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FAQ: lots of small tweaks plus a couple of new discussions.

Browse Source 
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
This commit is contained in:
Rob Pike 2011-08-06 07:41:55 +10:00
parent be269b2f6d
commit 93c4a246a4
1 changed files with 168 additions and 16 deletions
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184
doc/go_faq.html
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@ -8,6 +8,7 @@ What is the purpose of the project?</h3>
<p>
No major systems language has emerged in over a decade, but over that time
the computing landscape has changed tremendously. There are several trends:
</p>
<ul>
<li>
@ -26,11 +27,11 @@ are not well supported by popular systems languages.
<li>
The emergence of multicore computers has generated worry and confusion.
</ul>
</p>
<p>
We believe it's worth trying again with a new language, a concurrent,
garbage-collected language with fast compilation. Regarding the points above:
</p>
<ul>
<li>
@ -50,7 +51,6 @@ concurrent execution and communication.
By its design, Go proposes an approach for the construction of system
software on multicore machines.
</ul>
</p>
<h3 id="What_is_the_origin_of_the_name">
What is the origin of the name?</h3>
@ -105,7 +105,8 @@ and libraries from prototype to reality.
</p>
<p>
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.
</p>
<h3 id="creating_a_new_language">
@ -314,7 +315,16 @@ exceptional.
</p>
<p>
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 href="http://blog.golang.org/2011/07/error-handling-and-go.html">A
canonical error type, coupled
with Go's other features</a>, makes error
handling pleasant but quite different from that in other languages.
</p>
<p>
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?</h3>
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.
</p>
@ -390,14 +400,14 @@ Why goroutines instead of threads?</h3>
<p>
Goroutines are part of making concurrency easy to use. The idea, which has
been around for a while, is to multiplex independently executing
functions&mdash;coroutines, really&mdash;onto a set of threads.
functions&mdash;coroutines&mdash;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.
</p>
<p>
@ -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&mdash;having one or even zero methods
in an interface can express useful concepts.
Interfaces can be very lightweight&mdash;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&mdash;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
<p>
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.
</p>
<h3 id="methods_on_basics">
@ -588,6 +598,85 @@ the interface idea. Sometimes, though, they're necessary to resolve ambiguities
among similar interfaces.
</p>
<h3 id="t_and_equal_interface">
Why doesn't type T satisfy the Equal interface?</h3>
<p>
Consider this simple interface to represent an object that can compare
itself with another value:
</p>
<pre>
type Equaler interface {
Equal(Equaler) bool
}
</pre>
<p>
and this type, <code>T</code>:
</p>
<pre>
type T int
func (t T) Equal(u T) bool { return t == u } // does not satisfy Equaler
</pre>
<p>
Unlike the analogous situation in some polymorphic type systems,
<code>T</code> does not implement <code>Equaler</code>.
The argument type of <code>T.Equal</code> is <code>T</code>,
not literally the required type <code>Equaler</code>.
</p>
<p>
In Go, the type system does not promote the argument of
<code>Equal</code>; that is the programmer's responsibility, as
illustrated by the type <code>T2</code>, which does implement
<code>Equaler</code>:
</p>
<pre>
type T2 int
func (t T2) Equal(u Equaler) bool { return t == u.(T2) } // satisfies Equaler
</pre>
<p>
Even this isn't like other type systems, though, because in Go <em>any</em>
type that satisfies <code>Equaler</code> could be passed as the
argument to <code>T2.Equal</code>, and at run time we must
check that the argument is of type <code>T2</code>.
Some languages arrange to make that guarantee at compile time.
</p>
<p>
A related example goes the other way:
</p>
<pre>
type Opener interface {
Open(name) Reader
}
func (t T3) Open() *os.File
</pre>
<p>
In Go, <code>T3</code> does not satisfy <code>Opener</code>,
although it might in another language.
</p>
<p>
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.
</p>
<h3 id="convert_slice_of_interface">
Can I convert a []T to an []interface{}?</h3>
@ -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?</h3>
<pre>
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
</pre>
<p>
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 (<code>s</code> 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.
</p>
<p>
First, and most important, does the method need to modify the
receiver?
If it does, the receiver <em>must</em> 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 <code>pointerMethod</code> modifies
the fields of <code>s</code>,
the caller will see those changes, but <code>valueMethod</code>
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.
</p>
<p>
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.
</p>
<p>
Second is the consideration of efficiency. If the receiver is large,
a big <code>struct</code> for instance, it will be much cheaper to
use a pointer receiver.
</p>
<p>
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 <a href="#different_method_sets">method sets</a>
for details.
</p>
<p>
For types such as basic types, slices, and small <code>structs</code>,
a value receiver is very cheap so unless the semantics of the method
requires a pointer, a value receiver is efficient and clear.
</p>
<h3 id="new_and_make">
What's the difference between new and make?</h3>
@ -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).
</p>
<p>
In any case, Go can often be very competitive. See the blog post about
<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling
Go programs</a> for an informative example.
<h2 id="change_from_c">Changes from C</h2>
<h3 id="different_syntax">
@ -1165,7 +1304,9 @@ and <code>chan</code> keep things clear.
</p>
<p>
See the <a href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">Go's Declaration Syntax</a> article for more details.
See the article about
<a href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">Go's Declaration Syntax</a>
for more details.
</p>
<h3 id="no_pointer_arithmetic">
@ -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.
</p>
<p>
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
<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling
Go programs</a> for a worked example, including a demonstration of Go's
profiling tools.
</p>