Go言語: 1行の文章を指定の文字数で改行したい

go/docのToText関数を使うと一発でできる。

package main

import (
    "go/doc"
    "os"
)

func main() {
    text := "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum."
    doc.ToText(os.Stdout, text, "", "", 60)
}

出力結果

Lorem Ipsum is simply dummy text of the printing and
typesetting industry. Lorem Ipsum has been the industry's
standard dummy text ever since the 1500s, when an unknown
printer took a galley of type and scrambled it to make a
type specimen book. It has survived not only five centuries,
but also the leap into electronic typesetting, remaining
essentially unchanged. It was popularised in the 1960s with
the release of Letraset sheets containing Lorem Ipsum
passages, and more recently with desktop publishing software
like Aldus PageMaker including versions of Lorem Ipsum.

一旦変数に代入したい場合は bytes.NewBufferString() を使うといい

    out := bytes.NewBufferString("")
    doc.ToText(out, text, "", "", 60)