How to write template output to a file in Golang?

Question

I use the following code which work ok, but now I want to print the template to a file and tried the following but got error

package main

import (
    "html/template"
    "log"
    "os"
)

func main() {
    t := template.Must(template.New("").Parse(`{{- range .}}{{.}}:
    echo "from {{.}}"
{{end}}
`))
    t.Execute(os.Stdout, []string{"app1", "app2", "app3"})

    f, err := os.Create("./myfile")
    if err != nil {
        log.Println("create file: ", err)
        return
    }
    err = t.Execute(f, t)
    if err != nil {
        log.Print("execute: ", err)
        return
    }
    f.Close()
}

The error is:

execute: template: :1:10: executing "" at <.>: range can't iterate over {0xc00000e520 0xc00001e400 0xc0000b3000 0xc00009e0a2}

Answer 1

Use the array as the second argument, not the template itself.

package main

import (
        "html/template"
        "log"
        "os"
)

func main() {
        t := template.Must(template.New("").Parse(`{{- range .}}{{.}}:
        echo "from {{.}}"
{{end}}
`))
        t.Execute(os.Stdout, []string{"app1", "app2", "app3"})

        f, err := os.Create("./myfile")
        if err != nil {
                log.Println("create file: ", err)
                return
        }
        err = t.Execute(f, []string{"app1", "app2", "app3"})
        if err != nil {
                log.Print("execute: ", err)
                return
        }
        f.Close()
}

Output:

app1:
    echo "from app1"
app2:
    echo "from app2"
app3:
    echo "from app3"

And the content of myfile is,

app1:
    echo "from app1"
app2:
    echo "from app2"
app3:
    echo "from app3"

Answer 2

The parameter you pass to the template execution the second time should match what you pass for the first time.

First you do:

t.Execute(os.Stdout, []string{"app1", "app2", "app3"})

Second you do:

err = t.Execute(f, t)

You passed the template itself (t). Change it to:

err = t.Execute(f, []string{"app1", "app2", "app3"})

Your template iterates over the passed param (with a {{range}} action) which works when you pass a slice, and it won't work when passing the template, it's a pointer to a struct, it's not something the template engine could iterate over.

Answer 3

You give a wrong parameter:

err = t.Execute(f, t)

It should be

err = t.Execute(f,[]string{"app1", "app2", "app3"})