From 93c4a246a4c7bde4214fef0fad7fe08666fabb5b Mon Sep 17 00:00:00 2001 From: Rob Pike Date: Sat, 6 Aug 2011 07:41:55 +1000 Subject: [PATCH] FAQ: lots of small tweaks plus a couple of new discussions. Expand the conversations about pointers, memory, and garbage collection. Describe the lack of co/contravariant typing. Fixes #1929. Fixes #1930. R=dsymonds, r, mirtchovski, edsrzf, hanwen, rsc CC=golang-dev https://golang.org/cl/4852041 --- doc/go_faq.html | 184 +++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 168 insertions(+), 16 deletions(-) diff --git a/doc/go_faq.html b/doc/go_faq.html index 0bb3eef76c..d7d23567e9 100644 --- a/doc/go_faq.html +++ b/doc/go_faq.html @@ -8,6 +8,7 @@ What is the purpose of the project?

No major systems language has emerged in over a decade, but over that time the computing landscape has changed tremendously. There are several trends: +

-

We believe it's worth trying again with a new language, a concurrent, garbage-collected language with fast compilation. Regarding the points above: +

-

What is the origin of the name?

@@ -105,7 +105,8 @@ and libraries from prototype to reality.

-Many others have contributed ideas, discussions, and code. +Go became a public open source project on November 10, 2009. +Many people from the community have contributed ideas, discussions, and code.

@@ -314,7 +315,16 @@ exceptional.

-Go takes a different approach. Instead of exceptions, it has a couple +Go takes a different approach. For plain error handling, Go's multi-value +returns make it easy to report an error without overloading the return value. +A +canonical error type, coupled +with Go's other features, makes error +handling pleasant but quite different from that in other languages. +

+ +

+Go also has a couple of built-in functions to signal and recover from truly exceptional conditions. The recovery mechanism is executed only as part of a function's state being torn down after an error, which is sufficient @@ -372,7 +382,7 @@ Why build concurrency on the ideas of CSP?

Concurrency and multi-threaded programming have a reputation for difficulty. We believe the problem is due partly to complex designs such as pthreads and partly to overemphasis on low-level details -such as mutexes, condition variables, and even memory barriers. +such as mutexes, condition variables, and memory barriers. Higher-level interfaces enable much simpler code, even if there are still mutexes and such under the covers.

@@ -390,14 +400,14 @@ Why goroutines instead of threads?

Goroutines are part of making concurrency easy to use. The idea, which has been around for a while, is to multiplex independently executing -functions—coroutines, really—onto a set of threads. +functions—coroutines—onto a set of threads. When a coroutine blocks, such as by calling a blocking system call, the run-time automatically moves other coroutines on the same operating system thread to a different, runnable thread so they won't be blocked. The programmer sees none of this, which is the point. The result, which we call goroutines, can be very cheap: unless they spend a lot of time in long-running system calls, they cost little more than the memory -for the stack. +for the stack, which is just a few kilobytes.

@@ -473,8 +483,8 @@ that specifies a subset of its methods. Besides reducing the bookkeeping, this approach has real advantages. Types can satisfy many interfaces at once, without the complexities of traditional multiple inheritance. -Interfaces can be very lightweight—having one or even zero methods -in an interface can express useful concepts. +Interfaces can be very lightweight—an interface with +one or even zero methods can express a useful concept. Interfaces can be added after the fact if a new idea comes along or for testing—without annotating the original types. Because there are no explicit relationships between types @@ -494,7 +504,7 @@ stream ciphers. All these ideas stem from a single interface

It takes some getting used to but this implicit style of type -dependency is one of the most exciting things about Go. +dependency is one of the most productive things about Go.

@@ -588,6 +598,85 @@ the interface idea. Sometimes, though, they're necessary to resolve ambiguities among similar interfaces.

+

+Why doesn't type T satisfy the Equal interface?

+ +

+Consider this simple interface to represent an object that can compare +itself with another value: +

+ +
+type Equaler interface {
+	Equal(Equaler) bool
+}
+
+ +

+and this type, T: +

+ +
+type T int
+func (t T) Equal(u T) bool { return t == u } // does not satisfy Equaler
+
+ +

+Unlike the analogous situation in some polymorphic type systems, +T does not implement Equaler. +The argument type of T.Equal is T, +not literally the required type Equaler. +

+ +

+In Go, the type system does not promote the argument of +Equal; that is the programmer's responsibility, as +illustrated by the type T2, which does implement +Equaler: +

+ +
+type T2 int
+func (t T2) Equal(u Equaler) bool { return t == u.(T2) }  // satisfies Equaler
+
+ +

+Even this isn't like other type systems, though, because in Go any +type that satisfies Equaler could be passed as the +argument to T2.Equal, and at run time we must +check that the argument is of type T2. +Some languages arrange to make that guarantee at compile time. +

+ +

+A related example goes the other way: +

+ +
+type Opener interface {
+   Open(name) Reader
+}
+
+func (t T3) Open() *os.File
+
+ +

+In Go, T3 does not satisfy Opener, +although it might in another language. +

+ +

+While it is true that Go's type system does less for the programmer +in such cases, the lack of subtyping makes the rules about +interface satisfaction very easy to state: are the function's names +and signatures exactly those of the interface? +Go's rule is also easy to implement efficiently. +We feel these benefits offset the lack of +automatic type promotion. Should Go one day adopt some form of generic +typing, we expect there would be a way to express the idea of these +examples and also have them be statically checked. +

+

Can I convert a []T to an []interface{}?

@@ -736,17 +825,62 @@ makes a copy of the pointer, but again not the data it points to. Should I define methods on values or pointers? 
-func (s *MyStruct) someMethod() { } // method on pointer
-func (s MyStruct) someMethod() { }  // method on value
+func (s *MyStruct) pointerMethod() { } // method on pointer
+func (s MyStruct)  valueMethod()   { } // method on value

+For programmers unaccustomed to pointers, the distinction between these +two examples can be confusing, but the situation is actually very simple. When defining a method on a type, the receiver (s in the above -example) behaves exactly is if it were an argument to the method. Define the -method on a pointer type if you need the method to modify the data the receiver -points to. Otherwise, it is often cleaner to define the method on a value type. +example) behaves exactly as if it were an argument to the method. +Whether to define the receiver as a value or as a pointer is the same +question, then, as whether a function argument should be a value or +a pointer. +There are several considerations.

+

+First, and most important, does the method need to modify the +receiver? +If it does, the receiver must be a pointer. +(Slices and maps are reference types, so their story is a little +more subtle, but for instance to change the length of a slice +in a method the receiver must still be a pointer.) +In the examples above, if pointerMethod modifies +the fields of s, +the caller will see those changes, but valueMethod +is called with a copy of the caller's argument (that's the definition +of passing a value), so changes it makes will be invisible to the caller. +

+ +

+By the way, pointer receivers are identical to the situation in Java, +although in Java the pointers are hidden under the covers; it's Go's +value receivers that are unusual. +

+ +

+Second is the consideration of efficiency. If the receiver is large, +a big struct for instance, it will be much cheaper to +use a pointer receiver. +

+ +

+Next is consistency. If some of the methods of the type must have +pointer receivers, the rest should too, so the method set is +consistent regardless of how the type is used. +See the section on method sets +for details. +

+ +

+For types such as basic types, slices, and small structs, +a value receiver is very cheap so unless the semantics of the method +requires a pointer, a value receiver is efficient and clear. +

+ +

What's the difference between new and make?

@@ -1111,6 +1245,11 @@ isn't fast enough yet (even if it were, taking care not to generate unnecessary garbage can have a huge effect).

+

+In any case, Go can often be very competitive. See the blog post about +profiling +Go programs for an informative example. +

Changes from C

@@ -1165,7 +1304,9 @@ and chan keep things clear.

-See the Go's Declaration Syntax article for more details. +See the article about +Go's Declaration Syntax +for more details.

@@ -1252,3 +1393,14 @@ program helps everyone. Finally, concurrency aside, garbage collection makes interfaces simpler because they don't need to specify how memory is managed across them.

+ +

+On the topic of performance, keep in mind that Go gives the programmer +considerable control over memory layout and allocation, much more than +is typical in garbage-collected languages. A careful programmer can reduce +the garbage collection overhead dramatically by using the language well; +see the article about +profiling +Go programs for a worked example, including a demonstration of Go's +profiling tools. +