dotnet-fsharp: Adding Basic F# Skills by ycastorium · Pull Request #753 · dotnet/skills

ycastorium · 2026-06-11T13:04:30Z

Adds a set of dotnet-fsharp skills closes: #752

github-actions · 2026-06-11T13:04:43Z

Note

This PR is from a fork and modifies infrastructure files (eng/ or .github/).

Changes to infrastructure typically need to be submitted from a branch in dotnet/skills (not a fork) so that CI workflows run with the correct permissions and secrets.

Please consider recreating this PR from an upstream branch. If you don't have push access to dotnet/skills, ask a maintainer to push your branch for you.

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Pull request overview

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Copilot was unable to run its full agentic suite in this review.

Adds a new dotnet-fsharp plugin that provides curated skills for writing idiomatic F#, testing, error handling, domain modeling, async/task usage, type providers, scripts, formatting with Fantomas, and .NET interop—along with evaluation scenarios and marketplace registration.

Changes:

Introduces the dotnet-fsharp plugin manifest and registers it in repository/plugin marketplaces.
Adds a set of F# skill guides (SKILL.md) plus an idiom rewrite reference doc.
Adds evaluation scenarios (tests/dotnet-fsharp/**/eval.yaml) covering each skill area.

Reviewed changes

Copilot reviewed 36 out of 36 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/dotnet-fsharp/writing-idiomatic-fsharp/eval.yaml	Adds eval scenarios for idiomatic refactors and null-to-Option guidance
tests/dotnet-fsharp/testing-fsharp/eval.yaml	Adds eval scenarios for property-based testing recommendations
tests/dotnet-fsharp/fsharp-units-of-measure/eval.yaml	Adds eval scenarios for units-of-measure modeling and conversions
tests/dotnet-fsharp/fsharp-type-providers/eval.yaml	Adds eval scenarios for JSON type provider usage vs trivial parsing
tests/dotnet-fsharp/fsharp-scripts/eval.yaml	Adds eval scenarios for `.fsx` scripting and `#r "nuget:"` usage
tests/dotnet-fsharp/fsharp-project-organization/eval.yaml	Adds eval scenarios for compile-order issues and namespace vs module guidance
tests/dotnet-fsharp/fsharp-error-handling/eval.yaml	Adds eval scenarios for Result-based errors and applicative validation
tests/dotnet-fsharp/fsharp-domain-modeling/eval.yaml	Adds eval scenarios for smart constructors and DU-based state modeling
tests/dotnet-fsharp/fsharp-async-and-tasks/eval.yaml	Adds eval scenarios for removing `.Result` and running async work in parallel
tests/dotnet-fsharp/fsharp-active-patterns/eval.yaml	Adds eval scenarios for partial/parameterized active patterns
tests/dotnet-fsharp/format-fsharp-with-fantomas/eval.yaml	Adds eval scenarios for Fantomas install/check and `.editorconfig` configuration
tests/dotnet-fsharp/design-fsharp-for-dotnet-interop/eval.yaml	Adds eval scenarios for C#-friendly public API shaping
tests/dotnet-fsharp/convert-csharp-to-fsharp/eval.yaml	Adds eval scenarios for idiomatic ports from C# to F#
tests/dotnet-fsharp/authoring-computation-expressions/eval.yaml	Adds eval scenarios for authoring a Result computation expression
plugins/dotnet-fsharp/skills/writing-idiomatic-fsharp/references/csharpism-rewrites.md	Adds before/after catalog of common “C#-isms” and idiomatic rewrites
plugins/dotnet-fsharp/skills/writing-idiomatic-fsharp/SKILL.md	Adds “writing idiomatic F#” skill definition and workflow
plugins/dotnet-fsharp/skills/testing-fsharp/SKILL.md	Adds testing skill definition including FsCheck/Expecto guidance
plugins/dotnet-fsharp/skills/fsharp-units-of-measure/SKILL.md	Adds units-of-measure skill definition and examples
plugins/dotnet-fsharp/skills/fsharp-type-providers/SKILL.md	Adds type provider skill definition and FSharp.Data examples
plugins/dotnet-fsharp/skills/fsharp-scripts/SKILL.md	Adds `.fsx`/`dotnet fsi` scripting skill definition and directives workflow
plugins/dotnet-fsharp/skills/fsharp-project-organization/SKILL.md	Adds project ordering and namespace/module guidance
plugins/dotnet-fsharp/skills/fsharp-error-handling/SKILL.md	Adds Result/Option vs exceptions guidance and validation patterns
plugins/dotnet-fsharp/skills/fsharp-domain-modeling/SKILL.md	Adds “illegal states unrepresentable” modeling patterns
plugins/dotnet-fsharp/skills/fsharp-async-and-tasks/SKILL.md	Adds async/task guidance, bridging patterns, and cancellation notes
plugins/dotnet-fsharp/skills/fsharp-active-patterns/SKILL.md	Adds active patterns guidance and examples
plugins/dotnet-fsharp/skills/format-fsharp-with-fantomas/SKILL.md	Adds Fantomas install/config/CI check guidance
plugins/dotnet-fsharp/skills/design-fsharp-for-dotnet-interop/SKILL.md	Adds public API interop rules and patterns
plugins/dotnet-fsharp/skills/convert-csharp-to-fsharp/SKILL.md	Adds mapping/workflow for idiomatic conversions from C#
plugins/dotnet-fsharp/skills/authoring-computation-expressions/SKILL.md	Adds CE authoring guidance and a minimal Result builder
plugins/dotnet-fsharp/plugin.json	Introduces the plugin manifest for `dotnet-fsharp`
eng/known-domains.txt	Adds F# ecosystem domains for link allowlisting
README.md	Adds `dotnet-fsharp` to the plugin list
.github/plugin/marketplace.json	Registers `dotnet-fsharp` in GitHub marketplace config
.cursor-plugin/marketplace.json	Registers `dotnet-fsharp` in Cursor marketplace config
.claude-plugin/marketplace.json	Registers `dotnet-fsharp` in Claude marketplace config
.agents/plugins/marketplace.json	Registers `dotnet-fsharp` in agents marketplace config

Comments suppressed due to low confidence (1)

tests/dotnet-fsharp/testing-fsharp/eval.yaml:1

This scenario asks the agent to 'run it' but doesn’t assert exit_success or specify expect_tools (like other scenarios that require dotnet fsi). Adding exit_success and expect_tools: [\"bash\"] (and optionally an output assertion indicating the test actually executed) will make the evaluation enforce the 'run it' requirement rather than only checking for framework keywords.

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ycastorium · 2026-06-11T13:19:10Z

+match status with
+| Active -> "on"
+| _ -> "off"


False positive |, not ||. The syntax is valid as-written.

ycastorium · 2026-06-11T13:19:23Z

+match status with
+| Active -> "on"
+| Suspended -> "off"
+| Closed -> "off"


False positive — same as above, these cases use a single |. Valid F# syntax.

+| `if x <> null then f x else d` | `x \|> Option.map f \|> Option.defaultValue d` |
+| `let r = if c then a else b` (statement style) | `let r = if c then a else b` (used as a value, single expression) |
+| `g(f(x))` | `x \|> f \|> g` |


ycastorium · 2026-06-11T13:19:34Z

+  - name: "Accumulate validation errors instead of stopping at the first"
+    prompt: |
+      In F#, validate a form with a Name (non-empty) and an Age (between 0 and 130). When BOTH
+      are invalid, the caller must receive BOTH error messages, not just the first one. Implement
+      it and demonstrate the both-invalid case with dotnet fsi.
+    assertions:
+      - type: "exit_success"
+    expect_tools: ["bash"]
+    rubric:
+      - "Uses applicative validation (and! / a Validation type) so errors accumulate"
+      - "Does NOT use Result.bind / let! chaining that would short-circuit on the first error"
+      - "The both-invalid run reports two errors"
+    timeout: 180


Added an output_matches assertion requiring both the Name and Age errors to appear. Skipped the let! guard: applicative validation in F# uses let! ... and!, so it would reject correct code. The no-short-circuit check stays in the rubric.

ycastorium · 2026-06-11T13:09:25Z

@dotnet-policy-service agree

Copilot

Pull request overview

Copilot reviewed 36 out of 36 changed files in this pull request and generated 6 comments.

ycastorium · 2026-06-11T13:24:21Z

@@ -0,0 +1,211 @@
+# C#-ism to idiomatic F# rewrites
+
+A before/after catalog for the most common ways C# habits leak into F#. Each pair compiles.


Softened the intro: the pairs are minimal fragments, not full programs. (The || part is a false positive — these cases already use a single |.)

ycastorium · 2026-06-11T13:24:23Z

+match status with
+| Active -> "on"
+| _ -> "off"


Softened the intro: the pairs are minimal fragments, not full programs. (The || part is a false positive — these cases already use a single |.)

ycastorium · 2026-06-11T13:24:24Z

+match status with
+| Active -> "on"
+| Suspended -> "off"
+| Closed -> "off"


Softened the intro: the pairs are minimal fragments, not full programs. (The || part is a false positive — these cases already use a single |.)

ycastorium · 2026-06-11T13:24:26Z

+`result { }` ships in libraries such as FsToolkit.ErrorHandling, or can be authored with
+`authoring-computation-expressions`.


Added attribution: validation { } and List.sequenceResultM now noted as FsToolkit.ErrorHandling, not FSharp.Core.

ycastorium · 2026-06-11T13:24:27Z

+    validation {
+        let! name = validateName input
+        and! age = validateAge input
+        return { Name = name; Age = age }
+    }


Added attribution: validation { } and List.sequenceResultM now noted as FsToolkit.ErrorHandling, not FSharp.Core.

ycastorium · 2026-06-11T13:24:29Z

+let parseAll lines =
+    lines
+    |> List.map parseLine          // string list -> Result<Row,string> list
+    |> List.sequenceResultM        // -> Result<Row list, string>


Added attribution: validation { } and List.sequenceResultM now noted as FsToolkit.ErrorHandling, not FSharp.Core.

AbhitejJohn · 2026-06-16T16:22:56Z

@T-Gro : Gentle ping to help review.

T-Gro

Thank you for this contribution.

There is massive overlap with existing F# docs and style guide.

I would like you to research one of two options also w.r.t. to test results on the provided scenarios:

Make the skills a lot slimmer and just point them to docs
Just use the mslearn MCP
If there are practical reasons for not using any of the above, let embed metadata so that .md here can evolve as docs are evolving. (= I am after future maintenance of this content. Someone will have to do it). i.e. admit this is a copy, list the practical reasons why a copy is better than MCP/URL, and allow automatic sync.

If there is legit content not covered in the docs, do keep it in here, I am definitely happy for any of that.

Also, to make review easier, it would be better if you shared original prompt that went into generating this - so that we can review the methodology of assembling this instead of reading the full set.

It is also great you did the eval scenarios - they will be helpful in comparing the options I listed above, great job 👍 . It should also show which skills are not providing much value over nothing (just leaning on model's training data), MsLearn MCP or slim skills just pointing to docs.

T-Gro · 2026-06-18T08:27:01Z

One more thought, maybe consider porting #236 over to this PR as well ?

github-actions · 2026-07-20T22:32:41Z

This PR has been automatically marked as stale because it has no activity for 30 days. It will be closed if no further activity occurs within another 7 days of this comment. If it is closed, you may reopen it anytime when you're ready again.

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github-actions · 2026-07-27T05:33:06Z

This pull request has been automatically closed because it has been open for more than 30 days with no recent activity.

If you believe this work is still relevant, please feel free to reopen or create a new pull request. Thank you for your contribution!

dotnet-fsharp: Adding Basic F# Skills

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Copilot AI review requested due to automatic review settings June 11, 2026 13:04

ycastorium requested review from JanKrivanek, ViktorHofer, dbreshears and timheuer as code owners June 11, 2026 13:04

Merge branch 'main' into f_sharp_is_also_dotnet

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dotnet-fsharp: address PR review feedback on skill and evals

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Copilot AI review requested due to automatic review settings June 11, 2026 13:17

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dotnet-fsharp: attribute FsToolkit constructs, soften compile claim

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JanKrivanek requested a review from T-Gro June 11, 2026 17:21

T-Gro suggested changes Jun 18, 2026

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JanKrivanek mentioned this pull request Jul 22, 2026

Fold "Close Stale PRs" into a deterministic pr-triage stale sweep #928

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github-actions Bot closed this Jul 27, 2026

		@@ -0,0 +1,211 @@
		# C#-ism to idiomatic F# rewrites

		A before/after catalog for the most common ways C# habits leak into F#. Each pair compiles.

		`result { }` ships in libraries such as FsToolkit.ErrorHandling, or can be authored with
		`authoring-computation-expressions`.

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