非同期エフェクトの追加

code:fsharp

type AsyncResult<'success, 'failure> = Async<Result<'success, 'failure>>

module AsyncResult =

let map f (x:AsyncResult<_,_>) : AsyncResult<_,_> =

Async.map (Result.map f) x

let mapError f (x:AsyncResult<_,_>) : AsyncResult<_,_> =

Async.map (Result.mapError f) x

let retn x : AsyncResult<_,_> =

x |> Result.Ok |> Async.retn

let bind

(f: 'a -> AsyncResult<'b,'c>) (xAsyncResult : AsyncResult<_, _>) :AsyncResult<_,_> =

async {

let! xResult = xAsyncResult

match xResult with

| Ok x -> return! f x

| Error err -> return (Error err)

}

let ofResult x : AsyncResult<_,_> =

x |> Async.retn

type AsyncResultBuilder() =

member this.Return(x) = AsyncResult.retn x

member this.Bind(x, f) = AsyncResult.bind f x

let asyncResult = AsyncResultBuilder()

検証のステップは、以下のように置き換えられる

code:fsharp

let validateOrder: ValidateOrder =

fun checkProductCodeExists checkAddressExists unvalidatedOrder ->

asyncResult {

let! orderId =

unvalidatedOrder.OrderId

|> OrderId.create

|> Result.mapError ValidationError

|> AsyncResult.ofResult

let! customerInfo =

unvalidatedOrder.CustomerInfo

|> toCustomerInfo

|> AsyncResult.ofResult

let! checkedShippingAddress =

unvalidatedOrder.ShippingAddress

|> toCheckedAddress checkAddressExists

let! shippingAddress =

checkedShippingAddress

|> toAddress

|> AsyncResult.ofResult

let! billingAddress = ...

let! lines =

unvalidatedOrder.Lines

|> List.map (toValidatedOrderLine checkAddressExists)

|> Result.sequence

|> AsyncResult.ofResult

// すべてのフィールドの準備ができたら、それを元にレコード型を作成して返す

let validatedOrder: ValidatedOrder = {

OrderId = orderId

CustomerInfo = customerInfo

ShippingAddress = shippingAddress

BillingAddress = billingAddress

OrderLines = lines

}

return validatedOrder

}

主な変更点

住所の検証を 2 つの部分に分けた

理由

code:fsharp

type CheckAddressExists =

UnvalidatedAddress -> AsyncResult<CheckedAddress, AddressValidationError>

AddressValidationError

code:fsharp

and AddressValidationError =

| InvalidFormat of string

| AddressNotFound of string

このシグネチャを適合させたい

toCheckedAddress

code:fsharp

let toCheckedAddress (checkAddress: CheckAddressExists) address =

address

|> checkAddress

|> AsyncResult.mapError (fun addrError ->

match addrError with

| AddressNotFound -> ValidationError "Address not found"

| InvalidFormat -> ValidationError "Address has bad format"

code:fsharp

let placeOrder: PlaceOrder =

fun unvalidatedOrder ->

asyncResult {

let! validatedOrder =

validateOrder checkProductCodeExists checkAddressExists unvalidatedOrder

|> AsyncResult.mapError PlaceOrderError.Validation

let! pricedOrder =

priceOrder getProductPrice validatedOrder

|> AsyncResult.ofResult

|> AsyncResult.mapError PlaceOrderError.Pricing

let acknowledgmentOption = ...

let events = ...

return events

}