docopt.go 奇怪的错误消息

Question

使用docopt.go重构旧项目并最小化代码 程序看起来像这样

package main
import (
    "fmt"
    "github.com/docopt/docopt.go"
)
const Version = `2.0`
const Usage = `
Usage:
    serve [--port] <dir>
    serve help | --help
    serve --version 
Options:
    -p, --port       port for the sever to listen on
    -h, --help       display help information
    -v, --version    display Version
`
func check(err error) {
    if err != nil {
        panic(err)
    }
}
func main() {
    args, err := docopt.Parse(Usage, nil, true, Version, false)
    check(err)
    port := args["[--port]"].(string)
    fmt.Println(args)
    fmt.Println(port)v
}

但是当我运行程序时go run ./serve.go help期待帮助消息，我收到这个

panic: interface conversion: interface is nil, not string
goroutine 1 [running]:
main.main()
    /Users/jburns/Development/Gopath/src/github.com/nyumal/serve/serve.go:31 +0x148
goroutine 2 [runnable]:
runtime.forcegchelper()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/proc.go:90
runtime.goexit()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1
goroutine 3 [runnable]:
runtime.bgsweep()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/mgc0.go:82
runtime.goexit()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1
goroutine 4 [runnable]:
runtime.runfinq()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/malloc.go:712
runtime.goexit()
    /usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1
exit status 2

运行go run ./serve.go --port 5000它返回相同的内容，但运行go run ./serve.go --port 5000 .返回

Usage:
    serve [--port] <dir>
    serve help | --help
    serve --version
exit status 1

我哪里做错了？

Answer 1

你需要为 port 声明一个参数：

const Usage = `
Usage:
    serve [--port=<arg>] <dir>
    serve help | --help
    serve --version 
Options:
    -p, --port=<arg> port for the sever to listen on
    -h, --help       display help information
    -v, --version    display Version

使用两个值类型断言来处理未设置端口的情况：

port, ok := args["--port"].(string)
if ok {
   // port is set
}

此外，从地图键周围删除"[]"。