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FAQ: variant types, unions

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Fixes #1935.

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/4850044
This commit is contained in:
Rob Pike 2011-08-06 11:21:59 +10:00
parent 93c4a246a4
commit 7d87f3d233
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doc/go_faq.html
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@ -695,6 +695,42 @@ for i, v := range t {
}
</pre>
<h3 id="unions">
Why are there no untagged unions, as in C?</h3>
<p>
Untagged unions would violate Go's memory safety
guarantees.
</p>
<h3 id="variant_types">
Why does Go not have variant types?</h3>
<p>
Variant types, also known as algebraic types, provide a way to specify
that a value might take one of a set of other types, but only those
types. A common example in systems programming would specify that an
error is, say, a network error, a security error or an application
error and allow the caller to discriminate the source of the problem
by examining the type of the error. Another example is a syntax tree
in which each node can be a different type: declaration, statement,
assignment and so on.
</p>
<p>
We considered adding variant types to Go, but after discussion
decided to leave them out because they overlap in confusing ways
with interfaces. What would happen if the elements of a variant type
were themselves interfaces?
</p>
<p>
Also, some of what variant types address is already covered by the
language. The error example is easy to express using an interface
value to hold the error and a type switch to discriminate cases. The
syntax tree example is also doable, although not as elegantly.
</p>
<h2 id="values">Values</h2>
<h3 id="conversions">
@ -1212,6 +1248,60 @@ that includes more powerful run-time support. We believe that with some effort
the size of Go binaries can be reduced.
</p>
<h3 id="unused_variables_and_imports">
Can I stop these complaints about my unused variable/import?</h3>
<p>
The presence of an unused variable may indicate a bug, while
unused imports just slow down compilation.
Accumulate enough unused imports in your code tree and
things can get very slow.
For these reasons, Go allows neither.
</p>
<p>
When developing code, it's common to create these situations
temporarily and it can be annoying to have to edit them out before the
program will compile.
</p>
<p>
Some have asked for a compiler option to turn those checks off
or at least reduce them to warnings.
Such an option has not been added, though,
because compiler options should not affect the semantics of the
language and because the Go compiler does not report warnings, only
errors that prevent compilation.
</p>
<p>
There are two reasons for having no warnings. First, if it's worth
complaining about, it's worth fixing in the code. (And if it's not
worth fixing, it's not worth mentioning.) Second, having the compiler
generate warnings encourages the implementation to warn about weak
cases that can make compilation noisy, masking real errors that
<em>should</em> be fixed.
</p>
<p>
It's easy to address the situation, though. Use the blank identifier
to let unused things persist while you're developing.
</p>
<pre>
import "unused"
// This declaration marks the import as used by referencing an
// item from the package.
var _ = unused.Item // TODO: Delete before committing!
func main() {
debugData := debug.Profile()
_ = debugData // Used only during debugging.
....
}
</pre>
<h2 id="Performance">Performance</h2>
<h3 id="Why_does_Go_perform_badly_on_benchmark_x">