Go: abstract iterable - go

Question

Suppose I want to have a method that should either return a chan or a slice. For example, I need a chan if I want to "follow" a file as new lines come, and a slice if I just want to read and return existing lines.
In both cases I will only have to iterate through this return value. Here is an abstract example in Python (which has nothing to do with files but sort of shows the idea):
def get_iterable(self):
if self.some_flag:
return (x for x in self.some_iterable)
return [x for x in self.some_iterable]
def do_stuff(self):
items = self.get_iterable()
for item in items:
self.process(item)
Now, I have a difficulty doing this in Go. I suppose I should look for something like an "iterable interface" which I should return, but I failed to google up some ready-to-use solutions (sorry if it's just my poor googling skills).
What is the best way to do what I want? Or, maybe, the whole design is "bad" for Go and I should consider something else?

Answer 1

Or, maybe, the whole design is "bad" for Go and I should consider something else?
While you could build some interface on top of the types so that you can deal with them as if they were the same I would say it's a poor choice. The far simpler one is to take advantage of multiple return types and define your func with chan myType, []myType, error for it's return then just use 3 way if-else to check for error, followed by chan or slice. Read of the channel like you normally would, iterate the slice like you normally would. Put the code that does work on myType in a helper method so you can call it from both control flows.
My money says this is no more code and it's also far more straight forward. I don't have to read through some abstraction to understand that I have a channel and the inherit complications that come along with it (chan and a slice are incongruous so trying to model them the same sounds like a nightmare), instead you just have an extra step in the programs control flow.

Answer 2

I'm kinda late to the party, but if you really need some "abstract iterable", you could create an interface like this:
type Iterable interface {
Next() (int, error)
}
(Inspired by sql.Rows.)
Then, you could use it like this:
for n, err := iter.Next(); err != nil; n, err = iter.Next() {
fmt.Println(n)
}

Answer 3

For iteration I usually follow pattern found in sql.Rows and bufio.Scanner. Both have a next-equivalent function returning bool, indicating whether next item has been successfully fetched. Then there's a separate method to access the value and error. This pattern lets you write very clean for loops without complex conditions (and without using break or continue statements) and moves error handling outside of the loop.
If you were to abstract your line input, you could for example create an interface like this:
type LineScanner interface {
Scan() bool
Text() string
Err() error
}
This would give you and abstract line source reader. As a bonus, by using exactly these method names you would make bufio.Scanner instantly implementing your interface, so you could use it along with your own types, for example tail-like reader mentioned in your question.
Fuller example:
package main
import (
"bufio"
"fmt"
"strings"
)
type LineScanner interface {
Scan() bool
Text() string
Err() error
}
func main() {
var lr LineScanner
// Use scanner from bufio package
lr = bufio.NewScanner(strings.NewReader("one\ntwo\nthree!\n"))
// Alternatively you can provide your own implementation of LineScanner,
// for example tail-like, blocking on Scan() until next line appears.
// Very clean for loop, isn't it?
for lr.Scan() {
// Handle next line
fmt.Println(lr.Text())
}
// Check if no error while reading
if lr.Err() != nil {
fmt.Println("Error:", lr.Err())
}
}
http://play.golang.org/p/LRbGWj9_Xw