小编zer*_*ing的帖子

我有以下数据类型（如列表）：

  data List a =
      Nil
    | Cons a (List a)
    deriving (Eq, Show)

并创建了一个实例Monoid：

  instance Monoid (List a) where
    mempty = Nil
    mappend Nil ys = ys
    mappend (Cons x xs) ys = Cons x (mappend xs ys) 

我尝试编写一个实例来Arbitrary测试后者，但无法完成它：

  instance Arbitrary a => Arbitrary (List a) where
    arbitrary = do  

请帮助我完成该功能。

我有以下类型定义:

  data Hello a b =  Hi a
                  | Sali b
                  deriving (Show, Eq) 

它不是一个实例,Applicative但我仍然可以用pure它来转换Applicative为什么？

*ExercisesTraversable Data.Monoid Control.Applicative> :t pure $ Hi 34
pure $ Hi 34 :: (Num a, Applicative f) => f (Hello a b)

*ExercisesTraversable Data.Monoid Control.Applicative> pure $ Hi 34
Hi 34

当我尝试:

*ExercisesTraversable Data.Monoid Control.Applicative> (Sali (*2)) <*> (Sali 4)

<interactive>:23:1: error:
    * Non type-variable argument
        in the constraint: Applicative (Hello a)
      (Use FlexibleContexts to permit this)
    * When …

我想使用应用函数并尝试如下：

*ReaderExercise Control.Applicative> (+4) <*> (+3)

然后收到以下错误消息：

<interactive>:51:11: error:
    * Occurs check: cannot construct the infinite type: a ~ a -> b
      Expected type: (a -> b) -> a
        Actual type: a -> a
    * In the second argument of `(<*>)', namely `(+ 3)'
      In the expression: (+ 4) <*> (+ 3)
      In an equation for `it': it = (+ 4) <*> (+ 3)
    * Relevant bindings include
        it :: (a -> b) -> b (bound at <interactive>:51:1)

我期望的是一个带有一个参数的返回函数。
无限类型是什么意思？

我从haskellbook中有以下代码片段,它逐步显示monad变换器将如何解包:

module OuterInner where

  import Control.Monad.Trans.Except
  import Control.Monad.Trans.Maybe
  import Control.Monad.Trans.Reader

  -- We only need to use return once
  -- because it's one big Monad
  embedded :: MaybeT (ExceptT String (ReaderT () IO)) Int
  embedded = return 1

  maybeUnwrap :: ExceptT String (ReaderT () IO) (Maybe Int)
  maybeUnwrap = runMaybeT embedded

  -- Next
  eitherUnwrap :: ReaderT () IO (Either String (Maybe Int))
  eitherUnwrap = runExceptT maybeUnwrap

  -- Lastly
  readerUnwrap :: () -> IO (Either String (Maybe Int))
  readerUnwrap = runReaderT eitherUnwrap …

在Haskell中,我可以将参数约束到类型类,例如:

Prelude> f x y = x * y
Prelude> :t f
f :: Num a => a -> a -> a

我试图在Scala以下方面做同样的事情:

def sendMessage[A: Array[Byte], B: Array[Byte]](producer: Eval[KafkaProducer[A, B]])(pr: Eval[ProducerRecord[A, B]]) =  

但编译器抱怨:

[error] /home/developer/Desktop/scala/PureProducer/src/main/scala/TheProducer.scala:39:20: Array[Byte] does not take type parameters
[error]   def sendMessage[A: Array[Byte], B: Array[Byte]](producer: Eval[KafkaProducer[A, B]])(pr: Eval[ProducerRecord[A, B]]) =  

如何约束Scala中的类型参数？

我有以下编译器抱怨的代码：

val state: KTable[String, String]  = builder
  .table("BARY-PATH", Materialized.as("PATH-STORE"))

错误信息：

[error] /home/developer/Desktop/microservices/paths-stream/src/main/scala/io/khinkali/PathsTopology.scala:23:8: overloaded method value table with alternatives:
[error]   [K, V](x$1: String, x$2: org.apache.kafka.streams.kstream.Materialized[K,V,org.apache.kafka.streams.state.KeyValueStore[org.apache.kafka.common.utils.Bytes,Array[Byte]]])org.apache.kafka.streams.kstream.KTable[K,V] <and>
[error]   [K, V](x$1: String, x$2: org.apache.kafka.streams.Consumed[K,V])org.apache.kafka.streams.kstream.KTable[K,V]
[error]  cannot be applied to (String, org.apache.kafka.streams.kstream.Materialized[Nothing,Nothing,Nothing])
[error]       .table("BARY-PATH", Materialized.as("PATH-STORE"))
[error]        ^

然后我尝试：

val state: KTable[String, String]  = builder
  .table[String, String]("BARY-PATH", Materialized[String, String,KeyValueStore[org.apache.kafka.common.utils.Bytes, Array[Byte]]].as("PATH-STORE"))

编译器仍然抱怨：

[error] /home/developer/Desktop/microservices/paths-stream/src/main/scala/io/khinkali/PathsTopology.scala:24:43: object org.apache.kafka.streams.kstream.Materialized is not a value
[error]       .table[String, String]("BARY-PATH", Materialized[String, String,KeyValueStore[org.apache.kafka.common.utils.Bytes, Array[Byte]]].as("PATH-STORE"))

我阅读了API 文档，但无法弄清楚，我做错了什么？

方法实现：

 * Materialize a {@link StateStore} …

我有以下函数,它做递归:

  @tailrec
  private def pool[F[_]: Monad, A]
  : Consumer[String, String] => (Vector[KkConsumerRecord] => F[A]) => IO[Unit]
  = consumer => cb => {
    val records: ConsumerRecords[String, String] = consumer.poll(Long.MaxValue)
    val converted = records.iterator().asScala.map(rec => {
      KkConsumerRecord(rec.key(), rec.value(), rec.offset(), rec.partition(), rec.topic())
    })

    val vec = converted.foldLeft(Vector.empty[KkConsumerRecord]) { (b, a) =>
      a +: b
    }
    cb(vec)
    pool(consumer)(cb)
  }

编译器抱怨:

[error] /home/developer/Desktop/microservices/bary/kafka-api/src/main/scala/io/khinkali/Consumer/KkConsumer.scala:57:10: type mismatch;
[error]  found   : org.apache.kafka.clients.consumer.Consumer[String,String]
[error]  required: cats.Monad[?]
[error]     pool(consumer)(cb)
[error]          ^
[error] two errors found

我究竟做错了什么？

我想在SAP Web IDE上的ES6中编写代码并按照指令https://blogs.sap.com/2017/11/01/next-generation-javascript-in-ui5-using-sap-web-ide/。

但是如您所见，我仍然收到错误消息。

我究竟做错了什么？

我正在使用Webpack4来构建我的React应用程序，其webpack.config.js外观如下：

const HtmlWebPackPlugin = require("html-webpack-plugin");
const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const path = require('path');

module.exports = {
  entry: ['@babel/polyfill', './src/index.js'],
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'bundle.js'
  },
  module: {
    rules: [{
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: "babel-loader",
          options: {
            presets: ["@babel/preset-env", "@babel/preset-react"]
          }
        }
      },
      {
        test: /\.html$/,
        use: [{
          loader: "html-loader",
          options: {
            minimize: true
          }
        }]
      },
      {
        test: /\.scss$/,
        use: [{
          loader: MiniCssExtractPlugin.loader
        }, {
          loader: "style-loader"
        }, { …

我尝试使用如下粘性属性:

<html lang="en">

<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <meta name="viewport" content="width=device-width,initial-scale=1,shrink-to-fit=no">
    <meta name="theme-color" content="#000000">
    <title>Cockpit</title>
    <style>
    .topbar-container {
  width: 100%;
  height: 100px;
  position: sticky;
  background-color: #2d3e50;
  z-index: 999; }

.navi-container {
  width: 100%;
  height: 50px;
  background-color: #119C7E;
  position: sticky;
  z-index: 999; }

.portfolio-container {
  width: 992px;
  position: absolute;
  top: 130px; }

.app-container {
  display: flex;
  width: 100%;
  height: 100%;
  flex-direction: column;
  top: 0px;
  position: relative;
  align-items: center; }

* {
  padding: 0;
  margin: 0; }

html,
body {
  height: 100%; …