Ask HN: JS's for-of loop conceptually flawed, or is it just me?

2 points by antrion 9 years ago | 9 comments

When working with generators, two identical for-of loops don't show the same behaviour. Specifically, the first one will behave as expected, whereas the second one will terminate immediately. What is this?

I know that the iterator `g` will signal the value `done` after the first loop, but this behaviour was unexpected and really counterintuitive to me

Note: contrary to this example, for-of loops of arrays will work as I expected. It just doesn't make sense

Code example:

> function* gen() { yield* [1,2,3] }

undefined

> let g = gen()

undefined

> for (let el of g) console.log(el)

undefined

> for (let el of g) console.log(el)

undefined

edit: layout

tantalor 9 years ago |

Same in python,

    >>> def gen(): yield 1; yield 2; yield 3
    ... 
    >>> g = gen()
    >>> for el in g: print el
    ... 
    1
    2
    3
    >>> for el in g: print el
    ... 
    >>>

detaro 9 years ago |

You reached the end of the iterator with the first loop, so there aren't any elements left in it to iterate over. That's consistent to how iterators work in other languages at least.

antrion 9 years ago | |

I know that as per the iterator spec this is normal behaviour. It just seems to me that the behaviour that follows from this is really quite strange. When I want to be able to loop over the contents twice, how am I supposed to to that?

detaro 9 years ago | | |

Depends on what your generator does. If it just does simple calculations, just generate a second instance of it.

Otherwise there are constructions that provide a virtual copy of an iterator and cache the values they still need for the second loop.

Or just don't use an iterator if it doesn't fit the model you need, or turn it into an array before.

(Part of) the idea behind generators is that you don't store all results, because they could be a long or possibly even infinite sequence. E.g. in Python the basic iterator is range(X), which returns the numbers from 0 to X-1. If X=100000, it is a bad idea to generate a giant list of all those numbers, just because you want to know how often you ran through a loop.

tantalor 9 years ago | | |

Store the contents of the generator in memory (an array).

Or make a second instance of the generator.

antrion 9 years ago | |

And is it also that hard to loop twice in said languages?