How to create methods for string type in Golang

Question

I'm planning to create some method extension for the string type.

I need to write all my extension methods into a separate package.

here is my hierarchy.

root
|    main.go
|    validation
     |   validate.go

on the main.go I would like to have, "abcd".Required()

main.go

package main

import (
    "fmt"
    "./validation"
)

func main() {
    fmt.Println( "abcd".Required() )
}

validate.go

package validation

func (s string) Required() bool {

    if s != "" {
        return true
    }

    return false
}

When I run it, will get an error.

error

cannot define new methods on non-local type string

I found some answers in other questions on StackOverflow but they don't exactly talk about the string type & having the method in a different package file.

As the error says: you can't define methods on the string type. You may however create a new type (e.g. type mystring string), and add methods to the new type. — icza
– icza, Commented Jan 24, 2019 at 13:02
@icza yes, that's true. i've tired that but still doesn't work as i expected. — usr784638
– usr784638, Commented Jan 25, 2019 at 4:10

Iain Duncan · Accepted Answer · 2019-01-24 17:17:11Z

7

In your validate.go create a new type String:

package validation

type String string

func (s String) Required() bool {

    return s != ""
}

And then work with validation.String objects in your main:

package main

import (
    "fmt"

    "./validation"
)

func main() {
    fmt.Println(validation.String("abcd").Required())
}

An executable example with it all in the same file here:

https://play.golang.org/p/z_LcTZ6Qvfn

edited Jan 24, 2019 at 17:17

answered Jan 24, 2019 at 13:36

Iain Duncan

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4 Comments

Gavin Over a year ago

return s != ""

usr784638 Over a year ago

@IainDuncan this works. tnx. but I wonder how can we have validation.String("abcd").Reqired() much simpler.

usr784638 Over a year ago

How can we actually accept any type not only string in this example?

Iain Duncan Over a year ago

@usr784638 if it is treating for the zero value of any field then you could do that via reflection like this question google.com/url?sa=t&source=web&rct=j&url=https://…

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