Szathmáry László honlapja @ DEIK | Nim2 / Python's generator-like iterator

Python has generators. yield pauses, the state is frozen, and the next call resumes. Let's see how to do it in Nim.

Python vs Nim
def counter(start): i = start while True: yield i i += 1 a = counter(0) b = counter(100) print(next(a)) # 0 print(next(a)) # 1 print(next(b)) # 100 print(next(a)) # 2 print(next(b)) # 101	proc counter(start: int): iterator(): int = return iterator(): int = var i = start while true: yield i inc i let a = counter(0) let b = counter(100) echo a() # 0 echo a() # 1 echo b() # 100 echo a() # 2 echo b() # 101

a and b are independent, each remembers its own i.

In Nim we use the following pattern: we wrap the iterator in a factory proc that captures the parameters and returns an anonymous iterator. The proc captures the start parameter, and each call to counter() creates a fresh independent instance.

More info here.

Example

Let's see another example. We have a text and we want to process it line by line. The iterator returns the next line. When there are no more lines, it returns an empty string.

let TEXT = "aa\nbbb\ncccc\ndd\n"

proc next_line(text: string, start: int): iterator(): string =
return iterator(): string =
var
start = start
i = start
while i < text.len:
while text[i] != '\n':
i += 1
yield text[start ..< i]
start = i + 1
i = start

proc main() =
let s = next_line(TEXT, 0) # process TEXT, starting at index position 0
echo s() # aa
echo s() # bbb
echo s() # cccc
echo s() # dd
echo "'" & s() & "'" # empty string
echo "'" & s() & "'" # empty string

main()