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Move Types before Declarations and Scopes.

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This is the only change in this CL: only rearrangement, no content change, so subsequent edits will be easier to understand.

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Rob Pike 2009-02-23 19:26:07 -08:00
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@ -538,473 +538,6 @@ literal.
</p>
<hr/>
<h2>Declarations and Scope</h2>
<p>
A declaration binds an identifier to a language entity such as
a variable or function and specifies properties such as its type.
Every identifier in a program must be declared.
</p>
<pre class="grammar">
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl .
</pre>
<p>
The <i>scope</i> of an identifier is the extent of source text within which the
identifier denotes the bound entity. No identifier may be declared twice in a
single scope, but inner blocks can declare a new entity with the same
identifier, in which case the scope created by the outer declaration excludes
that created by the inner.
</p>
<p>
There are levels of scoping in effect before each source file is compiled.
In order from outermost to innermost:
</p>
<ol>
<li>The <i>universe</i> scope contains all predeclared identifiers.</li>
<li>An implicit scope contains only the package name.</li>
<li>The <i>package-level</i> scope surrounds all declarations at the
top level of the file, that is, outside the body of any
function or method. That scope is shared across all
source files within the package (§Packages), allowing
package-level identifiers to be shared between source
files.</li>
</ol>
<p>
The scope of an identifier depends on the entity declared:
</p>
<ol>
<li> The scope of predeclared identifiers is the universe scope.</li>
<li> The scope of an (identifier denoting a) type, function or package
extends from the point of the identifier in the declaration
to the end of the innermost surrounding block.</li>
<li> The scope of a constant or variable extends textually from
the end of the declaration to the end of the innermost
surrounding block. If the variable is declared in the
<i>init</i> statement of an <code>if </code>, <code>for</code>,
or <code>switch </code> statement, the
innermost surrounding block is the block associated
with that statement.</li>
<li> The scope of a parameter or result is the body of the
corresponding function.</li>
<li> The scope of a field or method is selectors for the
corresponding type containing the field or method (§Selectors).</li>
<li> The scope of a label is a unique scope emcompassing
the body of the innermost surrounding function, excluding
nested functions. Labels do not conflict with variables.</li>
</ol>
<h3>Predeclared identifiers</h3>
<p>
The following identifiers are implicitly declared in the outermost scope:
</p>
<pre class="grammar">
Basic types:
bool byte float32 float64 int8 int16 int32 int64 string uint8 uint16 uint32 uint64
Platform-specific convenience types:
float int uint uintptr
Constants:
true false iota nil
Functions:
cap convert len make new panic panicln print println typeof (TODO: typeof??)
Packages:
sys unsafe (TODO: does sys endure?)
</pre>
<h3>Exported identifiers</h3>
<p>
By default, identifiers are visible only within the package in which they are declared.
Some identifiers are <i>exported</i> and can be referenced using
<i>qualified identifiers</i> in other packages (§Qualified identifiers).
If an identifier satisfies these two conditions:
</p>
<ol>
<li>the first character of the identifier's name is a Unicode upper case letter;
<li>the identifier is declared at the package level or is a field or method of a type
declared at the top level;
</ol>
<p>
it will be exported automatically.
</p>
<h3>Const declarations</h3>
<p>
A constant declaration binds a list of identifiers (the names of
the constants) to the values of a list of constant expressions
(§Constant expressions). The number of identifiers must be equal
to the number of expressions, and the n<sup>th</sup> identifier on
the left is bound to value of the n<sup>th</sup> expression on the
right.
</p>
<pre class="grammar">
ConstDecl = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
ConstSpecList = ConstSpec { ";" ConstSpec } [ ";" ] .
ConstSpec = IdentifierList [ CompleteType ] [ "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
CompleteType = Type .
</pre>
<p>
If the type (CompleteType) is omitted, the constants take the
individual types of the corresponding expressions, which may be
``ideal integer'' or ``ideal float'' (§Ideal number). If the type
is present, all constants take the type specified, and the types
of all the expressions must be assignment-compatible
with that type.
</p>
<pre>
const Pi float64 = 3.14159265358979323846
const E = 2.718281828
const (
size int64 = 1024;
eof = -1;
)
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
const u, v float = 0, 3 // u = 0.0, v = 3.0
</pre>
<p>
Within a parenthesized <code>const</code> declaration list the
expression list may be omitted from any but the first declaration.
Such an empty list is equivalent to the textual substitution of the
first preceding non-empty expression list. Omitting the list of
expressions is therefore equivalent to repeating the previous list.
The number of identifiers must be equal to the number of expressions
in the previous list. Together with the <code>iota</code> constant generator
(§Iota) this mechanism permits light-weight declaration of sequential values:
</p>
<pre>
const (
Sunday = iota;
Monday;
Tuesday;
Wednesday;
Thursday;
Friday;
Partyday;
numberOfDays; // this constant is not exported
)
</pre>
<h3>Iota</h3>
<p>
Within a constant declaration, the predeclared pseudo-constant
<code>iota</code> represents successive integers. It is reset to 0
whenever the reserved word <code>const</code> appears in the source
and increments with each semicolon. It can be used to construct a
set of related constants:
</p>
<pre>
const ( // iota is reset to 0
c0 = iota; // c0 == 0
c1 = iota; // c1 == 1
c2 = iota // c2 == 2
)
const (
a = 1 << iota; // a == 1 (iota has been reset)
b = 1 << iota; // b == 2
c = 1 << iota; // c == 4
)
const (
u = iota * 42; // u == 0 (ideal integer)
v float = iota * 42; // v == 42.0 (float)
w = iota * 42; // w == 84 (ideal integer)
)
const x = iota; // x == 0 (iota has been reset)
const y = iota; // y == 0 (iota has been reset)
</pre>
<p>
Within an ExpressionList, the value of each <code>iota</code> is the same because
it is only incremented at a semicolon:
</p>
<pre>
const (
bit0, mask0 = 1 << iota, 1 << iota - 1; // bit0 == 1, mask0 == 0
bit1, mask1; // bit1 == 2, mask1 == 1
bit2, mask2; // bit2 == 4, mask2 == 3
)
</pre>
<p>
This last example exploits the implicit repetition of the
last non-empty expression list.
</p>
<h3>Type declarations</h3>
<p>
A type declaration binds an identifier, the <i>type name</i>,
to a new type. <font color=red>TODO: what exactly is a "new type"?</font>
</p>
<pre class="grammar">
TypeDecl = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
TypeSpec = identifier Type .
</pre>
<pre>
type IntArray [16] int
type (
Point struct { x, y float };
Polar Point
)
type TreeNode struct {
left, right *TreeNode;
value Point;
}
type Comparable interface {
cmp(Comparable) int
}
</pre>
<h3>Variable declarations</h3>
<p>
A variable declaration creates a variable, binds an identifier to it and
gives it a type and optionally an initial value.
The variable type must be a complete type (§Types).
</p>
<pre class="grammar">
VarDecl = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
</pre>
<pre>
var i int
var U, V, W float
var k = 0
var x, y float = -1.0, -2.0
var (
i int;
u, v, s = 2.0, 3.0, "bar"
)
</pre>
<p>
If there are expressions, their number must be equal
to the number of identifiers, and the n<sup>th</sup> variable
is initialized to the value of the n<sup>th</sup> expression.
Otherwise, each variable is initialized to the <i>zero</i>
of the type (§Program initialization and execution).
The expressions can be general expressions; they need not be constants.
</p>
<p>
Either the type or the expression list must be present. If the
type is present, it sets the type of each variable and the expressions
(if any) must be assignment-compatible to that type. If the type
is absent, the variables take the types of the corresponding
expressions.
</p>
<p>
If the type is absent and the corresponding expression is a constant
expression of ideal integer or ideal float type, the type of the
declared variable is <code>int</code> or <code>float</code>
respectively:
</p>
<pre>
var i = 0 // i has type int
var f = 3.1415 // f has type float
</pre>
<h3>Short variable declarations</h3>
A <i>short variable declaration</i> uses the syntax
<pre class="grammar">
SimpleVarDecl = IdentifierList ":=" ExpressionList .
</pre>
and is shorthand for the declaration syntax
<pre class="grammar">
"var" IdentifierList = ExpressionList .
</pre>
<pre>
i, j := 0, 10;
f := func() int { return 7; }
ch := new(chan int);
</pre>
<p>
Unlike regular variable declarations, short variable declarations
can be used, by analogy with tuple assignment (§Assignments), to
receive the individual elements of a multi-valued expression such
as a call to a multi-valued function. In this form, the ExpressionLIst
must be a single such multi-valued expression, the number of
identifiers must equal the number of values, and the declared
variables will be assigned the corresponding values.
</p>
<pre>
count, error := os.Close(fd); // os.Close() returns two values
</pre>
<p>
Short variable declarations may appear only inside functions.
In some contexts such as the initializers for <code>if</code>,
<code>for</code>, or <code>switch</code> statements,
they can be used to declare local temporary variables (§Statements).
</p>
<h3>Function declarations</h3>
<p>
A function declaration binds an identifier to a function (§Function types).
</p>
<pre class="grammar">
FunctionDecl = "func" identifier Signature [ Block ] .
</pre>
<pre>
func min(x int, y int) int {
if x &lt; y {
return x;
}
return y;
}
</pre>
<p>
A function must be declared or forward-declared before it can be invoked (§Forward declarations).
Implementation restriction: Functions can only be declared at the package level.
</p>
<h3>Method declarations</h3>
<p>
A method declaration binds an identifier to a method,
which is a function with a <i>receiver</i>.
</p>
<pre class="grammar">
MethodDecl = "func" Receiver identifier Signature [ Block ] .
Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .
</pre>
<p>
The receiver type must be a type name or a pointer to a type name,
and that name is called the <i>receiver base type</i> or just <i>base type</i>.
The base type must not be a pointer type and must be
declared in the same source file as the method.
The method is said to be <i>bound</i> to the base type
and is visible only within selectors for that type
(§Type declarations, §Selectors).
</p>
<p>
All methods bound to a base type must have the same receiver type,
either all pointers to the base type or all the base type itself.
Given type <code>Point</code>, the declarations
</p>
<pre>
func (p *Point) Length() float {
return Math.sqrt(p.x * p.x + p.y * p.y);
}
func (p *Point) Scale(factor float) {
p.x = p.x * factor;
p.y = p.y * factor;
}
</pre>
<p>
bind the methods <code>Length</code> and <code>Scale</code>
to the base type <code>Point</code>.
</p>
<p>
If the
receiver's value is not referenced inside the the body of the method,
its identifier may be omitted in the declaration. The same applies in
general to parameters of functions and methods.
</p>
<p>
Methods can be declared
only after their base type is declared or forward-declared, and invoked
only after their own declaration or forward-declaration (§Forward declarations).
Implementation restriction: They can only be declared at package level.
</p>
<h3>Forward declarations</h3>
<p>
Mutually-recursive types struct or interface types require that one be
<i>forward declared</i> so that it may be named in the other.
A forward declaration of a type omits the block containing the fields
or methods of the type.
</p>
<pre>
type List struct // forward declaration of List
type Item struct {
value int;
next *List;
}
type List struct {
head, tail *Item
}
</pre>
<p>
A forward-declared type is incomplete (§Types)
until it is fully declared. The full declaration must follow
before the end of the block containing the forward declaration.
</p>
<p>
Functions and methods may similarly be forward-declared by omitting their body.
</p>
<pre>
func F(a int) int // forward declaration of F
func G(a, b int) int {
return F(a) + F(b)
}
func F(a int) int {
if a <= 0 { return 0 }
return G(a-1, b+1)
}
</pre>
<hr/>
<h2>Types</h2>
A type specifies the set of values that variables of that type may assume
@ -1803,6 +1336,474 @@ different declarations.
<hr/>
<h2>Declarations and Scope</h2>
<p>
A declaration binds an identifier to a language entity such as
a variable or function and specifies properties such as its type.
Every identifier in a program must be declared.
</p>
<pre class="grammar">
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl .
</pre>
<p>
The <i>scope</i> of an identifier is the extent of source text within which the
identifier denotes the bound entity. No identifier may be declared twice in a
single scope, but inner blocks can declare a new entity with the same
identifier, in which case the scope created by the outer declaration excludes
that created by the inner.
</p>
<p>
There are levels of scoping in effect before each source file is compiled.
In order from outermost to innermost:
</p>
<ol>
<li>The <i>universe</i> scope contains all predeclared identifiers.</li>
<li>An implicit scope contains only the package name.</li>
<li>The <i>package-level</i> scope surrounds all declarations at the
top level of the file, that is, outside the body of any
function or method. That scope is shared across all
source files within the package (§Packages), allowing
package-level identifiers to be shared between source
files.</li>
</ol>
<p>
The scope of an identifier depends on the entity declared:
</p>
<ol>
<li> The scope of predeclared identifiers is the universe scope.</li>
<li> The scope of an (identifier denoting a) type, function or package
extends from the point of the identifier in the declaration
to the end of the innermost surrounding block.</li>
<li> The scope of a constant or variable extends textually from
the end of the declaration to the end of the innermost
surrounding block. If the variable is declared in the
<i>init</i> statement of an <code>if </code>, <code>for</code>,
or <code>switch </code> statement, the
innermost surrounding block is the block associated
with that statement.</li>
<li> The scope of a parameter or result is the body of the
corresponding function.</li>
<li> The scope of a field or method is selectors for the
corresponding type containing the field or method (§Selectors).</li>
<li> The scope of a label is a unique scope emcompassing
the body of the innermost surrounding function, excluding
nested functions. Labels do not conflict with variables.</li>
</ol>
<h3>Predeclared identifiers</h3>
<p>
The following identifiers are implicitly declared in the outermost scope:
</p>
<pre class="grammar">
Basic types:
bool byte float32 float64 int8 int16 int32 int64 string uint8 uint16 uint32 uint64
Platform-specific convenience types:
float int uint uintptr
Constants:
true false iota nil
Functions:
cap convert len make new panic panicln print println typeof (TODO: typeof??)
Packages:
sys unsafe (TODO: does sys endure?)
</pre>
<h3>Exported identifiers</h3>
<p>
By default, identifiers are visible only within the package in which they are declared.
Some identifiers are <i>exported</i> and can be referenced using
<i>qualified identifiers</i> in other packages (§Qualified identifiers).
If an identifier satisfies these two conditions:
</p>
<ol>
<li>the first character of the identifier's name is a Unicode upper case letter;
<li>the identifier is declared at the package level or is a field or method of a type
declared at the top level;
</ol>
<p>
it will be exported automatically.
</p>
<h3>Const declarations</h3>
<p>
A constant declaration binds a list of identifiers (the names of
the constants) to the values of a list of constant expressions
(§Constant expressions). The number of identifiers must be equal
to the number of expressions, and the n<sup>th</sup> identifier on
the left is bound to value of the n<sup>th</sup> expression on the
right.
</p>
<pre class="grammar">
ConstDecl = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
ConstSpecList = ConstSpec { ";" ConstSpec } [ ";" ] .
ConstSpec = IdentifierList [ CompleteType ] [ "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
CompleteType = Type .
</pre>
<p>
If the type (CompleteType) is omitted, the constants take the
individual types of the corresponding expressions, which may be
``ideal integer'' or ``ideal float'' (§Ideal number). If the type
is present, all constants take the type specified, and the types
of all the expressions must be assignment-compatible
with that type.
</p>
<pre>
const Pi float64 = 3.14159265358979323846
const E = 2.718281828
const (
size int64 = 1024;
eof = -1;
)
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
const u, v float = 0, 3 // u = 0.0, v = 3.0
</pre>
<p>
Within a parenthesized <code>const</code> declaration list the
expression list may be omitted from any but the first declaration.
Such an empty list is equivalent to the textual substitution of the
first preceding non-empty expression list. Omitting the list of
expressions is therefore equivalent to repeating the previous list.
The number of identifiers must be equal to the number of expressions
in the previous list. Together with the <code>iota</code> constant generator
(§Iota) this mechanism permits light-weight declaration of sequential values:
</p>
<pre>
const (
Sunday = iota;
Monday;
Tuesday;
Wednesday;
Thursday;
Friday;
Partyday;
numberOfDays; // this constant is not exported
)
</pre>
<h3>Iota</h3>
<p>
Within a constant declaration, the predeclared pseudo-constant
<code>iota</code> represents successive integers. It is reset to 0
whenever the reserved word <code>const</code> appears in the source
and increments with each semicolon. It can be used to construct a
set of related constants:
</p>
<pre>
const ( // iota is reset to 0
c0 = iota; // c0 == 0
c1 = iota; // c1 == 1
c2 = iota // c2 == 2
)
const (
a = 1 << iota; // a == 1 (iota has been reset)
b = 1 << iota; // b == 2
c = 1 << iota; // c == 4
)
const (
u = iota * 42; // u == 0 (ideal integer)
v float = iota * 42; // v == 42.0 (float)
w = iota * 42; // w == 84 (ideal integer)
)
const x = iota; // x == 0 (iota has been reset)
const y = iota; // y == 0 (iota has been reset)
</pre>
<p>
Within an ExpressionList, the value of each <code>iota</code> is the same because
it is only incremented at a semicolon:
</p>
<pre>
const (
bit0, mask0 = 1 << iota, 1 << iota - 1; // bit0 == 1, mask0 == 0
bit1, mask1; // bit1 == 2, mask1 == 1
bit2, mask2; // bit2 == 4, mask2 == 3
)
</pre>
<p>
This last example exploits the implicit repetition of the
last non-empty expression list.
</p>
<h3>Type declarations</h3>
<p>
A type declaration binds an identifier, the <i>type name</i>,
to a new type. <font color=red>TODO: what exactly is a "new type"?</font>
</p>
<pre class="grammar">
TypeDecl = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
TypeSpec = identifier Type .
</pre>
<pre>
type IntArray [16] int
type (
Point struct { x, y float };
Polar Point
)
type TreeNode struct {
left, right *TreeNode;
value Point;
}
type Comparable interface {
cmp(Comparable) int
}
</pre>
<h3>Variable declarations</h3>
<p>
A variable declaration creates a variable, binds an identifier to it and
gives it a type and optionally an initial value.
The variable type must be a complete type (§Types).
</p>
<pre class="grammar">
VarDecl = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
</pre>
<pre>
var i int
var U, V, W float
var k = 0
var x, y float = -1.0, -2.0
var (
i int;
u, v, s = 2.0, 3.0, "bar"
)
</pre>
<p>
If there are expressions, their number must be equal
to the number of identifiers, and the n<sup>th</sup> variable
is initialized to the value of the n<sup>th</sup> expression.
Otherwise, each variable is initialized to the <i>zero</i>
of the type (§Program initialization and execution).
The expressions can be general expressions; they need not be constants.
</p>
<p>
Either the type or the expression list must be present. If the
type is present, it sets the type of each variable and the expressions
(if any) must be assignment-compatible to that type. If the type
is absent, the variables take the types of the corresponding
expressions.
</p>
<p>
If the type is absent and the corresponding expression is a constant
expression of ideal integer or ideal float type, the type of the
declared variable is <code>int</code> or <code>float</code>
respectively:
</p>
<pre>
var i = 0 // i has type int
var f = 3.1415 // f has type float
</pre>
<h3>Short variable declarations</h3>
A <i>short variable declaration</i> uses the syntax
<pre class="grammar">
SimpleVarDecl = IdentifierList ":=" ExpressionList .
</pre>
and is shorthand for the declaration syntax
<pre class="grammar">
"var" IdentifierList = ExpressionList .
</pre>
<pre>
i, j := 0, 10;
f := func() int { return 7; }
ch := new(chan int);
</pre>
<p>
Unlike regular variable declarations, short variable declarations
can be used, by analogy with tuple assignment (§Assignments), to
receive the individual elements of a multi-valued expression such
as a call to a multi-valued function. In this form, the ExpressionLIst
must be a single such multi-valued expression, the number of
identifiers must equal the number of values, and the declared
variables will be assigned the corresponding values.
</p>
<pre>
count, error := os.Close(fd); // os.Close() returns two values
</pre>
<p>
Short variable declarations may appear only inside functions.
In some contexts such as the initializers for <code>if</code>,
<code>for</code>, or <code>switch</code> statements,
they can be used to declare local temporary variables (§Statements).
</p>
<h3>Function declarations</h3>
<p>
A function declaration binds an identifier to a function (§Function types).
</p>
<pre class="grammar">
FunctionDecl = "func" identifier Signature [ Block ] .
</pre>
<pre>
func min(x int, y int) int {
if x &lt; y {
return x;
}
return y;
}
</pre>
<p>
A function must be declared or forward-declared before it can be invoked (§Forward declarations).
Implementation restriction: Functions can only be declared at the package level.
</p>
<h3>Method declarations</h3>
<p>
A method declaration binds an identifier to a method,
which is a function with a <i>receiver</i>.
</p>
<pre class="grammar">
MethodDecl = "func" Receiver identifier Signature [ Block ] .
Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .
</pre>
<p>
The receiver type must be a type name or a pointer to a type name,
and that name is called the <i>receiver base type</i> or just <i>base type</i>.
The base type must not be a pointer type and must be
declared in the same source file as the method.
The method is said to be <i>bound</i> to the base type
and is visible only within selectors for that type
(§Type declarations, §Selectors).
</p>
<p>
All methods bound to a base type must have the same receiver type,
either all pointers to the base type or all the base type itself.
Given type <code>Point</code>, the declarations
</p>
<pre>
func (p *Point) Length() float {
return Math.sqrt(p.x * p.x + p.y * p.y);
}
func (p *Point) Scale(factor float) {
p.x = p.x * factor;
p.y = p.y * factor;
}
</pre>
<p>
bind the methods <code>Length</code> and <code>Scale</code>
to the base type <code>Point</code>.
</p>
<p>
If the
receiver's value is not referenced inside the the body of the method,
its identifier may be omitted in the declaration. The same applies in
general to parameters of functions and methods.
</p>
<p>
Methods can be declared
only after their base type is declared or forward-declared, and invoked
only after their own declaration or forward-declaration (§Forward declarations).
Implementation restriction: They can only be declared at package level.
</p>
<h3>Forward declarations</h3>
<p>
Mutually-recursive types struct or interface types require that one be
<i>forward declared</i> so that it may be named in the other.
A forward declaration of a type omits the block containing the fields
or methods of the type.
</p>
<pre>
type List struct // forward declaration of List
type Item struct {
value int;
next *List;
}
type List struct {
head, tail *Item
}
</pre>
<p>
A forward-declared type is incomplete (§Types)
until it is fully declared. The full declaration must follow
before the end of the block containing the forward declaration.
</p>
<p>
Functions and methods may similarly be forward-declared by omitting their body.
</p>
<pre>
func F(a int) int // forward declaration of F
func G(a, b int) int {
return F(a) + F(b)
}
func F(a int) int {
if a <= 0 { return 0 }
return G(a-1, b+1)
}
</pre>
<hr/>
<h2>Expressions</h2>
An expression specifies the computation of a value via the application of