Add fsharp-project-structure skill

.github/CODEOWNERS

@@ -16,6 +16,9 @@
 /plugins/dotnet/skills/csharp-scripts/ @dotnet/roslyn
 /tests/dotnet/csharp-scripts/ @dotnet/roslyn
 
+/plugins/dotnet/skills/fsharp-project-structure/ @dotnet/fsharp @T-Gro
+/tests/dotnet/fsharp-project-structure/ @dotnet/fsharp @T-Gro
+
 /plugins/dotnet/skills/dotnet-pinvoke/ @dotnet/appmodel
 /tests/dotnet/dotnet-pinvoke/ @dotnet/appmodel
 

plugins/dotnet/skills/fsharp-project-structure/SKILL.md

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+---
+name: fsharp-project-structure
+description: "F# .fsproj file ordering, adding .fs/.fsi files, fixing FS0039 FS0010 FS0034 compilation order errors, signature files."
+---
+
+# F# Project File Structure
+
+F# compiles `<Compile Include>` items sequentially, top to bottom. A file can only reference types/modules from files listed **above** it in the .fsproj.
+
+## Inputs
+
+| Input | Required | Description |
+|-------|----------|-------------|
+| .fsproj file | Yes | The F# project file to modify |
+
+## File compilation order
+
+- File B can use types from file A only if A is listed BEFORE B in the .fsproj
+- Entry point (`Program.fs` or `[<EntryPoint>]`) must be LAST
+- When adding a file: check its `open` declarations, insert AFTER the last dependency but BEFORE any consumer
+- Wrong order symptom: `FS0039 "The type/value/namespace 'X' is not defined"` where X exists in another project file
+
+Example ordering:
+```xml
+<ItemGroup>
+  <Compile Include="Domain.fs" />
+  <Compile Include="Services.fs" />
+  <Compile Include="Program.fs" />
+</ItemGroup>
+```
+
+## Signature files (.fsi)
+
+- A `.fsi` defines the public API contract for its companion `.fs` file
+- Contains type signatures and `val` declarations — no implementation
+- `.fsi` MUST appear immediately BEFORE its `.fs` in the Compile list
+- If `.fsi` exists, public members in `.fs` not declared in `.fsi` become internal
+- `FS0034` (ValueNotContained): signature and implementation don't match
+
+Example with signatures:
+```xml
+<ItemGroup>
+  <Compile Include="Domain.fsi" />
+  <Compile Include="Domain.fs" />
+  <Compile Include="Services.fsi" />
+  <Compile Include="Services.fs" />
+  <Compile Include="Program.fs" />
+</ItemGroup>
+```
+
+## Workflow
+
+1. Identify where the new file fits in the dependency chain
+2. Add `<Compile Include>` at the correct position in .fsproj
+3. Run `dotnet build` to verify
+
+## Pitfalls
+
+| Pitfall | Fix |
+|---------|-----|
+| New file appended after Program.fs | Insert before Program.fs, after its dependencies |
+| `.fsi` placed AFTER its `.fs` | Must be immediately BEFORE |
+| Deleting `.fsi` to "fix" FS0034 | Update the `.fsi` to match the new API instead |
+| Circular dependency between files | Split one file into two |
+| Modifying source to work around order | Reorder `<Compile>` items in .fsproj instead |

tests/dotnet/fsharp-project-structure/BrokenOrder/BrokenOrder.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="Services.fs" />
+    <Compile Include="Domain.fs" />
+    <Compile Include="Program.fs" />
+  </ItemGroup>
+</Project>

tests/dotnet/fsharp-project-structure/BrokenOrder/Domain.fs

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+module BrokenOrder.Domain
+
+type OrderId = OrderId of int
+
+type OrderItem = {
+    Name: string
+    Quantity: int
+    Price: decimal
+}
+
+type Order = {
+    Id: OrderId
+    CustomerName: string
+    Items: OrderItem list
+}

tests/dotnet/fsharp-project-structure/BrokenOrder/Program.fs

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+open BrokenOrder.Domain
+open BrokenOrder.Services
+
+let order = {
+    Id = OrderId 1
+    CustomerName = "Alice"
+    Items = [
+        { Name = "Widget"; Quantity = 2; Price = 9.99m }
+        { Name = "Gadget"; Quantity = 1; Price = 24.99m }
+    ]
+}
+
+let total = processOrder order
+printfn "Order total: $%M" total

tests/dotnet/fsharp-project-structure/BrokenOrder/Services.fs

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+module BrokenOrder.Services
+
+open BrokenOrder.Domain
+
+let calculateTotal (order: Order) =
+    order.Items
+    |> List.sumBy (fun item -> item.Price * decimal item.Quantity)
+
+let processOrder (order: Order) =
+    let total = calculateTotal order
+    printfn "Processing order for %s: $%M" order.CustomerName total
+    total

tests/dotnet/fsharp-project-structure/OrderService/Domain.fs

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+module OrderService.Domain
+
+type OrderId = OrderId of int
+
+type OrderItem = {
+    Name: string
+    Quantity: int
+    Price: decimal
+}
+
+type Order = {
+    Id: OrderId
+    CustomerName: string
+    Items: OrderItem list
+}

tests/dotnet/fsharp-project-structure/OrderService/OrderService.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="Domain.fs" />
+    <Compile Include="Services.fs" />
+    <Compile Include="Program.fs" />
+  </ItemGroup>
+</Project>

tests/dotnet/fsharp-project-structure/OrderService/Program.fs

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+open OrderService.Domain
+open OrderService.Services
+
+let order = {
+    Id = OrderId 1
+    CustomerName = "Alice"
+    Items = [
+        { Name = "Widget"; Quantity = 2; Price = 9.99m }
+        { Name = "Gadget"; Quantity = 1; Price = 24.99m }
+    ]
+}
+
+let total = processOrder order
+printfn "Order total: $%M" total

tests/dotnet/fsharp-project-structure/OrderService/Services.fs

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+module OrderService.Services
+
+open OrderService.Domain
+
+let calculateTotal (order: Order) =
+    order.Items
+    |> List.sumBy (fun item -> item.Price * decimal item.Quantity)
+
+let processOrder (order: Order) =
+    let total = calculateTotal order
+    printfn "Processing order for %s: $%M" order.CustomerName total
+    total

tests/dotnet/fsharp-project-structure/eval.yaml

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+scenarios:
+  - name: "Add a module to an F# project"
+    prompt: |
+      Add a Validation.fs module to the F# project at OrderService/ that validates
+      orders before processing. It should check that the order has at least one item
+      and that the customer name is not empty. Return a Result<Order, string> with
+      a descriptive error message on failure. Then update Program.fs to validate the
+      order before processing it, printing the error if validation fails.
+    setup:
+      copy_test_files: true
+    assertions:
+      - type: "exit_success"
+      - type: "file_contains"
+        path: "OrderService/OrderService.fsproj"
+        value: "Validation.fs"
+    rubric:
+      - "Created Validation.fs with a validation function returning Result<Order, string>"
+      - "Inserted Compile Include=\"Validation.fs\" AFTER Domain.fs and BEFORE Program.fs in the .fsproj"
+      - "Did NOT place Validation.fs after Program.fs in the Compile item list"
+      - "Updated Program.fs to call the validation function before processing"
+      - "Ran dotnet build and it succeeded"
+    timeout: 120
+
+  - name: "Fix broken file order causing FS0039"
+    prompt: |
+      The F# project at BrokenOrder/ fails to build. Diagnose and fix the issue.
+    setup:
+      copy_test_files: true
+    assertions:
+      - type: "exit_success"
+      - type: "file_contains"
+        path: "BrokenOrder/BrokenOrder.fsproj"
+        value: "Domain.fs"
+    rubric:
+      - "Ran dotnet build and observed FS0039 or similar 'not defined' errors"
+      - "Identified that the failure is caused by wrong file order in the .fsproj — Services.fs is listed before Domain.fs"
+      - "Reordered Compile items so Domain.fs appears before Services.fs in the .fsproj"
+      - "Did NOT modify any .fs source files to work around the ordering issue"
+      - "Ran dotnet build after the fix and confirmed it succeeds"
+    timeout: 120
+
+  - name: "Add a signature file to define public API"
+    prompt: |
+      Add a signature file (Domain.fsi) for the Domain module in the F# project at
+      OrderService/ to explicitly define its public API surface. The signature should
+      expose all types and keep the module's public contract clear. Make sure the
+      project still builds after adding the signature file.
+    setup:
+      copy_test_files: true
+    assertions:
+      - type: "exit_success"
+      - type: "file_contains"
+        path: "OrderService/OrderService.fsproj"
+        value: "Domain.fsi"
+    rubric:
+      - "Created Domain.fsi with type signatures matching the types in Domain.fs"
+      - "Inserted Compile Include=\"Domain.fsi\" immediately BEFORE Domain.fs in the .fsproj"
+      - "Did NOT place Domain.fsi after Domain.fs in the Compile item list"
+      - "The .fsi file contains type declarations (not implementation code)"
+      - "The project builds successfully with the signature file present"
+    timeout: 120