func letter_frequency(File file) {
    file.read.chars.grep{.match(/[[:alpha:]]/)} \
        .group_by {|letter| letter.downcase}    \
        .map_val  {|_, val| val.len}            \
        .sort_by  {|_, val| -val}
}

var top = letter_frequency(File(__FILE__))
top.each{|pair| say "#{pair[0]}: #{pair[1]}"}