Question

我是哈斯克尔的新手。在哈斯克尔做这样的事情会有什么好办法？

19:18:36,042 ERROR [org.jboss.msc.service.fail] (MSC service thread 1-4) MSC000001: Failed to start service jboss.deployment.unit."cargo-tracker.war".PARSE: org.jboss.msc.service.StartException in service jboss.deployment.unit."cargo-tracker.war".PARSE: WFLYSRV0153: Failed to process phase PARSE of deployment "cargo-tracker.war"
    at org.jboss.as.server.deployment.DeploymentUnitPhaseService.start(DeploymentUnitPhaseService.java:154)
    at org.jboss.msc.service.ServiceControllerImpl$StartTask.startService(ServiceControllerImpl.java:1948)
    at org.jboss.msc.service.ServiceControllerImpl$StartTask.run(ServiceControllerImpl.java:1881)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Caused by: org.jboss.msc.service.ServiceNotFoundException: Service service jboss.ejb.default-resource-adapter-name-service not found
    at org.jboss.msc.service.ServiceContainerImpl.getRequiredService(ServiceContainerImpl.java:669)
    at org.jboss.as.ejb3.deployment.processors.MessageDrivenComponentDescriptionFactory.getDefaultResourceAdapterName(MessageDrivenComponentDescriptionFactory.java:274)
    at org.jboss.as.ejb3.deployment.processors.MessageDrivenComponentDescriptionFactory.processMessageBeans(MessageDrivenComponentDescriptionFactory.java:154)
    at org.jboss.as.ejb3.deployment.processors.MessageDrivenComponentDescriptionFactory.processAnnotations(MessageDrivenComponentDescriptionFactory.java:81)
    at org.jboss.as.ejb3.deployment.processors.AnnotatedEJBComponentDescriptionDeploymentUnitProcessor.processAnnotations(AnnotatedEJBComponentDescriptionDeploymentUnitProcessor.java:57)
    at org.jboss.as.ejb3.deployment.processors.AbstractDeploymentUnitProcessor.deploy(AbstractDeploymentUnitProcessor.java:76)
    at org.jboss.as.server.deployment.DeploymentUnitPhaseService.start(DeploymentUnitPhaseService.java:147)
    ... 5 more

19:18:36,050 ERROR [org.jboss.as.controller.management-operation] (Controller Boot Thread) WFLYCTL0013: Operation ("deploy") failed - address: ([("deployment" => "cargo-tracker.war")]) - failure description: {
    "WFLYCTL0080: Failed services" => {"jboss.deployment.unit.\"cargo-tracker.war\".PARSE" => "org.jboss.msc.service.StartException in service jboss.deployment.unit.\"cargo-tracker.war\".PARSE: WFLYSRV0153: Failed to process phase PARSE of deployment \"cargo-tracker.war\"
    Caused by: org.jboss.msc.service.ServiceNotFoundException: Service service jboss.ejb.default-resource-adapter-name-service not found"},
    "WFLYCTL0412: Required services that are not installed:" => ["jboss.deployment.unit.\"cargo-tracker.war\".PARSE"],
    "WFLYCTL0180: Services with missing/unavailable dependencies" => undefined
}
19:18:36,080 INFO  [org.jboss.as.server] (ServerService Thread Pool -- 34) WFLYSRV0010: Deployed "cargo-tracker.war" (runtime-name : "cargo-tracker.war")
19:18:36,083 INFO  [org.jboss.as.controller] (Controller Boot Thread) WFLYCTL0183: Service status report
WFLYCTL0186:   Services which failed to start:      service jboss.deployment.unit."cargo-tracker.war".PARSE: org.jboss.msc.service.StartException in service jboss.deployment.unit."cargo-tracker.war".PARSE: WFLYSRV0153: Failed to process phase PARSE of deployment "cargo-tracker.war"

Answer 1

Haskell是功能和声明性语言。这意味着通常没有太多“做某事”。还有更多计算某些东西并将其返回。

这可能看起来像挑剔，但是例如在Haskell中，无法设置变量两次：一旦你为它分配一个表达式（本身不是一个值），您无法将其设置为不同的值。

如果您想要返回某些内容，通常会使用模式匹配。例如：

f :: Bool -> String
f True = "Yes"
f False = "No"

这在Java / C＃/ ...中有点等同于：

public String f (boolean var1) {
    if(var1) {
        return "Yes";
    } else {
        return "No";
    }
}

请注意，Haskell也可以 lazy ：如果你返回一个函数调用或者任何东西，你不会立即评估该函数调用：只有那样才会评估一个调用是必要的。

如何进行I / O可能会出现问题。为此，有一个I / O monad的概念。 monad是一种函数式编程技术，可以执行某种评估顺序。

但是函数式编程因此需要与命令式编程不同的“思维模式”：你不会想到命令方面的程序一个接一个地完成，但更多的是在一起组合函数以生成给定输入的输出。像通常是数学家或物理学家一样。例如，你给出一个函数，给定物体的质量和速度，计算该物体的动能。

Answer 2

Haskell有if-then-else条件。

我能写的最接近你的代码是这样的：

let var = length "hello" == 5
in if var then "ok" else "no"

请注意，这样的条件更类似于C或Java的var ? "ok" : "no"表达式，而不是if()...语句，但这是因为Haskell功能正常，所以它没有＆＃ 34;陈述＆＃34;，只有表达。

任何Haskell教程都应该涵盖这一点。如果你想学习Haskell，我建议你读一个。试图从其他语言转换成语是一种糟糕的策略。

简单检查haskell

2 个答案: