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name: fsharp description: "Write, review, or modernize F# code in .NET repositories with functional-first design, algebraic data types, pattern matching, pipelines, async workflows, project ordering, and C# interop. USE FOR: F# .fs/.fsproj code; discriminated unions, records, options, results, pattern matching, computation expressions, or functional domain modeling; strongly typed AI-generated .NET code. DO NOT USE FOR: C# language modernization; FSI-only exploratory scripts with no project changes. INVOKES: inspect .fsproj file order and SDK settings, edit F# source and project files, and run dotnet build/test or dotnet fsi validation commands when changes are made." compatibility: "Requires a .NET SDK with F# support and an F# project, script, or interop boundary."

F# for .NET

Trigger On

• the task touches .fs, .fsx, .fsi, or .fsproj files
• domain logic benefits from records, discriminated unions, options, results, or exhaustive pattern matching
• generated code needs a strongly typed functional model instead of nullable primitive state
• F# code must interoperate with C# or other .NET libraries
• the repository needs project setup, source ordering, or validation for F# code

Do Not Use For

• C# language feature selection; use modern-csharp
• analyzer-only, formatter-only, or CI-only work with no F# language decisions
• one-off REPL or scripting exploration that does not affect project code; use fsi
• non-.NET functional languages

Project Setup

Use the .NET SDK templates when adding F# projects.

dotnet new classlib -lang "F#" -o src/Domain
dotnet new console -lang "F#" -o src/App
dotnet new xunit -lang "F#" -o tests/Domain.Tests
dotnet sln add src/Domain/Domain.fsproj tests/Domain.Tests/Domain.Tests.fsproj
dotnet add tests/Domain.Tests/Domain.Tests.fsproj reference src/Domain/Domain.fsproj
dotnet build

FSharp.Core is normally supplied by the F# SDK project. Add or pin it explicitly only when the repo has a package policy that requires deterministic package versions.

dotnet add src/Domain/Domain.fsproj package FSharp.Core

Source Ordering

F# compiles files in the order listed in the project file. Define types and modules before files that consume them.

<ItemGroup>
  <Compile Include="Domain.fs" />
  <Compile Include="Validation.fs" />
  <Compile Include="Program.fs" />
</ItemGroup>

When adding a file, update the .fsproj intentionally. Do not assume wildcard ordering.

Workflow

1. Inspect the existing .fsproj, .fs, .fsi, and .fsx files to determine compile order, public boundaries, and whether the code is F#-first or C#-facing.
2. Choose the smallest F# model that represents the invariant: records for named product data, discriminated unions for closed state, option for absence, and Result for expected failures.
3. Add or edit source files in dependency order, then update the .fsproj compile list before consumers reference the new code.
4. Review public API shape for .NET interop. Translate F#-specific internal models to DTOs, methods, or Try* patterns when C# callers need a stable surface.
5. Run dotnet build, targeted tests, and any relevant dotnet fsi probes before returning the final result.

Current Upstream Notes

• The August 2026 F# overview continues to emphasize functional-first programming on .NET with records, discriminated unions, pattern matching, units of measure, type providers, and interop. Keep F# guidance domain-model oriented rather than translating C# object patterns mechanically.
• Use fsi for exploratory scripts, but move durable code into ordered .fsproj files before it becomes production behavior.

Practical Patterns

Model State With Records And Unions

Prefer a type that names every valid state over loose strings, nullable values, or parallel booleans.

module Orders

type OrderId = private OrderId of string

module OrderId =
    let tryCreate value =
        if System.String.IsNullOrWhiteSpace value then
            Error "Order id is required."
        else
            Ok (OrderId value)

type Payment =
    | Card of last4: string
    | Wire of iban: string
    | PurchaseOrder of number: string

type Order =
    { Id: OrderId
      Customer: string
      Payment: Payment }

let describePayment payment =
    match payment with
    | Card last4 -> quot;card ending {last4}"
    | Wire iban -> quot;wire transfer {iban}"
    | PurchaseOrder number -> quot;purchase order {number}"

Compose Validation With Result

Return Result<'T,'Error> when callers must handle failure and when failures are part of the domain.

module Pricing

type Price =
    private
    | Price of decimal

module Price =
    let tryCreate amount =
        if amount < 0m then
            Error "Price cannot be negative."
        else
            Ok (Price amount)

    let value (Price amount) = amount

type Line =
    { Sku: string
      Quantity: int
      UnitPrice: Price }

let tryCreateLine sku quantity amount =
    match Price.tryCreate amount with
    | Ok price when quantity > 0 ->
        Ok { Sku = sku; Quantity = quantity; UnitPrice = price }
    | Ok _ ->
        Error "Quantity must be positive."
    | Error message ->
        Error message

Read And Transform Data

Use pipelines for readable transformations, but stop before the pipeline hides error handling or allocation cost.

open System.IO

let readActiveUsers path =
    File.ReadLines path
    |> Seq.skip 1
    |> Seq.choose (fun line ->
        match line.Split(',') with
        | [| id; name; "active" |] -> Some {| Id = id; Name = name |}
        | _ -> None)
    |> Seq.toList

Write A Small .NET Boundary

Make public interop APIs easy for C# callers. Hide F#-specific details behind functions, methods, or DTO records when needed.

namespace Company.Domain

type InvoiceDto =
    { Id: string
      Total: decimal
      IsPaid: bool }

module Invoice =
    let markPaid invoice =
        { invoice with IsPaid = true }

Interop Guidance

• Keep F# discriminated unions inside F# boundaries unless C# consumers are expected to understand their generated shape.
• Use records or explicit classes for public cross-language DTOs.
• Use option internally; translate to nullable annotations, Try* methods, or Result at C#-first public boundaries.
• Avoid throwing for expected domain failures. Use exceptions for unexpected infrastructure failures.
• Prefer task { } or Task-returning APIs at .NET interop boundaries; use Async<'T> when the code is F#-first.

Review Checklist

• The .fsproj compile order matches dependency order.
• All union cases are handled explicitly; wildcard branches are justified.
• Public types have stable names and shapes for downstream .NET consumers.
• Domain failures are modeled with option or Result, not undocumented null.
• Pipelines remain readable and do not hide repeated enumeration of expensive sequences.
• Tests cover at least one success and one failure path for each domain constructor or validator.

Validate

Run the narrowest relevant checks after changes:

dotnet build
dotnet test
dotnet fsi scripts/check.fsx

If adding a new F# file, also inspect the project file diff to confirm the compile order is deliberate.

Sources

• https://learn.microsoft.com/dotnet/fsharp/what-is-fsharp
• https://learn.microsoft.com/dotnet/fsharp/get-started/get-started-command-line
• https://learn.microsoft.com/dotnet/fsharp/language-reference/discriminated-unions
• https://learn.microsoft.com/dotnet/fsharp/language-reference/pattern-matching
• https://learn.microsoft.com/dotnet/fsharp/language-reference/results