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spec: add new language for alias declarations

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For #16339.

Change-Id: I7d912ea634bbfacfc0217f97dccb270fde06f16b
Reviewed-on: https://go-review.googlesource.com/30601
Reviewed-by: Russ Cox <rsc@golang.org>
This commit is contained in:
Robert Griesemer 2016-10-04 10:06:14 -07:00
parent 09692182fa
commit aff37662d1
1 changed files with 69 additions and 6 deletions
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75
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of October 19, 2016",
"Subtitle": "Version of October 27, 2016",
"Path": "/ref/spec"
}-->
@ -1726,8 +1726,12 @@ the left is bound to the value of the <i>n</i>th expression on the
right.
</p>
<p>
For constant <i>aliases</i>, see the section on <a href="#Alias_declarations">alias declarations</a>.
</p>
<pre class="ebnf">
ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstDecl = "const" ( ConstSpec | AliasSpec | "(" { ( ConstSpec | AliasSpec ) ";" } ")" ) .
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
@ -1846,8 +1850,12 @@ and operations defined for the existing type are also defined for the new type.
The new type is <a href="#Type_identity">different</a> from the existing type.
</p>
<p>
For type <i>aliases</i>, see the section on <a href="#Alias_declarations">alias declarations</a>.
</p>
<pre class="ebnf">
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeDecl = "type" ( TypeSpec | AliasSpec | "(" { ( TypeSpec | AliasSpec ) ";" } ")" ) .
TypeSpec = identifier Type .
</pre>
@ -1928,8 +1936,12 @@ A variable declaration creates one or more variables, binds corresponding
identifiers to them, and gives each a type and an initial value.
</p>
<p>
For variable <i>aliases</i>, see the section on <a href="#Alias_declarations">alias declarations</a>.
</p>
<pre class="ebnf">
VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarDecl = "var" ( VarSpec | AliasSpec | "(" { ( VarSpec | AliasSpec ) ";" } ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
</pre>
@ -1976,6 +1988,7 @@ inside a <a href="#Function_declarations">function body</a> if the variable is
never used.
</p>
<h3 id="Short_variable_declarations">Short variable declarations</h3>
<p>
@ -2006,7 +2019,7 @@ _, y, _ := coord(p) // coord() returns three values; only interested in y coord
<p>
Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
variables provided they were originally declared earlier in the same block
(or the parameter lists if the block is the function body) with the same type,
(or the parameter lists if the block is the function body) with the same type,
and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
As a consequence, redeclaration can only appear in a multi-variable short declaration.
Redeclaration does not introduce a new variable; it just assigns a new value to the original.
@ -2034,8 +2047,12 @@ A function declaration binds an identifier, the <i>function name</i>,
to a function.
</p>
<p>
For function <i>aliases</i>, see the section on <a href="#Alias_declarations">alias declarations</a>.
</p>
<pre class="ebnf">
FunctionDecl = "func" FunctionName ( Function | Signature ) .
FunctionDecl = "func" ( FunctionName ( Function | Signature ) ) | AliasSpec .
FunctionName = identifier .
Function = Signature FunctionBody .
FunctionBody = Block .
@ -2148,6 +2165,52 @@ However, a function declared this way is not a method.
</p>
<h3 id="Alias_declarations">Alias declarations</h3>
<p>
An alias declaration binds an identifier, the <i>alias</i>, to a
<a href="#Constant_declarations">constant</a>,
<a href="#Type_declarations">type</a>,
<a href="#Variable_declarations">variable</a>, or
<a href="#Function_declarations">function</a>
denoted by a <a href="#Qualified_identifiers">qualified identifier</a> and
declared in a different package.
</p>
<pre class="ebnf">
AliasSpec = identifier "=&gt;" QualifiedIdent .
</pre>
<p>
The effect of referring to a constant, type, variable, or function by an alias
is indistinguishable from referring to it by its original name.
For example, the type denoted by a type alias and the aliased type are
<a href="#Type_identity">identical</a>.
</p>
<p>
An alias declaration may appear only as a form of constant, type, variable,
or function declaration at the package level, and the aliased entity must be
a constant, type, variable, or function respectively. Alias declarations inside
functions are not permitted.
</p>
<pre>
const (
G = 6.67408e-11 // regular and alias declarations may be grouped
Pi =&gt; math.Pi // same effect as: Pi = math.Pi
)
type Struct =&gt; types.Struct // re-export of types.Struct
func sin =&gt; math.Sin // non-exported shortcut for frequently used function
</pre>
<p>
An alias declaration may not refer to package <a href="#Package_unsafe">unsafe</a>.
</p>
<h2 id="Expressions">Expressions</h2>
<p>