--- name: swe-programming-fsharp description: F# coding standards from authoritative docs/explanation/software-engineering/programming-languages/f-sharp/ documentation --- # F# Coding Standards ## Purpose Progressive disclosure of F# coding standards for agents writing F# code. **Authoritative Source**: [docs/explanation/software-engineering/programming-languages/f-sharp/README.md](../../../docs/explanation/software-engineering/programming-languages/f-sharp/README.md) **Usage**: Auto-loaded for agents when writing F# code. Provides quick reference to idioms, best practices, and antipatterns. ## Prerequisite Knowledge **IMPORTANT**: This skill provides **demo-specific style guides**, not educational tutorials. **You MUST understand F# fundamentals before using these standards.** Complete the demo F# learning path first: **What this skill covers**: demo naming conventions, framework choices, repository-specific patterns, how to apply F# knowledge in THIS codebase. **What this skill does NOT cover**: F# syntax, language fundamentals, generic patterns (those are in crud-fs-ts-nextjs). ## Quick Standards Reference ### Naming Conventions **Modules/Types/DUs**: PascalCase - `ZakatCalculator`, `MurabahaContract`, `PaymentResult` **Functions/Values**: camelCase - `calculateZakat`, `totalAmount`, `validateContract` **DU Cases**: PascalCase - `Due`, `BelowNisab`, `ValidationError` **Predicate functions**: `isValid`, `hasPayments` (boolean-returning functions prefixed with `is`/`has`) ### Discriminated Unions for Domain Modeling ```fsharp // CORRECT: DU for domain states (exhaustive) type ZakatResult = | Due of amount: decimal | BelowNisab | ValidationError of message: string // CORRECT: Exhaustive pattern matching (compiler enforced) let handleResult result = match result with | Due amount -> sprintf "Zakat due: %M" amount | BelowNisab -> "Below nisab threshold" | ValidationError msg -> sprintf "Error: %s" msg ``` ### Railway-Oriented Programming ```fsharp // CORRECT: Result type for error handling let calculateZakat (wealth: decimal) (nisab: decimal) : Result = if wealth < 0m then Error "Wealth cannot be negative" elif wealth >= nisab then Ok (wealth * 0.025m) else Ok 0m // CORRECT: Computation expression for chaining let processPayment (wealth: decimal) (nisab: decimal) = result { let! zakatAmount = calculateZakat wealth nisab let! validated = validateAmount zakatAmount return! saveZakat validated } ``` ### Pipeline Operator ```fsharp // CORRECT: Use |> for readable pipelines let totalZakat = wealthAmounts |> List.filter (fun w -> w >= nisabThreshold) |> List.map (fun w -> w * 0.025m) |> List.sum // CORRECT: Function composition with >> let calculateAndValidate = calculateZakat >> validateZakat ``` ### Records (Immutable by Default) ```fsharp // CORRECT: Record type for value objects type ZakatCalculation = { Wealth: decimal Nisab: decimal Amount: decimal CalculationDate: DateOnly } // CORRECT: Record copy expression (non-destructive update) let updated = { calculation with Amount = newAmount } ``` ### Async Workflows ```fsharp // CORRECT: F# async computation expression let calculateAsync wealth nisab = async { let! nisabValue = repository.GetNisabAsync() let result = calculateZakat wealth nisabValue return result } // CORRECT: Running async let result = calculateAsync 10000m 5000m |> Async.RunSynchronously // CORRECT: Task interop let taskAsync = calculateAsync 10000m 5000m |> Async.StartAsTask ``` ### Fantomas Formatting (MANDATORY) ```fsharp // CORRECT: Fantomas-formatted code let calculate (wealth: decimal) (nisab: decimal) = if wealth >= nisab then wealth * 0.025m else 0m // Run: dotnet fantomas . (formats all F# files) // Pre-commit: fantomas --check . (fails if not formatted) ``` ### Testing with Expecto ```fsharp open Expecto let zakatTests = testList "ZakatCalculator" [ test "calculates 2.5% when above nisab" { let result = calculateZakat 10000m 5000m Expect.equal result (Ok 250m) "Should return 2.5% of wealth" } test "returns 0 when below nisab" { let result = calculateZakat 1000m 5000m Expect.equal result (Ok 0m) "Should return 0 below nisab" } ] [] let main args = runTestsWithCLIArgs [] args zakatTests ``` ### Property-Based Testing with FsCheck ```fsharp open FsCheck let zakatProperties = testList "ZakatCalculator properties" [ testProperty "zakat is always non-negative" <| fun (wealth: decimal) -> let nisab = 5000m match calculateZakat (abs wealth) nisab with | Ok amount -> amount >= 0m | Error _ -> true testProperty "zakat is exactly 2.5% when above nisab" <| fun (wealth: decimal) -> wealth > 5000m ==> (calculateZakat wealth 5000m = Ok (wealth * 0.025m)) ] ``` ## Comprehensive Documentation **Authoritative Index**: [docs/explanation/software-engineering/programming-languages/f-sharp/README.md](../../../docs/explanation/software-engineering/programming-languages/f-sharp/README.md) ### Mandatory Standards (All F# Code MUST Follow) 1. **[Coding Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/coding-standards.md)** - F# naming conventions, module organization, pipeline idioms 2. **[Testing Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/testing-standards.md)** - Expecto, FsCheck property-based testing, AltCover coverage 3. **[Code Quality Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/code-quality-standards.md)** - Fantomas, FSharpLint, exhaustive pattern matching 4. **[Build Configuration](../../../docs/explanation/software-engineering/programming-languages/f-sharp/build-configuration.md)** - .fsproj file order, dotnet CLI, Nx integration ### Context-Specific Standards (Apply When Relevant) 1. **[Error Handling Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/error-handling-standards.md)** - Result type, railway-oriented programming, computation expressions 2. **[Concurrency Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/concurrency-standards.md)** - Async workflows, MailboxProcessor, Task interop 3. **[Functional Programming Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/functional-programming-standards.md)** - Computation expressions, monads, applicatives 4. **[Type Safety Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/type-safety-standards.md)** - DUs, units of measure, phantom types 5. **[Performance Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/performance-standards.md)** - Tail recursion, sequences, lazy evaluation 6. **[Security Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/security-standards.md)** - Type-driven validation, Giraffe authentication 7. **[API Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/api-standards.md)** - Giraffe HttpHandler composition, Saturn routing 8. **[DDD Standards](../../../docs/explanation/software-engineering/programming-languages/f-sharp/ddd-standards.md)** - DU-based domain modeling, making illegal states unrepresentable ## Related Skills - docs-applying-content-quality - repo-practicing-trunk-based-development ## References - [F# README](../../../docs/explanation/software-engineering/programming-languages/f-sharp/README.md) - [Functional Programming](../../../repo-governance/development/pattern/functional-programming.md)