordan Harband <ljh...@gmail.com> wrote:
>>>
>>> Additionally, https://github.com/tc39/proposal-pattern-matching - switch
>>> statements are something I hope we'll soon be able to relegate to the
>>> dustbin of history.
>>>
>>> On Mon, Feb 25, 2019 at 6:01 AM David Koblas <da...@koblas.com> wrote:
>>>> I quite aware that it’s covered in do expressions. Personally I find do
>>>> expressions non-JavaScript in style and it’s also not necessarily going to
>>>> make it into the language.
>>>>
>>>> Hence why I wanted to put out there the idea of switch expressions.
>>>>
>>>> --David
>>>>
>>>>
>>>> On Feb 25, 2019, at 5:28 AM, N. Oxer <bluesh...@gmail.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> This would be covered by do expressions. You could just do:
>>>>
>>>> ```js
>>>> const category = do {
>>>> switch (...) {
>>>> ...
>>>> };
>>>> };
>>>> ```
>>>>
>>>> On Sun, Feb 24, 2019 at 10:42 AM David Koblas <da...@koblas.com> wrote:
>>>>> After looking at a bunch of code in our system noted that there are many
>>>>> cases where our code base has a pattern similar to this:
>>>>>
>>>>> let category = data.category;
>>>>>
>>>>> if (category === undefined) {
>>>>> // Even if Tax is not enabled, we have defaults for incomeCode
>>>>> switch (session.merchant.settings.tax.incomeCode) {
>>>>> case TaxIncomeCode.RENTS_14:
>>>>> category = PaymentCategory.RENT;
>>>>> break;
>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17:
>>>>> category = PaymentCategory.SERVICES;
>>>>> break;
>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17:
>>>>> category = PaymentCategory.SERVICES;
>>>>> break;
>>>>> }
>>>>> }
>>>>>
>>>>> I also bumped into a block of go code that also implemented similar
>>>>> patterns, which really demonstrated to me that there while you could go
>>>>> crazy with triary nesting there should be a better way. Looked at the
>>>>> pattern matching proposal and while could possibly help looked like it
>>>>> was overkill for the typical use case that I'm seeing. The most relevant
>>>>> example I noted was switch expressions from Java. When applied to this
>>>>> problem really create a simple result:
>>>>>
>>>>> const category = data.category || switch (setting.incomeCode) {
>>>>> case TaxIncomeCode.RENTS_14 => PaymentCategory.RENT;
>>>>> case TaxIncomeCode.ROYALTIES_COPYRIGHTS_12 =>
>>>>> PaymentCategory.ROYALTIES;
>>>>> case TaxIncomeCode.INDEPENDENT_PERSONAL_SERVICE_17 =>
>>>>> PaymentCategory.SERVICES;
>>>>> default => PaymentCategory.OTHER;
>>>>> }
>>>>>
>>>>> Note; the instead of using the '->' as Java, continue to use => and with
>>>>> the understanding that the right hand side is fundamentally function.
>>>>> So similar things to this are natural, note this proposal should remove
>>>>> "fall through" breaks and allow for multiple cases as such.
>>>>>
>>>>> const quarter = switch (foo) {
>>>>> case "Jan", "Feb", "Mar" => "Q1";
>>>>> case "Apr", "May", "Jun" => "Q2";
>>>>> case "Jul", "Aug", "Sep" => "Q3";
>>>>> case "Oct", "Nov", "Dec" => { return "Q4" };
>>>>> default => { throw new Error("Invalid Month") };
>>>>> }
>>>>>
>>>>> Also compared this to the do expression proposal, it also provides a
>>>>> substantial simplification, but in a way that is more consistent with
>>>>> the existing language. In one of their examples they provide an example
>>>>> of the Redux reducer
>>>>> https://redux.js.org/basics/reducers#splitting-reducers -- this would be
>>>>> a switch expression implementation.
>>>>>
>>>>> function todoApp(state = initialState, action) => switch
>>>>> (action.type) {
>>>>> case SET_VISIBILITY_FILTER => { ...state, visibilityFilter:
>>>>> action.filter };
>>>>> case ADD_TODO => {
>>>>> ...state, todos: [
>>>>> ...state.todos,
>>>>> {
>>>>> text: action.text,
>>>>> completed: false
>>>>> }
>>>>> ]
>>>>> };
>>>>> case TOGGLE_TODO => {
>>>>> ...state,
>>>>> todos: state.todos.map((todo, index) => (index ===
>>>>> action.index) ? { ...todo, completed: !todo.completed } : todo)
>>>>> };
>>>>> default => state;
>>>>> }
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> es-discuss mailing list
>>>>> es-discuss@mozilla.org
>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>> _______________________________________________
>>>> es-discuss mailing list
>>>> es-discuss@mozilla.org
>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss@mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss@mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
> _______________________________________________
> es-discuss mailing list
> es-discuss@mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
_______________________________________________
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