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spec: add type parameter types

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- add section on type parameters
- added two sections on the scope of type parameters
- expanded general section on types accordingly
- introduced the notion of a named type which will
  help in simplifying various rules (subsequent CLs)

Change-Id: I49c1ed7d6d4f951d751f0a3ca5dfb637e49829f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/365414
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2021-11-18 15:33:19 -08:00
parent 0c3b4a358a
commit a287c4aa38
1 changed files with 53 additions and 9 deletions
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62
doc/go_spec.html
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@ -787,32 +787,46 @@ If a variable has not yet been assigned a value, its value is the
<p>
A type determines a set of values together with operations and methods specific
to those values. A type may be denoted by a <i>type name</i>, if it has one,
or specified using a <i>type literal</i>, which composes a type from existing types.
to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
followed by <a href="#Instantiations">type arguments</a> if the type is parameterized.
A type may also be specified using a <i>type literal</i>, which composes a type
from existing types.
</p>
<pre class="ebnf">
Type = TypeName | TypeLit | "(" Type ")" .
Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
TypeName = identifier | QualifiedIdent .
TypeArgs = "[" TypeList [ "," ] "]" .
TypeList = Type { "," Type } .
TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
SliceType | MapType | ChannelType .
</pre>
<p>
The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
Others are introduced with <a href="#Type_declarations">type declarations</a>.
Others are introduced with <a href="#Type_declarations">type declarations</a>
or <a href="#Type_parameter_lists">type parameter lists</a>.
<i>Composite types</i>&mdash;array, struct, pointer, function,
interface, slice, map, and channel types&mdash;may be constructed using
type literals.
</p>
<p>
Predeclared types, defined types, and type parameters are called <i>named types</i>.
An alias denotes a named type if the type given in the alias declaration is a named type.
</p>
<h3 id="Underlying_types">Underlying types</h3>
<p>
Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
is one of the predeclared boolean, numeric, or string types, or a type literal,
the corresponding underlying
type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type
is the underlying type of the type to which <code>T</code> refers in its
<a href="#Type_declarations">type declaration</a>.
the corresponding underlying type is <code>T</code> itself.
Otherwise, <code>T</code>'s underlying type is the underlying type of the
type to which <code>T</code> refers in its <a href="#Type_declarations">type
declaration</a>. Accordingly, the underlying type of a type parameter is the
underlying type of its <a href="#Type_constraints">type constraint</a>, which
is always an interface.
</p>
<pre>
@ -827,12 +841,15 @@ type (
B3 []B1
B4 B3
)
func f[P any](x P) { … }
</pre>
<p>
The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
and <code>B2</code> is <code>string</code>.
The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
The underlying type of <code>P</code> is <code>interface{}</code>.
</p>
<h3 id="Method_sets">Method sets</h3>
@ -1706,6 +1723,25 @@ and a second goroutine receives them, the values are
received in the order sent.
</p>
<h3 id="Type_parameters">Type parameters</h3>
<p>
A <i>type parameter</i> is an (unqualified) type name declared in the
<a href="#Type_parameter_lists">type parameter list</a> of a
<a href="#Function_declarations">function declaration</a> or
<a href="#Type_definitions">type definition</a>; or in the receiver specification
of a <a href="#Method_declarations">method declaration</a> that is associated
with a parameterized type.
A type parameter acts as a place holder for an (as of yet) unknown type in the declaration;
the type parameter is replaced with a <i>type argument</i> upon
<a href="#Instantiations">instantiation</a> of the parameterized function or type.
</p>
<p>
The properties of a type parameter are determined by its
<a href="#Type_constraints">type constraint</a>.
</p>
<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
<h3 id="Type_identity">Type identity</h3>
@ -1983,6 +2019,15 @@ Go is lexically scoped using <a href="#Blocks">blocks</a>:
<li>The scope of an identifier denoting a method receiver, function parameter,
or result variable is the function body.</li>
<li>The scope of an identifier denoting a type parameter of a type-parameterized function
or declared by a method receiver is the function body and all parameter lists of the
function.
</li>
<li>The scope of an identifier denoting a type parameter of a parameterized type
begins after the name of the parameterized type and ends at the end
of the TypeSpec.</li>
<li>The scope of a constant or variable identifier declared
inside a function begins at the end of the ConstSpec or VarSpec
(ShortVarDecl for short variable declarations)
@ -5384,7 +5429,6 @@ TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClau
TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
TypeCaseClause = TypeSwitchCase ":" StatementList .
TypeSwitchCase = "case" TypeList | "default" .
TypeList = Type { "," Type } .
</pre>
<p>