Proposal: switch expressions
Isiah Meadows
isiahmeadows at gmail.com
Tue Feb 26 19:40:48 UTC 2019
You're not alone in wanting pattern matching to be expression-based:
https://github.com/tc39/proposal-pattern-matching/issues/116
-----
Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com
-----
Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com
On Tue, Feb 26, 2019 at 1:34 PM David Koblas <david at 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 <david at 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 <ljharb at 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 <david at 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 <blueshuk2 at 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 <david at 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 at mozilla.org
>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at 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 <ljharb at 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 <david at 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 <blueshuk2 at 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 <david at 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 at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>
>
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
>
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
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