@alfonsogarciacaro Are we ready to publish the beta version as stable?

We were trying to update packages in Fable.SignalR and came across the following problem.

If only Fable.SimpleJson is used everything compiles fine, but as soon as Fable.Promise is added as nuget package the compilation fails with following output:

Fable 4.1.4: F# to JavaScript compiler
Minimum fable-library version (when installed from npm): 1.1.1

Thanks to the contributor! @cmeeren
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Parsing TestSimpleJson.fsproj...
.> cmd /C dotnet restore TestSimpleJson.csproj -p:FABLE_COMPILER=true -p:FABLE_COMPILER_4=true -p:FABLE_COMPILER_JAVASCRIPT=true
  Determining projects to restore...
  Restored C:\Users\leko.tomas\playground\TestSimpleJson\TestSimpleJson.csproj (in 207 ms).
Project and references (12 source files) parsed in 3833ms

Started Fable compilation...
Fable compilation finished in 3523ms

.\fable_modules\Fable.SimpleJson.3.24.0\SimpleJson.fs(6,12): (6,18) error FSHARP: The namespace 'Import' is not defined. (code 39)
Compilation failed

Here is a smallest repro I could make. TestSimpleJson

• clone the repo
• dotnet tool restore
• dotnet fable .

We already found a similar issue here, but we couldn't find solution for this problem.

I was looking at some compiled JS code (due to a bug I had on a "catch" clause) and found out interestingly that fable-promise uses the form .then(success, error) way of handle exceptions
https://github.com/fable-compiler/fable-promise/blob/master/src/Promise.fs#L35-L45

usually the preferred form (afaik) is

.then(success).catch(error)

since this will allow exception bubbling (i.e. pass the exception to the next catch handler in the chain and so on until you get to the last) even if there's a throw inside .catch which sometimes is useful a graceful recovery and continue with async operations.

when you use .then(success, error) if the success handler throws or is rejected, the error handler will not be invoked, this rejection will be sent to the next .catch on the promise chain

I got an example here

https://codesandbox.io/s/promise-errors-cf439?file=/src/index.js

let me know if this is something we can look into I can try to save some time and send a PR later on

Some reading stuff on the matter (the first one is my always go-to resource)
https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html (tl;dr Advanced mistake 2)
https://dmitripavlutin.com/javascript-promises-then-vs-then-catch/#2-whats-the-difference

Promise.Parallel is an alias to promise.all in JavaScript Source. Why not name it all directly, especially when we need to name it Parallel with an upper P in order to avoid F# warning.

After a dotnet paket update I noticed that navigation with Feliz.Router.BasePath stopped working, or better, the hook stopped setting a new state with the current page.

I was able to track it down to Fable.Promise 3.1. When I downgraded to 2.2.2, it worked again.

I still have no idea what is happening, and whether Feliz.Router or the React.useElmish hook is also affected.

Description

While trying to port Ionide to Fable2 (Still with Fable.Core 2 but 3 seem to have the same problem) I remarked that the signatures of methods of Promise that were previously correct are now missing a big chunk of the functionalities.

Repro code

Let's read MDN and see what Promise.then handlers can do:

• returns a value, the promise returned by then gets resolved with the returned value as its value;
• doesn't return anything, the promise returned by then gets resolved with an undefined value;
• returns an already fulfilled promise, the promise returned by then gets fulfilled with that promise's value as its value;

Ok Fable exposes the javascript API, so given the documentation I wound expect that handlers are 'TBefore -> U3<unit, 'TResult, PromiseLike<'TResult>> but that's not the case:

    and [<AllowNullLiteral>] Promise<'T> =
        abstract ``then``: ?onfulfilled: ('T->'TResult) * ?onrejected: (obj->'TResult) -> Promise<'TResult>

Expected and actual results

Expected: Being able to use JS Api in a typed way
Actual: unbox, unbox, unbox

Related information

• Fable version: 2.3.7
• Fable.Core: 2.0.3
• Operating system: Windows 10

/home/gitlab-runner/builds/d5cd2d2a/0/a-team/eCarApp/src/Client/Utils.fs(42,22): error FS0041: Eine eindeutige Überladung für Methode 'Source' konnte nicht auf Basis von Typinformationen vor diesem Programmpunkt bestimmt werden. Unter Umständen ist eine Typanmerkung erforderlich.��Bekannter Argumenttyp: 'a��Kandidaten:� - member Promise.PromiseBuilder.Source : p:Fable.Core.JS.Promise<'T1> -> Fable.Core.JS.Promise<'T1>� - member Promise.PromiseBuilder.Source : ps:'a0 -> 'a0 when 'a0 :> seq<'a1> [/home/gitlab-runner/builds/d5cd2d2a/0/a-team/eCarApp/test/ComponentTests/ComponentTests.fsproj]

It says it wants a type hint. Why is that? This used to work.

FSC : error FS0193: Das Modul bzw. der Namespace "Promise" aus der Kompilierungseinheit "Fable.Promise" enthielt nicht den Wert "ValLinkagePartialKey(Run)". [/home/gitlab-runner/builds/d5cd2d2a/0/a-team/eCarApp/src/Client/Client.fsproj]

Sorry for german error message. It says it can't find ValLinkagePartialKey

See MDN. We could have something like:

let race2 (pr1: 'T1) (pr2: 'T2): Promise<Choice<'T1, 'T2>>
let race3 (pr1: 'T1) (pr2: 'T2) (pr3: 'T3): Promise<Choice<'T1, 'T2, 'T3>>
..

@alfonsogarciacaro

When upgrading to version 3.0.0-beta-001 in my project, it make Fable compilation fails.

C:/Users/mamangel/Documents/Workspace/Gitlab/fleet-view/fleet-mapping/Medio/Server/Tests/fableBuild/.fable/Fable.Fetch.2.1.0/Fetch.fs(351,12): (351,23) error FSHARP: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved. (code 72)

Yes, the error in Fetch.fs and not fable-promise but that's actually fable-promise which make it fails because if I everything works fine.

Also, when compiling the project via dotnet build I don't have the error so I still trying to find the source of the error.

As seen on TV:
http://www.fssnip.net/7XZ/title/Async-applicatives
https://devblogs.microsoft.com/dotnet/announcing-f-5/#applicative-computation-expressions
https://gist.github.com/cartermp/ac1f62acc605624945039d5620706fb5
https://gist.github.com/cartermp/8540e3fb1edf647f3c7ee8e4352c5941
https://github.com/fsharp/fslang-design/blob/master/FSharp-5.0/FS-1063-support-letbang-andbang-for-applicative-functors.md

Should be easier & more natural to do parallel operations (i suppose andFor does that ATM)