fsharp / fsharp/fslang-suggestions

Compiler should infer union case names from known type information

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#1,074 10 comments 19 reactions 0 assignees View on GitHub
area: unions
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Description

I propose we enable the compiler to infer the proper union case name in a `match` statement from known type information.

The existing way of approaching this problem in F# is that the compiler uses the last type that is declared. It's confusing b/c it can also matter which order your `open` statements are in.

Below is an example of code that should compile:

```fsharp
type A =
| Updating
| Done

type B =
| Updating
| Complete

type C = {
Status:A
}

[]
let main argv =

let c = { Status = Done }

match c.Status with
| Updating -> 0
| Done -> 1
```

## Pros and Cons

The advantages of making this adjustment to F# are:

1. It will be easier to refactor code
2. It will be easier to maintain code

The disadvantages of making this adjustment to F# are ...

I'm not sure

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): M

Related suggestions: (put links to related suggestions here)

I filed this issue: https://github.com/dotnet/fsharp/issues/12131

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design**
* [ ] I or my company would be willing to help implement and/or test this

> ** @dsyme said that this behavior is by design, but I don't think it would be breaking. code that did compile wouldn't break due to this change.

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Contributor guide

No contributing guide indexed for this repository

Research direction

Start with the F# example in the issue and the linked related suggestion. Determine how known type information in the match expression should select the union case name, and confirm that the example compiles without depending on declaration or open-statement order.

Written by the indexing model from the issue text.

Assessment

Tech stack
fsharp
Domain
compilers
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
25/100

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