Fixing the other Go loop bug
flak.tedunangst.com
flak.tedunangst.com
With immutables you simply map on players list and continue working with the copy.
Bugs avoided. Performance concerns - maybe they exist - but show me them first.
Apologies for doing a glorified +1 comment, but if you take the above comment plus the article's in-loop mutation algorithm and try it in different languages you get insights on who you are as a coder and what are the language's strengths.
Another detail is that the programmer in the article prefers an mutation iterator format while the language forces him/her to use an array/index. This has further impact on pointer vs value accesses.
Also by mapping on players list the above post means: just discard the previous one, create a new one however you want and make the new one the new source of truth. No need for mutability considerations.
I want to create it by re-using the memory of the previous one that I've just discarded, please (well, the one that will end up discarded, actually: it has to live until the map ends executing, hasn't it). Unfortunately, most of the languages' implementations/runtimes don't employ the Perceus algorithm.
If you really need it, you can always fall back to using mutables for this special list, and use immutables for the rest? This is probably the reason why languages don't implement the Perceus algorithm. If you really needed it you might as well do it by hand.
The real thing here is the definition of the Player type.
It should have a mutating method that resets it's score (a method defined on *Player).
Raw access on fields is often only ok for naive, trivial implementations. Methods are often needed anyway.
That's straightforward. Immutability doesn't make any sense here.
... Examples on how to avoid the bug ...
Notable is that you claim here is an opinion not an argument.
That immutability is not needed is also not an argument against it.
The point here is that working with immutables relieves you from some cognitive stress and repetitive work such as writing accessors and making fields private.
With mutability, it's still possible to make things immutable if properly encapsulated.
If everything is immutable by default and you need mutability for some reason, good luck.
The heuristic is simple too. If you need something to be mutable, you should deal with pointers. So *Player and []*Player here.
At the type/variable level, it's the opposite. We see it with const. Once it's in here, it's in there.
The Go code
var player Player
otherPlayer := player
otherPlayer.score = 0
This isn't something that should be fixed by the language. Instead, a linter rule that detects that the write is never read would catch this.
(You can still write the same bug in Rust or C++ accidentally.)
I honestly don't think you can. If you try to iterate over values instead of references, you'll take ownership of the vector and not be able to use it again.
Same with any C++ class for which there is no copy constructor.
For example, in Nim (it also prefers value semantics) this would result in a compile error:
var players = [1, 2, 3]
for player in players:
player = 0
var players = [1, 2, 3]
for player in players.mitems:
player = 0Because it's a variable. That's what you do with variables, you mutate them to hold different values. There are no "named, non-constant holders for values that you can't mutate directly" in Go.
For reasons it’s never really bothered me, but the *other* loop bug does bite me some times. This is the one where the loop values are values.
The common mistakes wiki lists the capture bug twice, but doesn’t mention this bug.
for _, *player := range players {
player.score = 0
}The author's title literally refers to it as a bug.
Go favors concurrency safety and concurrency efficiency. Therefore it has to copy loop values to be able to parallelize them. With a reference this would be unsafe to do.
Reads of memory locations larger than a single machine word are encouraged but not required to meet the same semantics as word-sized memory locations, observing a single allowed write w. For performance reasons, implementations may instead treat larger operations as a set of individual machine-word-sized operations in an unspecified order. This means that races on multiword data structures can lead to inconsistent values not corresponding to a single write. When the values depend on the consistency of internal (pointer, length) or (pointer, type) pairs, as can be the case for interface values, maps, slices, and strings in most Go implementations, such races can in turn lead to arbitrary memory corruption.
They don't yet parallelize loops, but the idea was there. Most others do it already
[0] https://github.com/golang/go/issues/24210#issuecomment-36979...