col.ToStrings未定义（类型Columns没有字段或方法ToStrings）

I am attempting to create a type that represents a slice of pointers to another type and define a method for it, my code looks similar to this, albeit a bit simplified for the example:

package column

type Column struct {

    name string
}

type Columns []*Column

func (c Column) ToString() string {

    return c.name
}

func (c Columns) ToStrings() []string {

    var strSlice []string

    for _, v := range c {

        strSlice = append(strSlice, v.ToString())
    }

    return strSlice
}

It is then called in a separate file like this:

import (
    c "main/column"
    "strings"
)

type Columns c.Columns

func ToString(col Columns) string {

    return strings.Join(col.ToStrings(), ",
")
}

However when I try to call the method ToStrings() on the exported "Columns" type I get this error:

columns.ToStrings undefined (type Columns has no field or method ToStrings)

It seems the compiler can't find the method ToStrings(). Is there a way around this? Why can't the compiler find the exported method defined for the "Columns" type?

type Columns c.Columns creates an entirely new type, with a new method set. The only reason to do this is to specifically remove the methods of the exiting type.