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113 lines
2.8 KiB
Markdown
113 lines
2.8 KiB
Markdown
---
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paths:
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- "**/*.fs"
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- "**/*.fsx"
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---
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# F# Coding Style
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> This file extends [common/coding-style.md](../common/coding-style.md) with F#-specific content.
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## Standards
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- Follow standard F# conventions and leverage the type system for correctness
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- Prefer immutability by default; use `mutable` only when justified by performance
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- Keep modules focused and cohesive
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## Types and Models
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- Prefer discriminated unions for domain modeling over class hierarchies
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- Use records for data with named fields
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- Use single-case unions for type-safe wrappers around primitives
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- Avoid classes unless interop or mutable state requires them
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```fsharp
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type EmailAddress = EmailAddress of string
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type OrderStatus =
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| Pending
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| Confirmed of confirmedAt: DateTimeOffset
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| Shipped of trackingNumber: string
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| Cancelled of reason: string
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type Order =
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{ Id: Guid
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CustomerId: string
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Status: OrderStatus
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Items: OrderItem list }
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```
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## Immutability
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- Records are immutable by default; use `with` expressions for updates
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- Prefer `list`, `map`, `set` over mutable collections
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- Avoid `ref` cells and mutable fields in domain logic
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```fsharp
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let rename (profile: UserProfile) newName =
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{ profile with Name = newName }
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```
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## Function Style
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- Prefer small, composable functions over large methods
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- Use the pipe operator `|>` to build readable data pipelines
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- Prefer pattern matching over if/else chains
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- Use `Option` instead of null; use `Result` for operations that can fail
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```fsharp
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let processOrder order =
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order
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|> validateItems
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|> Result.bind calculateTotal
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|> Result.map applyDiscount
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|> Result.mapError OrderError
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```
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## Async and Error Handling
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- Use `task { }` for interop with .NET async APIs
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- Use `async { }` for F#-native async workflows
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- Propagate `CancellationToken` through public async APIs
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- Prefer `Result` and railway-oriented programming over exceptions for expected failures
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```fsharp
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let loadOrderAsync (orderId: Guid) (ct: CancellationToken) =
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task {
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let! order = repository.FindAsync(orderId, ct)
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return
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order
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|> Option.defaultWith (fun () ->
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failwith $"Order {orderId} was not found.")
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}
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```
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## Formatting
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- Use `fantomas` for automatic formatting
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- Prefer significant whitespace; avoid unnecessary parentheses
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- Remove unused `open` declarations
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### Open Declaration Order
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Group `open` statements into four sections separated by a blank line, each section sorted lexically within itself:
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1. `System.*`
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2. `Microsoft.*`
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3. Third-party namespaces
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4. First-party / project namespaces
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```fsharp
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open System
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open System.Collections.Generic
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open System.Threading.Tasks
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open Microsoft.AspNetCore.Http
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open Microsoft.Extensions.Logging
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open FsCheck.Xunit
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open Swensen.Unquote
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open MyApp.Domain
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open MyApp.Infrastructure
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```
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