Things I Wish Someone Had Told Me About Go

http://openmymind.net/Things-I-Wish-Someone-Had-Told-Me-About-Go/

by latch • 13 years ago

74 44 13 years ago

jgrahamc• 13 years ago

But, without pointer arithmetics or, despite many claims, pass-by-reference, Go's "pointers" feels like a more flexible form of Java/Ruby/Python/Node/....

It's not clear from the first example if the writer actually understands pointers. The example code passes a pointer to a function and then modifies the pointer, not what it's pointing to. It was no surprise to me that this wouldn't modify the original thing the pointer was referencing. If they wanted to modify the pointer itself then they should have passed a * * User not a *User.

Go does, indeed, have pass-by-reference (use a pointer) or pass-by-value (don't use a pointer).

latch5• 13 years ago

I understand what a pointer is, but it's entirely possible that I still don't grok Go. Before writing that, I did try:

    func main() {
      u := &User{Name: "Leto"}
      Modify(&u)
    }

    func Modify(u **User) {
      u = &User{Name: "Paul"}
    }

With no luck.

zeeboo2• 13 years ago

Nope, looks like you don't understand pointers. You're still modifying the passed-by-value pointer to a pointer, and not the thing that it points to.

voidlogic7• 13 years ago

Is this what you want? http://play.golang.org/p/Pt1D_ICJu5

fmstephe3• 13 years ago

Enjoyed your article. I think that you are a little off the mark with respect to pointers though.

The simple solution to your first example is to simply write to the Name field of the struct being pointed too.

http://play.golang.org/p/D3t0Iv8oG4

If you want to do complex pointer meddling (and replace the entire struct) you need this syntax.

http://play.golang.org/p/tMwHC-k-bu

As a side note I have personally used pointers to interfaces for doing search tree reordering. You can really do a lot of useful pointer operations without pointer arithmetic.

(A link to a quadtree implementation which uses pointers to interfaces, on line 119) https://github.com/fmstephe/location_server/blob/master/quad...

roel_v• 13 years ago

"I understand what a pointer is,"

What are you trying to do, and how would you think it works in your examples? I assume you're trying to change the name of the object being pointed to, but you're just assigning a new value to a variable that is passed to a function. Therefore, when the function returns, that value is 'lost'. How could it ever change the value of a member of the object you're pointing to? I have never written a line of Go in my life, but with the knowledge that Go supports some form of pointers, it's quite easy to trace this back to how it would work in C or C++; it also wouldn't work there, either. Knowing what a pointer is is easy, it's just memorizing a definition - knowing how they work takes some experience.

drivebyacct24• 13 years ago

You're displaying some basic conceptual problems with pointers. A simple Go example:

    func main() {
      u := User{Name: "Leto"}
      Modify(&u)
    }

    func Modify(u *User) {
      // u = &User{Name: "Paul"}
      // you could, but why?
      u.Name = "Paul"
    }

I mean, if you read your code for main, you're saying "Declare u to be a pointer to a new User. Then call modify with the address of u... aka, the address of the pointer that points to the actual User object"...

This is pretty much basic pointer stuff, the same as you would do in C: runnable: http://codepad.org/TeUZi1Wp

irahul• 13 years ago

> I understand what a pointer is, but it's entirely possible that I still don't grok Go.

I don't know about you, but I understand pointers from C, and then learning assembly. Go's behavior isn't different from C here.

Also, elsewhere you are discussing pass by value/pass by reference. Technically, C/Go/Java are pass by value. Passing pointers and de-referencing it is how pass-by-ref is emulated. You will notice that the top comment has qualified pass-by-reference with "passing pointers". That is colloquial terminology.

voidlogic7• 13 years ago

Gos lack of pointer arithmetic is one of the design decisions that allows go to be memory safe (http://en.wikipedia.org/wiki/Memory_safe) . If you need need to be naughty with pointers you can use the "unsafe" package. But some environments like GAE or Go playground do not allow this.

Also some of the built-in types like channels and maps are reference types (so they are always passed by reference).

zeeboo2• 13 years ago

Everything in Go is passed by value, even the "reference types". Those types are called reference types because they hold a reference to some value in memory, exactly like a pointer.

If they were passed by reference, then we would expect this playground snip to print "false": http://play.golang.org/p/r6naknvzng

voidlogic7• 13 years ago

"Tip Or Don't Bother"

This is pretty true, although I would not bother using tip if writing a small command line utility or learning Go, but for a large project this is good advice. Execution time and memory usage are greatly improved in tip, at the cost you might run into bug that has not been fixed yet.

Here is the burn down to Go 1.1's release, when it hits bottom it releases. http://swtch.com/~rsc/go11.html# Go 1.1 RC should be out in April I believe.

Also if you need to pick a Go tip build to use this is useful: http://build.golang.org/

phasevar• 13 years ago

Just how much has execution time improved? I'd love to see some benchmarks.

voidlogic7• 13 years ago

If you read the golang-dev google groups mailing list you will see lots of micro-benchmarks posted for specific commits. The best test is to install both stable and tip on your machine and compile your application with both. In one of my application there was a 9% improvement, in another it was 53%. When Go 1.1 is released I presume the computer benchmarks game will be updated, which might make comparison, at lease with other languages, easier.

dewitt• 13 years ago

That pointer example is a little weird. A C/C++ pointer used that way wouldn't print "Leto" "Paul", either. All it would do is update the local variable to point to a new object, but the caller would still be pointing to the original. I think the poster meant to use a pointer to a pointer here (which would work as intended in both Go and C/C++).

NateDad3• 13 years ago

The author is obviously new to Go, and that's fine.... here's a clarification of interfaces:

Interface Pointers

Note that the author of a type that fulfills an interface and the consumer of a function that takes an interface both have full control over how much data is copied on a call to the function. You can control whether a type fulfills an interface, or whether a pointer to the type fulfills the interface. In Go, it is always easy to tell how much memory is being copied for any function call.

  type Jumper interface {
    Jump()
  }

  func MakeItJump(j Jumper) {
    // this function doesn't know what's wrapped inside the Jumper interface
    // and doesn't need to care
    j.Jump()
  }

  type Foo struct {
    data [10]int64 // array of 10 8 byte numbers = 80 bytes on a copy
  }
  func (f *Foo) Jump() {
    // we define Jump on Foo to take a copy of a pointer to a Foo.
    // this means that when we pass a *Foo into MakeItJump, 
    // just a single pointer value is copied into the interface, \
    // not the whole array inside Foo
  }

  type Bar uint8

  func (b Bar) Jump() {
    // we define Jump on Bar as taking a copy of the value of Bar
    // since we know it only needs to copy an 8 bit integer to do so
  }

matthavener2• 13 years ago

I didn't quite "get" the point of the pointer/copy option on interfaces, but this makes it crystal clear. This works around such a common problem in writing C++ templates (which is really the only equivalent polymorphism if you want to dispatch a function on a native type):

  template <class T> void doSomething(const T &expensiveCopy);

Versus:

  template <class T> void doSomething(T cheapCopy);

As usual, boost provides another crazy template hack to work around this issue:

http://boost.sourceforge.net/libs/utility/call_traits.htm

esrauch• 13 years ago

Can you explain why this is actually a problem? Why not just always have a const T&, is there some negative in the "cheap copy" case to not copy?

pkulak• 13 years ago

The "interface as a pointer" thing really threw me for a loop as well. It was bothering me so much (not knowing what was actually going on), and Googling was helping so little, that I actually hopped on the Go IRC channel. The guys on there set me straight in, literally, minutes. There's a great community around Go right now.

Other than that, though, everything has been pretty cut and dry. There really aren't many "gotchas" in Go.

ISL• 13 years ago

Whoa. Pre-coffee, I'd clicked on the link expecting to find a "List of things someone had told me about the strategy game Go".

Anyone have a link to such a list? Seems like it'd be fun.

Beldur• 13 years ago

One thing I learned was:

Every move you play is certainly not the best. You just need to focus on making the smallest mistakes as possible.

shurcooL• 13 years ago

I had the opposite problem.

When someone retweeted this[1], I thought it was about golang problems.

[1] https://twitter.com/gogameguru/status/309695255056351232

kin3tic• 13 years ago

Wow, this displays a severe misunderstanding of how pointers work in general. It's not like the syntax for it in Go is any different, this code would look very similar in anything that has real pointers (C, for example), certainly in regards to the notation and syntax for pointers and references and such.

'I understand what a pointer is'. Questionable.

drivebyacct24• 13 years ago

For reference, I posted this in a subthread above: the same example in C demonstrating using a pointer and passing it or declaring an object and passing it's address: http://codepad.org/TeUZi1Wp

latch5• 13 years ago

Same example in C# showing pass by reference does exist http://rextester.com/IYZE41714

toddh• 13 years ago

>func ServeHTTP(res http.ResponseWriter, req *http.Request)

I'd always wondered about that too. Thanks for one less mystery in the world.

Things I Wish Someone Had Told Me About Go

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