文字/模​​板：“无法使用0个结果调用方法/函数。”

How can I execute function in templates that returns no value? Here is example:

func main() {
    u, err := url.Parse("http://example.com/test?param1=true&param2=true")
    if err != nil {
        log.Fatal(err)
    }
    m := u.Query()
    m.Del("param1") // param1 successful deleted!
    u.RawQuery = m.Encode()
    fmt.Println(u.RawQuery)

    const tmpl = `
    {{$m := .Query}}
    {{$m.Del "param2"}} <!-- failed to delete param2! -->
    {{.RawQuery}}
    `
    t := template.Must(template.New("").Parse(tmpl))
    err = t.Execute(os.Stdout, u)
    if err != nil {
        log.Println("executing template:", err)
    }

}

I know that in templates shouldn't be much logic, but ignorance of running function that returns no value seems to me interesting issue.

Templates in Go are not like those in other languages (e.g. PHP). Use template.FuncMap to create custom functions for your templates.

package main

import (
    "fmt"
    "log"
    "net/url"
    "os"
    "text/template"
)

func main() {
    funcMap := template.FuncMap{
        "delete": deleteMap,
    }

    u, err := url.Parse("http://example.com/test?param1=true&param2=true")
    if err != nil {
        log.Fatal(err)
    }

    u = deleteMap(u, "param1") // works in regular code and templates

    fmt.Println(u.RawQuery)

    const tmpl = `
    {{$m := delete . "param2"}} <!-- WORKS! -->
    {{$m.RawQuery}}
    `
    t := template.New("").Funcs(funcMap)
    t = template.Must(t.Parse(tmpl))

    err = t.Execute(os.Stdout, u)

    if err != nil {
        log.Println("executing template:", err)
    }

}

func deleteMap(u *url.URL, key string) *url.URL {
    m := u.Query()
    m.Del(key) // key successful deleted!
    u.RawQuery = m.Encode()
    return u
}

Or, try the playground version.