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spec: various clarifications/fixes for method sets and interfaces
- fixed a typo in the method set section - express in the syntax that ~T denotes an underlying type - be more precise when talking about types vs type terms - refer to "unions" rather than "union expressions" - make it clear in the spec title that this is WIP Change-Id: I9b2c4b1f77bc50dd574ed6893bedd40529c320fc Reviewed-on: https://go-review.googlesource.com/c/go/+/365154 Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -1,16 +1,14 @@
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<!--{
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"Title": "The Go Programming Language Specification",
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"Subtitle": "Version of Nov 17, 2021",
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"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
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"Subtitle": "Version of Nov 18, 2021",
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"Path": "/ref/spec"
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}-->
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<h2>Draft Go 1.18 Specification - Work in Progress </h2>
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<h2>Earlier version</h2>
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<p>
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<strong>
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For the pre-Go1.18 spec see
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For the pre-Go1.18 specification without generics support see
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<a href="/doc/go1.17_spec.html">The Go Programming Language Specification</a>.
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</strong>
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</p>
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<h2 id="Introduction">Introduction</h2>
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@ -852,7 +850,7 @@ Every type has a (possibly empty) method set associated with it:
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<li>
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The method set of a <a href="#Pointer_types">pointer</a> <code>*T</code>
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to a defined type <code>*T</code>
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to a defined type <code>T</code>
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(where <code>T</code> is neither a pointer nor an interface)
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is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
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</li>
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@ -1271,7 +1269,8 @@ InterfaceElem = MethodElem | TypeElem .
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MethodElem = MethodName Signature .
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MethodName = identifier .
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TypeElem = TypeTerm { "|" TypeTerm } .
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TypeTerm = [ "~" ] Type .
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TypeTerm = Type | UnderlyingType .
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UnderlyingType = "~" Type .
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</pre>
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<p>
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@ -1415,9 +1414,9 @@ type ReadCloser interface {
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</pre>
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<p>
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Finally, in their most general form, an interface element may be an arbitrary type
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<code>T</code>, a type term of the form <code>~T</code>, or a union of type terms
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<code>T1 | T2 | … Tn</code>.
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Finally, in their most general form, an interface element may also be an arbitrary type term
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<code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
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or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
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Together with method specifications, these elements enable the precise
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definition of an interface's type set as follows:
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</p>
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@ -1434,7 +1433,7 @@ definition of an interface's type set as follows:
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whose method sets include that method.
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</li>
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<li>The type set of a non-interface type is the set consisting
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<li>The type set of a non-interface type term is the set consisting
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of just that type.
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</li>
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@ -1442,7 +1441,8 @@ definition of an interface's type set as follows:
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is the set of types whose underlying type is <code>T</code>.
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</li>
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<li>The type set of a <i>union</i> of terms <code>T1 | T2 | … Tn</code>
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<li>The type set of a <i>union</i> of terms
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<code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
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is the union of the type sets of the terms.
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</li>
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</ul>
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@ -1487,7 +1487,7 @@ interface {
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</pre>
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<p>
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Union expressions denote unions of type sets:
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Union elements denote unions of type sets:
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</p>
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<pre>
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@ -1500,7 +1500,7 @@ type Floats interface {
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</pre>
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<p>
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In a union expression, a term cannot be a type parameter, and the type sets of all
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In a union, a term cannot be a type parameter, and the type sets of all
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non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
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Given a type parameter <code>P</code>:
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</p>
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@ -1516,14 +1516,15 @@ interface {
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<p>
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Implementation restriction:
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A union expression with more than one term cannot contain interface types
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A union with more than one term cannot contain interface types
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with non-empty <a href="#Method_sets">method sets</a>.
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</p>
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<p>
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Interfaces that contain union or tilde terms (not just methods) may only be used
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as type constraints, or as elements of other interfaces used as constraints. They
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cannot be the types of values or variables, or components of other, non-interface types.
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Interfaces that contain non-interface types, terms of the form <code>~T</code>,
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or unions may only be used as type constraints, or as elements of other interfaces used
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as constraints. They cannot be the types of values or variables, or components of other,
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non-interface types.
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</p>
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<pre>
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