It means that every single type in the language has one extra value it may contain, 'nil', and your code will crash or behave erratically if it contains this value and you haven't written code to handle it. This has caused billions of dollars in software errors (null dereferences in C/C++, NullPointerExceptions in Java, etc.). See "Null References: The Billion Dollar Mistake" by Tony Hoare, the guy who invented it:
A better solution is an explicit optional type, like Maybe in Haskell, Option in Rust, or Optional in Swift. Modern Java code also tends to use the NullObject pattern a lot, combined with @NonNull attributes.
Beside the fact you're wrong (structs, arrays, bools, numeric values, strings and functions can't be nil, for instance), I'm always a little puzzled when I read the argument that "nil costs billions of $".
First, most of the expensive bugs in C/C++ programs are caused by undefined behaviors, making your program run innocently (or not, it's just a question of luck) when you dereference NULL or try to access a freed object or the nth+1 element of an array. "Crashing" and "running erratically" are far from being the same. If those bugs were caught up-front (just like Java or Go do), the cost would be much less. The Morris worm wouldn't have existed with bound-checking, for instance.
Second point, since we're about bound checking. Why is nil such an abomination but trying to access the first element of an empty list is not? Why does Haskell let me write `head []` (and fail at runtime) ? How is that different from a nil dereference exception ? People never complain about this, although in practice I'm pretty sure off-by-one errors are much more frequent than nil derefs (well, at least, in my code, they are).
> $1bn over the history of computing is about $2k per hour. I would not be astonished if a class of bugs cost that much across the industry.
It's not about knowing whether it's $1bn, or 10bn, or just a few millions. The question is to know whether fighting so hard to make these bugs (the "caught at runtime" version, not the "undefined consequences" version) impossible is worth the cost or not.
Can you guarantee that hiring a team of experienced Haskell developers (or pick any strongly-typed language of your choice) will cost me less than hiring a team of experienced Go developers (all costs included, i.e from development and maintenance cost to loss of business after a catastrophic bug)? Can you even give me an exemple of a business that lost tons of money because of some kind of NullPointerException ?
>fighting so hard to make these bugs ... impossible is worth the cost or not.
In this case the solution is trivial, just don't include null when you design the language. It's so easy in fact, that the only reason I can imagine Go has null, is because its designers weren't aware of the problem.
Not including null has consequences, you can't just keep your language as it is, remove null and say you're done.
What's the default value for a pointer in the absence of null? You can force the developer to assign a value to each and every pointer at the moment they are declared, rather than rely on a default value (and the same thing for every composite type containing a pointer), but then you must include some sort of ternary operator when initialization depends on some condition, but then you cannot be sure your ternary operator won't be abused, etc.
You can also go the Haskell way, and have a `None` value but force the user to be in a branch where you know for sure your pointer is not null/None before dereferencing it (via pattern matching or not). But then again you end up with a very different language, which will not necessarily be a better fit to the problem you are trying to solve (fast compile times, easy to make new programmers productive, etc.).
I think it has consequences on the design of the language, making it more complex and more prone to "clever" code, i.e code harder to understand when you haven't written it yourself (or you wrote it a rather long time ago). I've experienced it myself, I spent much more time in my life trying to understand complex code (complex in the way it is written) than to correct trivial NPEs.
That being aside, it is less easy to find developers proficient in a more complex language, and it is more expensive to hire a good developer and let him time to teach himself that language.
I'm not sure it costs "very much", though. I might be wrong. But that's the point: nobody knows for sure. I just think we all lack evidence about those points, although PL theory says avoiding NULL is better, there have been no studies to actually prove it in the "real-world" context. Start-ups using Haskell/OCaml/F#/Rust and the like don't seem to have an undisputable competitive advantage over the ones using "nullable" languages, for instance, or else the latter would simply not exist.
But a bunch types you do expect to work can: Slices, maps and channels.
var m map[string]bool
m["foo"] = 1 // Nil, panic
var a []string
a[0] = "x" // Nil, panic
var c chan int
<-c // Blocks forever
This violates the principle of least surprise. Go has a nicely defined concept of "zero value" (for example, ints are 0 and strings are empty) until you get to these.
The most surprising nil wart, however, is this ugly monster:
package main
import "log"
type Foo interface {
Bar()
}
type Baz struct{}
func (b Baz) Bar() {}
func main() {
var a *Baz = nil
var b Foo = a
fmt.Print(b == nil) // Prints false!
}
This happens is because interfaces are indirections. They are implemented as a pointer to a struct containing a type and a pointer to the real value. The interface value can be nil, but so can the internal pointer. They are different things.
I think supporting nils today is unforgivable, but the last one is just mind-boggling. There's no excuse.
I don't think you're right that interfaces are implemented as a pointer to a struct. The struct is inline like any other struct, and it contains a pointer to a type and a pointer to the value, like `([*Baz], nil)` in your example. The problem is that a nil interface in Go is compiled to `(nil, nil)` which is different.
I don't think using nil to represent uninitialized data is a major issue-- if it were possible to catch uninitialized but queried variables at compile-time, that could be an improvement, but we want to give the programmer control to declare and initialize variables separately.
Interesting, because (reading up on this) value types can not be nil.
How often does typical Go code use values vs. interfaces or pointers? It seems like the situation is pretty similar to modern C++, which also does not allow null for value or reference types (only pointers) and encourages value-based programming. Nil is still a problem there, but less of one than in, say, Java, where everything is a reference.
In my own experience, nil basically only shows up when I've failed to initialize something (like forgetting to loop over and make each channel in an array of channels), or when returning a nil error to indicate a function succeeded. I've never run into other interfaces being nil, but I also haven't worked with reflection and have relatively little Go experience (~6 months).
The code that I've written regularly uses interfaces and pointers, but I'd guess 80% works directly with values.
> I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.
