F# Patterns

This file extends common/patterns.md with F#-specific content.

Result Type for Error Handling

Use Result<'T, 'TError> with railway-oriented programming instead of exceptions for expected failures.

type OrderError =
    | InvalidCustomer of string
    | EmptyItems
    | ItemOutOfStock of sku: string

let validateOrder (request: CreateOrderRequest) : Result<ValidatedOrder, OrderError> =
    if String.IsNullOrWhiteSpace request.CustomerId then
        Error(InvalidCustomer "CustomerId is required")
    elif request.Items |> List.isEmpty then
        Error EmptyItems
    else
        Ok { CustomerId = request.CustomerId; Items = request.Items }

Option for Missing Values

Prefer Option<'T> over null. Use Option.map, Option.bind, and Option.defaultValue to transform.

let findUser (id: Guid) : User option =
    users |> Map.tryFind id

let getUserEmail userId =
    findUser userId
    |> Option.map (fun u -> u.Email)
    |> Option.defaultValue "unknown@example.com"

Discriminated Unions for Domain Modeling

Model business states explicitly. The compiler enforces exhaustive handling.

type PaymentState =
    | AwaitingPayment of amount: decimal
    | Paid of paidAt: DateTimeOffset * transactionId: string
    | Refunded of refundedAt: DateTimeOffset * reason: string
    | Failed of error: string

let describePayment = function
    | AwaitingPayment amount -> $"Awaiting payment of {amount:C}"
    | Paid (at, txn) -> $"Paid at {at} (txn: {txn})"
    | Refunded (at, reason) -> $"Refunded at {at}: {reason}"
    | Failed error -> $"Payment failed: {error}"

Computation Expressions

Use computation expressions to simplify sequential operations that may fail.

let placeOrder request =
    result {
        let! validated = validateOrder request
        let! inventory = checkInventory validated.Items
        let! order = createOrder validated inventory
        return order
    }

Module Organization

Group related functions in modules rather than classes
Use [<RequireQualifiedAccess>] to prevent name collisions
Keep modules small and focused on a single responsibility

[<RequireQualifiedAccess>]
module Order =
    let create customerId items = { Id = Guid.NewGuid(); CustomerId = customerId; Items = items; Status = Pending }
    let confirm order = { order with Status = Confirmed(DateTimeOffset.UtcNow) }
    let cancel reason order = { order with Status = Cancelled reason }

Dependency Injection

Define dependencies as function parameters or record-of-functions
Use interfaces sparingly, primarily at the boundary with .NET libraries
Prefer partial application for injecting dependencies into pipelines

type OrderDeps =
    { FindOrder: Guid -> Task<Order option>
      SaveOrder: Order -> Task<unit>
      SendNotification: Order -> Task<unit> }

let processOrder (deps: OrderDeps) orderId =
    task {
        match! deps.FindOrder orderId with
        | None -> return Error "Order not found"
        | Some order ->
            let confirmed = Order.confirm order
            do! deps.SaveOrder confirmed
            do! deps.SendNotification confirmed
            return Ok confirmed
    }

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