First of all, I realize that this might go somewhat against the grain of Nim.
And also it's not how I normally write code in Nim. I will start by explaining
very briefly what I am doing and what's my motivation but feel free to skip to
the last code example and the actual question following it. I'm currently
experimenting with closures. I know that I can capture the reference to an
object or an attributes of an object like so:
type
Book = ref object
title: string
var moby = Book(title: "Moby Dick")
let getTitle = proc(): string = moby.title
echo getTitle()
Run
What I find interesting is that `getTitle` does not directly depend on the
`Book` type. And also when I change the title of the book itself (i.e. `moby`)
the `getTitle` function will return the correct (current) title. I can likewise
implement a setter that can manipulate an attribute without an explicit
reference to the type:
let setTitle = proc(newTitle: string) = moby.title = newTitle
Run
Additionally, I can define these anonymous functions for (single, concrete)
objects of different types (say `Movie` and `Book`) and then have a homogeneous
access, meaning I can put multiple `getTitle` closures into a sequence
`seq[proc(): string]`, even if they act on different types of objects.
I'm currently using this approach in an experimental tweening library. I think
tweening really is a cross-cutting concern and I want to keep the coupling to
the objects being tweened very minimal.
Sidenote: This looks somewhat similar to implementing `getTitle[T](obj: T):
string` but I can't have a `seq[T]`. I also thought about tagged unions but as
far as I know these can't be easily extended.
Now on to my question. I tried this with a list of books and a for loop:
type
Book = ref object
title: string
var
books = @[
Book(title: "War and Peace"),
Book(title: "Of Mice and Men"),
Book(title: "Moby Dick")
]
titleReaders: seq[proc(): string]
for i in 0..2:
let w = proc(): string = books[i].title
echo "Getter called inside loop: ", w()
titleReaders.add(w)
for tr in titleReaders:
echo "Getter called outside loop: ", tr()
Run
Link: <https://play.nim-lang.org/#ix=3zRL>
Here the closure inside the for loop also captures the loop variable `i` which
equals 2 once the loop has finished. So all my title getters return "Moby
Dick". (And it's also the same with a `for b in books` type of loop.)
Is there a way to capture "the book itself" inside the loop, basically
resolving one level of reference? I thought maybe with `addr` or `[]` but I
couldn't get it to work. (I guess I could also write an alternative loop as a
macro that expands into separate statements but I would rather stick to
built-in loop constructs.)
