Just thought I'd run this one by you...
I used to often forget to specify the 2nd variable in a for-range:
sum := ""
for v := range []string{"1", "2", "3"} {
sum += v
}
luckilly (or because Go is so good at type safety) this generates the error:
invalid operation: sum += v (mismatched types string and int)
I used to do this quite a bit which lead (when using an int slice) to one
(maybe more that I haven't found yet :) bugs. But for someone coming from
a language with a for-range statement (like Python) it seems this type of
bug is even more common:
sum := 0
for v := range []int{1, 2, 3} {
sum += v // no error as they're both int
}
Ideally you write tests to catch these sorts of things, but even with 100%
coverage poor tests may not reveal the bug. Then, if you're lucky, you get
unexpected results or it could leave a nasty bug lurking in some
rarely-executed piece of code.
A small fix to the language would avoid this problem. If for-range indices
had a different type then the above code could produce an error like.
invalid operation: sum += v (mismatched types int and *for range index*)
The name of the new type could be something like *intforrangeindex*.
Perhaps something shorter, like *intindex*, if searching existing (Go 1)
code reveals no potential conflicts, which I suspect it wouldn't.
BTW My gut was that *intindex *should be a signed int type but maybe
unsigned would not be a problem.
This would be a proposal for Go 2 as it would not be backward compatible.
However, existing code could be easily converted (go fix ?) by just adding
a type cast. For example, if the above code was actually intended it could
be written as:
sum := 0
for v := range []int{1, 2, 3} {
sum += int(v)
}
I think this is a small but worthwhile addition to type safety.
Note that go vet does not currently detect this problem but maybe it
should. A language change is better though, since not all code is vetted.
I was also thinking that *intindex* could be a hidden internal type but of
course you can declare a loop index like this:
var i int // type would change to intindex in Go 2
for i = range []int{1, 2, 3} {
sum += i
}
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