Task: Word wrap
C#: namespace RosettaCode.WordWrap
{
    using System;
    using System.Collections.Generic;

    internal static class Program
    {
        private const string LoremIpsum = @"
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas varius sapien
vel purus hendrerit vehicula. Integer hendrerit viverra turpis, ac sagittis arcu
pharetra id. Sed dapibus enim non dui posuere sit amet rhoncus tellus
consectetur. Proin blandit lacus vitae nibh tincidunt cursus. Cum sociis natoque
penatibus et magnis dis parturient montes, nascetur ridiculus mus. Nam tincidunt
purus at tortor tincidunt et aliquam dui gravida. Nulla consectetur sem vel
felis vulputate et imperdiet orci pharetra. Nam vel tortor nisi. Sed eget porta
tortor. Aliquam suscipit lacus vel odio faucibus tempor. Sed ipsum est,
condimentum eget eleifend ac, ultricies non dui. Integer tempus, nunc sed
venenatis feugiat, augue orci pellentesque risus, nec pretium lacus enim eu
nibh.";

        private static void Main()
        {
            foreach (var lineWidth in new[] { 72, 80 })
            {
                Console.WriteLine(new string('-', lineWidth));
                Console.WriteLine(Wrap(LoremIpsum, lineWidth));
            }
        }

        private static string Wrap(string text, int lineWidth)
        {
            return string.Join(string.Empty,
                               Wrap(
                                   text.Split(new char[0],
                                              StringSplitOptions
                                                  .RemoveEmptyEntries),
                                   lineWidth));
        }

        private static IEnumerable<string> Wrap(IEnumerable<string> words,
                                                int lineWidth)
        {
            var currentWidth = 0;
            foreach (var word in words)
            {
                if (currentWidth != 0)
                {
                    if (currentWidth + word.Length < lineWidth)
                    {
                        currentWidth++;
                        yield return " ";
                    }
                    else
                    {
                        currentWidth = 0;
                        yield return Environment.NewLine;
                    }
                }
                currentWidth += word.Length;
                yield return word;
            }
        }
    }
}

Go: package main

import (
    "fmt"
    "strings"
)

func wrap(text string, lineWidth int) (wrapped string) {
    words := strings.Fields(text)
    if len(words) == 0 {
        return
    }
    wrapped = words[0]
    spaceLeft := lineWidth - len(wrapped)
    for _, word := range words[1:] {
        if len(word)+1 > spaceLeft {
            wrapped += "\n" + word
            spaceLeft = lineWidth - len(word)
        } else {
            wrapped += " " + word
            spaceLeft -= 1 + len(word)
        }
    }
    return
}

var frog = `
In olden times when wishing still helped one, there lived a king
whose daughters were all beautiful, but the youngest was so beautiful
that the sun itself, which has seen so much, was astonished whenever
it shone in her face.  Close by the king's castle lay a great dark
forest, and under an old lime-tree in the forest was a well, and when
the day was very warm, the king's child went out into the forest and
sat down by the side of the cool fountain, and when she was bored she
took a golden ball, and threw it up on high and caught it, and this
ball was her favorite plaything.`

func main() {
    fmt.Println("wrapped at 80:")
    fmt.Println(wrap(frog, 80))
    fmt.Println("wrapped at 72:")
    fmt.Println(wrap(frog, 72))
}