The purpose of this function is to be as memory efficient as possible when operating on files with many, many lines, in Golang

ForeachLine.go

import (
  "bufio"
  "os"
)

// readWholeLine returns a single line (without the ending \n) from the input buffered reader.
// An error is returned iff there is an error with the buffered reader.
func readWholeLine(r *bufio.Reader) (string, error) {
  var (isPrefix bool = true
       err error = nil
       line, ln []byte
      )
  for isPrefix && err == nil {
      line, isPrefix, err = r.ReadLine()
      ln = append(ln, line...)
  }
  return string(ln),err
}

// ForeachLine opens the file specified in filename, and reads each line until the \n, one at a time.
// It then invokes the callback method (called withLine) with the string gotten from that line.
// The purpose of this function is to be as memory efficient as possible when operating on files with many, many lines.
//
// Example:
// ForeachLine("/Users/myuser/Desktop/longlonglong.log", func(line string) {
//   exampleFunction(line) // ... do whatever I need to with line
// })
func ForeachLine(filename string, withLine func(string)) {
  f, err := os.Open(filename)
  if err != nil { log.Fatal(err) }
  
  r := bufio.NewReader(f)
  s, e := readWholeLine(r)
  for e == nil {
    withLine(s)
    s,e = readWholeLine(r)
  }
}