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type switches

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Rob Pike 2009-10-12 21:18:23 -07:00
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doc/effective_go.html
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@ -628,6 +628,28 @@ func Compare(a, b []byte) int {
}
</pre>
<p>
A switch can also be used to discover the dynamic type of an interface
variable. Such a <em>type switch</em> uses the syntax of a type
assertion with the keyword <code>type</code> inside the parentheses.
If the switch declares a variable in the expression, the variable will
have the corresponding type in each clause.
</p>
<pre>
switch t := interfaceValue.(type) {
default:
fmt.Printf("unexpected type");
case bool:
fmt.Printf("boolean %t\n", t);
case int:
fmt.Printf("integer %d\n", t);
case *bool:
fmt.Printf("pointer to boolean %t\n", *t);
case *int:
fmt.Printf("pointer to integer %d\n", *t);
}
</pre>
<h2 id="functions">Functions</h2>
<h3 id="multiple-returns">Multiple return values</h3>
@ -1350,101 +1372,9 @@ By the way, the idea of using <code>Write</code> on a slice of bytes
is implemented by <code>bytes.Buffer</code>.
</p>
<h2>More to come</h2>
<h2>Interfaces</h2>
<!---
<h2 id="idioms">Idioms</h2>
<h3 id="buffer-slice">Use parallel assignment to slice a buffer</h3>
<pre>
header, body, checksum := buf[0:20], buf[20:n-4], buf[n-4:n];
</pre>
<h2 id="errors">Errors</h2>
<h3 id="error-returns">Return <code>os.Error</code>, not <code>bool</code></h3>
<p>
Especially in libraries, functions tend to have multiple error modes.
Instead of returning a boolean to signal success,
return an <code>os.Error</code> that describes the failure.
Even if there is only one failure mode now,
there may be more later.
</p>
<h3 id="error-context">Return structured errors</h3>
Implementations of <code>os.Error</code> should
describe the error and provide context.
For example, <code>os.Open</code> returns an <code>os.PathError</code>:
<a href="http://go/godoc/src/pkg/os/file.go">http://go/godoc/src/pkg/os/file.go</a>:
<pre>
// PathError records an error and the operation and
// file path that caused it.
type PathError struct {
Op string;
Path string;
Error Error;
}
func (e *PathError) String() string {
return e.Op + &quot; &quot; + e.Path + &quot;: &quot; + e.Error.String();
}
</pre>
<p>
<code>PathError</code>'s <code>String</code> formats
the error nicely, including the operation and file name
tha failed; just printing the error generates a
message, such as
</p>
<pre>
open /etc/passwx: no such file or directory
</pre>
<p>
that is useful even if printed far from the call that
triggered it.
</p>
<p>
Callers that care about the precise error details can
use a type switch or a type guard to look for specific
errors and extract details. For <code>PathErrors</code>
this might include examining the internal <code>Error</code>
to see if it is <code>os.EPERM</code> or <code>os.ENOENT</code>,
for instance.
</p>
<h2 id="types">Programmer-defined types</h2>
<p>Packages that export only a single type can
shorten <code>NewTypeName</code> to <code>New</code>;
the vector constructor is
<code>vector.New</code>, not <code>vector.NewVector</code>.
</p>
<p>
A type that is intended to be allocated
as part of a larger struct may have an <code>Init</code> method
that must be called explicitly.
Conventionally, the <code>Init</code> method returns
the object being initialized, to make the constructor trivial:
</p>
<a href="xxx">go/src/pkg/container/vector/vector.go</a>:
<pre>
func New(len int) *Vector {
return new(Vector).Init(len)
}
</pre>
<h2 id="interfaces">Interfaces</h2>
<h3 id="accept-interface-values">Accept interface values</h3>
buffered i/o takes a Reader, not an os.File. XXX
@ -1473,6 +1403,116 @@ XXX
<h3 id="fdsa">Use anonymous fields to incorporate an implementation</h3>
XXX
--->
<h2 id="errors">Errors</h2>
<p>
Library routines must often return some sort of error indication to
the caller. As mentioned earlier, Go's multivalue return makes it
easy to return a detailed error description alongside the normal
return value. By convention, errors have type <code>os.Error</code>,
a simple interface.
</p>
<pre>
type Error interface {
String() string;
}
</pre>
<p>
A library writer is free to implement this interface with a
richer model under the covers, making it possible not only
to see the error but also to provide some context.
For example, <code>os.Open</code> returns an <code>os.PathError</code>.
</p>
<pre>
// PathError records an error and the operation and
// file path that caused it.
type PathError struct {
Op string; // "open", "unlink", etc.
Path string; // The associated file.
Error Error; // Returned by the system call.
}
func (e *PathError) String() string {
return e.Op + " " + e.Path + ": " + e.Error.String();
}
</pre>
<p>
<code>PathError</code>'s <code>String</code> generates
a string like this:
</p>
<pre>
open /etc/passwx: no such file or directory
</pre>
<p>
Such an error, which includes the problematic file name, the
operation, and the operating system error it triggered, is useful even
if printed far from the call that caused it;
it is much more informative than the plain
"no such file or directory".
</p>
<p>
Callers that care about the precise error details can
use a type switch or a type assertion to look for specific
errors and extract details. For <code>PathErrors</code>
this might include examining the internal <code>Error</code>
field for recoverable failures.
</p>
<pre>
for try := 0; try < 2; try++ {
file, err := os.Open(filename, os.O_RDONLY, 0);
if err == nil {
return
}
if e, ok := err.(*os.PathError); ok &amp;&amp; e.Error == os.ENOSPC {
deleteTempFiles(); // Recover some space.
continue
}
return
}
</pre>
<h2>Testing</h2>
<h2>More to come</h2>
<!---
<h2 id="idioms">Idioms</h2>
<h3 id="buffer-slice">Use parallel assignment to slice a buffer</h3>
<pre>
header, body, checksum := buf[0:20], buf[20:n-4], buf[n-4:n];
</pre>
<h2 id="types">Programmer-defined types</h2>
<p>Packages that export only a single type can
shorten <code>NewTypeName</code> to <code>New</code>;
the vector constructor is
<code>vector.New</code>, not <code>vector.NewVector</code>.
</p>
<p>
A type that is intended to be allocated
as part of a larger struct may have an <code>Init</code> method
that must be called explicitly.
Conventionally, the <code>Init</code> method returns
the object being initialized, to make the constructor trivial:
</p>
<a href="xxx">go/src/pkg/container/vector/vector.go</a>:
<pre>
func New(len int) *Vector {
return new(Vector).Init(len)
}
</pre>
<h2>Data-Driven Programming</h2>