Add new Units type to ease UoM conversions

[roboz0r]

Description

Creates a new Units type to meet some of the goals in fsharp/fslang-suggestions#892

• RFC FS-1148 - Ease conversion between Units of Measure (UoM) and undecorated numerals and simplify casting fsharp/fslang-design#784

The type adds Units.Add, Units.Remove, and Units.Cast methods for supported primitive types to simplify UoM conversions. It also adds Units.Add*, Units.Remove*, and Units.Cast* e.g. Units.AddArray to perform UoM conversions on common collection types without allocations.

I haven't added any tests as the code doesn't actually do anything, it's just a wrapper over retype. Happy to add anything you think would be helpful.

Checklist

• Test cases added
• Performance benchmarks added in case of performance changes
• Release notes entry updated

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/FSharp.Core	docs/release-notes/.FSharp.Core/9.0.100.md

[vzarytovskii]

Few questions:

1. What bothers me is that this is not how we typically add functions to fsharp core - a type with static methods with a bunch of overloads
2. I think it contradicts a bit with what's approved in the suggestion - what functions and in which part of corelib.
3. It is missing a bunch of tests and documentation
4. What's the difference in assembly size before and after the addition?

I think before the implementation we need to start with RFC, and once that is approved and discussed, we can proceed with implementation

[roboz0r]

1. Agree it's not typical for fsharp core code but I found the ergonomics and discoverability much nicer to have 15 (primitive + 4 collections * 3) methods with 13 overloads than having 195 differently named methods.

• The naming and overall API are open to discussion but I felt it better to put out a concrete implementation to serve as a starting point for discussion.

2. The API design was inspired by Ease conversion between Units of Measure (UoM) and undecorated numerals and simplify casting fsharp/fslang-suggestions#892 (comment) the most recent comment in the that was received broadly positively from the few thumbs up. Agree that it is somewhat inconsistent with the initial description but I tried to synthesize with all the comments while avoiding the generic conversion which would require a language change.

• If you feel it would be better placed in LanguagePrimitives then that's an easy change to make, though as a noob I always felt the LanguagePrimitives module was intended as compiler internal helpers

3. Definitely still need to add the documentation. I intentionally held off on this, anticipating that the API specific were likely to change.

• What did you have in mind for the tests? I don't think there's any logic to test so would it just be a series of tests with explicit type annotations and if they compile then it's good?

4. Comparing the resulting size of FSharp.Core.dll (built with ./Build.cmd -c Release) is 2,250KB to 2,281KB

[vzarytovskii]

I think we should still start with rfc for it, so everyone agrees on specifics.

[roboz0r]

I put together a draft RFC here fsharp/fslang-design#784 is there something else I missed?

[vzarytovskii]

I put together a draft RFC here fsharp/fslang-design#784 is there something else I missed?

That looks like a good start, now we need some comments on it. I'll probably be also good to have all types next to examples so people get a good understanding how many methods/functions are added

[T-Gro]

@roboz0r : We have dicussed this addition and have an idea on how to bring the same functionality with a new compiler intrinsic, instead of adding many new functions to FSharp.Core.
Vlad will ping you on Discord on how this could work.

This would especially mean a lot smaller size footprint.