Custom command-line flags with flag.Value and encoding.TextUnmarshaler

gistfile1.go

package main

// This example shows how to create a custom command-line flag by implementing
// the flag.Value interface. The flag accepts a comma-separated list of values
// and stores the contents in a DomainList type, which has the underlying type
// []string.
//
// Use it like:
// go run main.go -domains="example.com, example.org"

import (
    "flag"
    "fmt"
    "strings"
)

type DomainList []string

func (d *DomainList) String() string {
    return strings.Join(*d, ",")
}

func (d *DomainList) Set(value string) error {
    domains := strings.Split(value, ",")
    for i, domain := range domains {
        domains[i] = strings.TrimSpace(domain)
    }
    *d = domains
    return nil
}

func main() {
    var domains DomainList
    
    flag.Var(&domains, "domains", "comma-separated list of domains")
    flag.Parse()
    
    fmt.Printf("Domains: %v\n", domains)
}

gistfile2.go

package main

// This example shows how to create a custom command-line flag by implementing
// the encoding.TextUnmarshaler interface. The flag accepts a comma-separated 
// list of values and stores the contents in a DomainList type, which has the
//  underlying type []string.
//
// Use it like:
// go run main.go -domains="example.com, example.org"

import (
    "flag"
    "fmt"
    "strings"
)

type DomainList []string

func (d *DomainList) UnmarshalText(text []byte) error {
    domains := strings.Split(string(text), ",")
    for i, domain := range domains {
        domains[i] = strings.TrimSpace(domain)
    }
    *d = domains
    return nil
}

func (d DomainList) MarshalText() ([]byte, error) {
    return []byte(strings.Join(d, ",")), nil
}

func main() {
    var domains DomainList
    
    flag.TextVar(&domains, "domains", DomainList{}, "comma-separated list of domains")
    flag.Parse()
    
    fmt.Printf("Domains: %v\n", domains)
}