Question

我有这3个monad变形金刚

{
  "name": "www-admin",
  "version": "0.0.0",
  "license": "MIT",
  "scripts": {
    "ng": "ng",
    "start": "ng serve",
    "build": "ng build",
    "test": "ng test",
    "lint": "ng lint",
    "e2e": "ng e2e"
  },
  "private": true,
  "dependencies": {
    "@agm/core": "^1.0.0-beta.0",
    "@angular/animations": "^4.1.3",
    "@angular/cdk": "^2.0.0-beta.11",
    "@angular/common": "^4.0.0",
    "@angular/compiler": "^4.0.0",
    "@angular/core": "^4.0.0",
    "@angular/forms": "^4.0.0",
    "@angular/http": "^4.0.0",
    "@angular/material": "^2.0.0-beta.6",
    "@angular/platform-browser": "^4.0.0",
    "@angular/platform-browser-dynamic": "^4.0.0",
    "@angular/router": "^4.0.0",
    "@ngui/auto-complete": "^0.14.4",
    "@ngx-translate/core": "^7.0.0",
    "@ngx-translate/http-loader": "^0.1.0",
    "@types/lodash": "^4.14.107",
    "@types/moment": "^2.13.0",
    "angular-2-dropdown-multiselect": "^1.6.0",
    "angular-2-local-storage": "^1.0.1",
    "angular2-hotkeys": "^2.0.3",
    "angular2-modal": "^2.0.3",
    "angular2-moment": "^1.8.0",
    "angular2-toaster": "^4.0.1",
    "angular4-carousel": "^3.1.8",
    "bootstrap": "^3.3.7",
    "core-js": "^2.4.1",
    "jquery": "^3.3.1",
    "lodash": "^4.17.5",
    "moment": "^2.18.1",
    "mydatepicker": "^2.0.31",
    "ng-drag-drop": "^4.0.1",
    "ng-http-loader": "^0.3.3",
    "ng-pick-datetime": "^5.0.0-beta.15",
    "ng-sidebar": "^5.1.0",
    "ng2-auto-complete": "^0.12.0",
    "ng2-bootstrap": "^1.6.3",
    "ng2-bootstrap-modal": "https://github.com/Sky4CE/ng2-bootstrap-modal/tarball/v.1.0.2",
    "ng2-dragula": "^1.5.0",
    "ng2-tooltip": "0.0.7",
    "ngx-bootstrap": "^1.6.6",
    "ngx-chips": "^1.5.2",
    "ngx-tooltip": "0.0.9",
    "rxjs": "^5.1.0",
    "ts-md5": "^1.2.2",
    "zone.js": "^0.8.4"
  },
  "devDependencies": {
    "@angular/cli": "^1.6.2",
    "@angular/compiler-cli": "^4.0.0",
    "@types/jasmine": "2.5.38",
    "@types/jquery": "^3.3.1",
    "@types/node": "~6.0.60",
    "codelyzer": "~2.0.0",
    "jasmine-core": "~2.5.2",
    "jasmine-spec-reporter": "~3.2.0",
    "karma": "~1.4.1",
    "karma-chrome-launcher": "~2.1.1",
    "karma-cli": "~1.0.1",
    "karma-coverage-istanbul-reporter": "^0.2.0",
    "karma-jasmine": "~1.1.0",
    "karma-jasmine-html-reporter": "^0.2.2",
    "protractor": "~5.1.0",
    "ts-node": "~2.0.0",
    "tslint": "~4.5.0",
    "typescript": "2.3.0"
  }
}

我想将相应的完整类型（即确切的相应类型）提升为这些类型。因此，对于T，我想将Future [Option [Int]]放入其中。对于E，我想将Future（Either [String，Int]）提升，对于P，我想将（Future [Either [String，Option [Int]]]）提升到其中。

我编写了这段代码，并进行了编译。除了我需要一种更简洁的方法来实现相同的目的。

type T[A] = OptionT[Future, A]
type E[A] = EitherT[Future, String, A]
type P[A] = OptionT[E, A]

我正在使用Cats 1.1.0和Scala 2.12.3。

asInstanceOf很烦人。但是如果我将最后一行更改为

val x : T[Int] = OptionT(Future(Option(10)))
val y : E[Int] = EitherT(Future(Right(10).asInstanceOf[Either[String, Int]]))
val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)).asInstanceOf[Either[String, Option[Int]]])))

我收到此编译器错误

val z : P[Int] = OptionT(EitherT(Future(Right(Option(10)))))

Answer 1

您可以使用import cats.implicits._的隐式方法然后你可以写类似

val z: P[Int] = OptionT(EitherT(Future(10.some.asRight[String])))

当然，您也可以编写自己的隐式

implicit class EitherFuture[A, B](val e: Future[A Either B]) extends AnyVal {
  def asEitherT: EitherT[Future, A, B] = EitherT(e)
}

implicit class OptionEitherT[A](val e: EitherT[Future, String, Option[A]]) extends AnyVal {
  def asOptionT = OptionT(e)
}

val zz: P[Int] = Future(10.some.asRight[String]).asEitherT.asOptionT

Answer 2

尝试为Right提供类型参数：

val z : P[Int] = OptionT(EitherT(Future(Right[String,Option[Int]](Option(10)))))

没有类型参数，当您执行Right(1)时，scala会推断Either[Nothing,Int]。

将全部型号提升到monad变压器中

2 个答案: