doc/go_lang_faq.html

<!-- The Go Programming Language Design FAQ -->

<h2 id="origins">Origins</h2>

<h3 id="history">
What is the history of the project?</h3>
<p>
Robert Griesemer, Rob Pike and Ken Thompson started sketching the
goals for a new language on the white board on September 21, 2007.
Within a few days the goals had settled into a plan to do something
and a fair idea of what it would be.  Design continued part-time in
parallel with unrelated activities.  By January 2008, Ken started work
on a compiler with which to explore ideas; it generated C code as its
output.  By mid-year the language had become a full-time project and
had settled enough to attempt a production compiler.  Meanwhile, Ian
Taylor had read the draft specification and written an independent GCC
front end.  Russ Cox joined in late 2008 and helped move the language
and libraries from prototype to reality.
</p>

<h3 id="creating_a_new_language">
Why are you creating a new language?</h3>
<p>
Go was born out of frustration with existing languages and
environments for systems programming.  Programming had become too
difficult and the choice of languages was partly to blame.  One had to
choose either efficient compilation, efficient execution, or ease of
programming; all three were not available in the same commonly
available language.  Programmers who could were choosing ease over
safety and efficiency by moving to dynamically typed languages such as
Python and JavaScript rather than C++ or, to a lesser extent, Java.
</p>
<p>
Go is an attempt to combine the ease of programming of an interpreted,
dynamically typed
language with the efficiency and type safety of a compiled language.
It also aims to be modern, with support for networked and multicore
computing.  Finally, it is intended to be <i>fast</i>: it should take
at most a few seconds to build a large executable on a single computer.
To meet these goals required addressing a number of
linguistic issues: an expressive but lightweight type system;
concurrency and garbage collection; rigid dependency specification;
and so on.  These cannot be addressed well by libraries or tools; a new
language was called for.
</p>


<h3 id="ancestors">
What are Go's ancestors?</h3>
<p>
Go is mostly in the C family (basic syntax),
with significant input from the Pascal/Modula/Oberon
family (declarations, packages),
plus some ideas from languages
inspired by Tony Hoare's CSP,
such as Newsqueak and Limbo (concurrency).
However, it is a new language across the board.
In every respect the language was designed by thinking
about what programmers do and how to make programming, at least the
kind of programming we do, more effective, which means more fun.
</p>

<h3 id="principles">
What are the guiding principles in the design?</h3>
<p>
Programming today involves too much bookkeeping, repetition, and
clerical work.  As Dick Gabriel says, &ldquo;Old programs read
like quiet conversations between a well-spoken research worker and a
well-studied mechanical colleague, not as a debate with a compiler.
Who'd have guessed sophistication bought such noise?&rdquo;
The sophistication is worthwhile&mdash;no one wants to go back to
the old languages&mdash;but can it be more quietly achieved?
</p>
<p>
Go attempts to reduce the amount of typing in both senses of the word.
Throughout its design, we have tried to reduce clutter and
complexity.  There are no forward declarations and no header files;
everything is declared exactly once.  Initialization is expressive,
automatic, and easy to use.  Syntax is clean and light on keywords.
Stuttering (<code>foo.Foo* myFoo = new(foo.Foo)</code>) is reduced by
simple type derivation using the <code>:=</code>
declare-and-initialize construct.  And perhaps most radically, there
is no type hierarchy: types just <i>are</i>, they don't have to
announce their relationships.  These simplifications allow Go to be
expressive yet comprehensible without sacrificing, well, sophistication.
</p>
<p>
Another important principle is to keep the concepts orthogonal.
Methods can be implemented for any type; structures represent data;
interfaces represent abstraction; and so on.  Orthogonality makes it
easier to understand what happens when things combine.
</p>


<h2 id="change_from_c">Changes from C</h2>

<h3 id="different_syntax">
Why is the syntax so different from C?</h3>
<p>
Other than declaration syntax, the differences are not major and stem
from two desires.  First, the syntax should feel light, without too
many mandatory keywords, repetition, or arcana.  Second, the language
has been designed to be easy to analyze
and can be parsed without a symbol table.  This makes it much easier
to build tools such as debuggers, dependency analyzers, automated
documentation extractors, IDE plug-ins, and so on.  C and its
descendants are notoriously difficult in this regard.
</p>

<h3 id="declarations_backwards">
Why are declarations backwards?</h3>
<p>
They're only backwards if you're used to C. In C, the notion is that a
variable is declared like an expression denoting its type, which is a
nice idea, but the type and expression grammars don't mix very well and
the results can be confusing; consider function pointers.  Go mostly
separates expression and type syntax and that simplifies things (using
prefix <code>*</code> for pointers is an exception that proves the rule).  In C,
the declaration
</p>
<pre>
	int* a, b;
</pre>
<p>
declares <code>a</code> to be a pointer but not <code>b</code>; in Go
</p>
<pre>
	var a, b *int;
</pre>
<p>
declares both to be pointers.  This is clearer and more regular.
Also, the <code>:=</code> short declaration form argues that a full variable
declaration should present the same order as <code>:=</code> so
</p>
<pre>
	var a uint64 = 1;
</pre>
has the same effect as
<pre>
	a := uint64(1);
</pre>
<p>
Parsing is also simplified by having a distinct grammar for types that
is not just the expression grammar; keywords such as <code>func</code>
and <code>chan</code> keep things clear.
</p>

<h3 id="no_pointer_arithmetic">
Why is there no pointer arithmetic?</h3>
<p>
Safety.  Without pointer arithmetic it's possible to create a
language that can never derive an illegal address that succeeds
incorrectly.  Compiler and hardware technology have advanced to the
point where a loop using array indices can be as efficient as a loop
using pointer arithmetic.  Also, the lack of pointer arithmetic can
simplify the implementation of the garbage collector.
</p>

<h3 id="inc_dec">
Why are <code>++</code> and <code>--</code> statements and not expressions?  And why postfix, not prefix?</h3>
<p>
Without pointer arithmetic, the convenience value of pre- and postfix
increment operators drops.  By removing them from the expression
hierarchy altogether, expression syntax is simplified and the messy
issues around order of evaluation of <code>++</code> and <code>--</code>
(consider <code>f(i++)</code> and <code>p[i] = q[++i]</code>)
are eliminated as well.  The simplification is
significant.  As for postfix vs. prefix, either would work fine but
the postfix version is more traditional; insistence on prefix arose
with the STL, a library for a language whose name contains, ironically, a
postfix increment.
</p>

<h3 id="garbage_collection">
Why do garbage collection?  Won't it be too expensive?</h3>
<p>
One of the biggest sources of bookkeeping in systems programs is
memory management.  We feel it's critical to eliminate that
programmer overhead, and advances in garbage collection
technology in the last few years give us confidence that we can
implement it with low enough overhead and no significant
latency.  (The current implementation is a plain mark-and-sweep
collector but a replacement is in the works.)
</p>
<p>
Another point is that a large part of the difficulty of concurrent
and multi-threaded programming is memory management;
as objects get passed among threads it becomes cumbersome
to guarantee they become freed safely.
Automatic garbage collection makes concurrent code far easier to write.
Of course, implementing garbage collection in a concurrent environment is
itself a challenge but meeting it once rather than in every
program helps everyone.
</p>
<p>
Finally, concurrency aside, garbage collection makes interfaces
simpler because they don't need to specify how memory is managed across them.
</p>

<h2 id="absent_features">Absent features</h2>

<h3 id="generics">
Why does Go not have generic types?</h3>
<p>
Generics may well be added at some point.  We don't feel an urgency for
them, although we understand some programmers do.
</p>
<p>
Generics are convenient but they come at a cost in
complexity in the type system and run-time.  We haven't yet found a
design that gives value proportionate to the complexity, although we
continue to think about it.  Meanwhile, Go's built-in maps and slices,
plus the ability to use the empty interface to construct containers
(with explicit unboxing) mean in many cases it is possible to write
code that does what generics would enable, if less smoothly.
</p>
<p>
This remains an open issue.
</p>

<h3 id="exceptions">
Why does Go not have exceptions?</h3>
<p>
Exceptions are a similar story.  A number of designs for exceptions
have been proposed but each adds significant complexity to the
language and run-time.  By their very nature, exceptions span functions and
perhaps even goroutines; they have wide-ranging implications.  There
is also concern about the effect they would have on the
libraries.  They are, by definition, exceptional yet experience with
other languages that support them show they have profound effect on
library and interface specification.  It would be nice to find a design
that allows them to be truly exceptional without encouraging common
errors to turn into special control flow requiring every programmer to
compensate.
</p>
<p>
Like generics, exceptions remain an open issue.
</p>

<h3 id="assertions">
Why does Go not have assertions?</h3>
<p>
This is answered in the general <a href="go_faq.html#Where_is_assert">FAQ</a>.
</p>

<h2 id="types">Types</h2>

<h3 id="inheritance">
Why is there no type inheritance?</h3>
<p>
Object-oriented programming, at least in the best-known languages,
involves too much discussion of the relationships between types,
relationships that often could be derived automatically.  Go takes a
different approach.
</p>
<p>
Rather than requiring the programmer to declare ahead of time that two
types are related, in Go a type automatically satisfies any interface
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 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
and interfaces, there is no type hierarchy to manage.
</p>
<p>
It's possible to use these ideas to construct something analogous to
type-safe Unix pipes.  For instance, see how <code>fmt.Fprintf</code>
enables formatted printing to any output, not just a file, or how the
<code>bufio</code> package can be completely separate from file I/O,
or how the <code>crypto</code> packages stitch together block and
stream ciphers.  All these ideas stem from a single interface
(<code>io.Writer</code>) representing a single method
(<code>Write</code>).  And that's only scratching the surface.
</p>
<p>
It takes some getting used to but this implicit style of type
dependency is one of the most exciting things about Go.
</p>

<h3 id="methods_on_basics">
Why is <code>len</code> a function and not a method?</h3>
<p>
We debated this issue but decided
implementing <code>len</code> and friends as functions was fine in practice and
didn't complicate questions about the interface (in the Go type sense)
of basic types.
</p>

<h3 id="overloading">
Why does Go not support overloading of methods and operators?</h3>
<p>
Method dispatch is simplified if it doesn't need to do type matching as well.
Experience with other languages told us that having a variety of
methods with the same name but different signatures was occasionally useful
but that it could also be confusing and fragile in practice.  Matching only by name
and requiring consistency in the types was a major simplifying decision
in Go's type system.
</p>
<p>
Regarding operator overloading, it seems more a convenience than an absolute
requirement.  Again, things are simpler without it.
</p>

<h2 id="values">Values</h2>

<h3 id="conversions">
Why does Go not provide implicit numeric conversions?</h3>
<p>
The convenience of automatic conversion between numeric types in C is
outweighed by the confusion it causes.  When is an expression unsigned?
How big is the value?  Does it overflow?  Is the result portable, independent
of the machine on which it executes?
It also complicates the compiler; &ldquo;the usual arithmetic conversions&rdquo;
are not easy to implement and inconsistent across architectures.
For reasons of portability, we decided to make things clear and straightforward
at the cost of some explicit conversions in the code.
The definition of constants in Go&mdash;arbitrary precision values free
of signedness and size annotations&mdash;ameliorates matters considerably,
though.
</p>
<p>
A related detail is that, unlike in C, <code>int</code> and <code>int64</code>
are distinct types even if <code>int</code> is a 64-bit type.  The <code>int</code>
type is generic; if you care about how many bits an integer holds, Go
encourages you to be explicit.
</p>

<h3 id="builtin_maps">
Why are maps built in?</h3>
<p>
The same reason strings are: they are such a powerful and important data
structure that providing one excellent implementation with syntactic support
makes programming more pleasant.  We believe that Go's implementation of maps
is strong enough that it will serve for the vast majority of uses.
If a specific application can benefit from a custom implementation, it's possible
to write one but it will not be as convenient to use; this seems a reasonable tradeoff.
</p>


<h3 id="map_keys">
Why don't maps allow structs and arrays as keys?</h3>
<p>
Map lookup requires an equality operator, which structs and arrays do not implement.
They don't implement equality because equality is not well defined on such types;
there are multiple considerations involving shallow vs. deep comparison, pointer vs.
value comparison, how to deal with recursive structures, and so on.
We may revisit this issue&mdash;and implementing equality for structs and arrays
will not invalidate any existing programs&mdash;but without a clear idea of what
equality of structs and arrays should mean, it was simpler to leave it out for now.
</p>

<h3 id="references">
Why are maps, slices, and channels references while arrays are values?</h3>
<p>
There's a lot of history on that topic.  Early on, maps and channels
were syntactically pointers and it was impossible to declare or use a
non-pointer instance.  Also, we struggled with how arrays should work.
Eventually we decided that the strict separation of pointers and
values made the language harder to use.  Introducing reference types,
including slices to handle the reference form of arrays, resolved
these issues.  Reference types add some regrettable complexity to the
language but they have a large effect on usability: Go became a more
productive, comfortable language when they were introduced.
</p>

<h2 id="concurrency">Concurrency</h2>

<h3 id="csp">
Why build concurrency on the ideas of CSP?</h3>
<p>
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.
Higher-level interfaces enable much simpler code, even if there are still
mutexes and such under the covers.
</p>
<p>
One of the most successful models for providing high-level linguistic support
for concurrency comes from Hoare's Communicating Sequential Processes, or CSP.
Occam and Erlang are two well known languages that stem from CSP.
Go's concurrency primitives derive from a different part of the family tree
whose main contribution is the powerful notion of channels as first class objects.
</p>

<h3 id="goroutines">
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.
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.
</p>
<p>
To make the stacks small, Go's run-time uses segmented stacks.  A newly
minted goroutine is given a few kilobytes, which is almost always enough.
When it isn't, the run-time allocates (and frees) extension segments automatically.
The overhead averages about three cheap instructions per function call.
It is practical to create hundreds of thousands of goroutines in the same
address space.  If goroutines were just threads, system resources would
run out at a much smaller number.
</p>

<h3 id="atomic_maps">
Why are map operations not defined to be atomic?</h3>

<p>
After long discussion it was decided that the typical use of maps did not require
safe access from multiple threads, and in those cases where it did, the map was
probably part of some larger data structure or computation that was already
synchronized.  Therefore requiring that all map operations grab a mutex would slow
down most programs and add safety to few.  This was not an easy decision,
however, since it means uncontrolled map access can crash the program.
</p>

<p>
The language does not preclude atomic map updates.  When required, such
as when hosting an untrusted program, the implementation could interlock
map access.
</p>