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- modified type switches (replacement for CL 32659)
- takes into account new scoping rules DELTA=52 (21 added, 7 deleted, 24 changed) OCL=33967 CL=33982
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@ -3272,9 +3272,9 @@ case x < 0: return -x
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default: return x
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default: return x
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}
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}
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switch { // missing expression means "true"
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switch { // missing expression means "true"
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case x < y: f1();
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case x < y: f1();
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case x < z: f2();
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case x < z: f2();
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case x == 4: f3();
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case x == 4: f3();
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}
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}
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</pre>
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</pre>
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@ -3283,9 +3283,8 @@ case x == 4: f3();
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<p>
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<p>
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A type switch compares types rather than values. It is otherwise similar
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A type switch compares types rather than values. It is otherwise similar
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to an expression switch. It is introduced by special
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to an expression switch. It is marked by a special switch expression which
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notation in the form of a simple declaration whose right hand side
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has the form of a <a href="#Type_assertions">type assertion</a>
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has the form of a type assertion (§<a href="#Type_assertions">Type assertions</a>)
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using the reserved word <code>type</code> rather than an actual type.
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using the reserved word <code>type</code> rather than an actual type.
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Cases then match literal types against the dynamic type of the expression
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Cases then match literal types against the dynamic type of the expression
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in the type assertion.
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in the type assertion.
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@ -3293,23 +3292,30 @@ in the type assertion.
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<pre class="ebnf">
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<pre class="ebnf">
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TypeSwitchStmt = "switch" [ [ SimpleStmt ] ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
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TypeSwitchStmt = "switch" [ [ SimpleStmt ] ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
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TypeSwitchGuard = identifier ":=" Expression "." "(" "type" ")" .
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TypeSwitchGuard = [ identifier ":=" ] Expression "." "(" "type" ")" .
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TypeCaseClause = TypeSwitchCase ":" [ StatementList ] .
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TypeCaseClause = TypeSwitchCase ":" [ StatementList ] .
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TypeSwitchCase = "case" Type | "default" .
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TypeSwitchCase = "case" Type | "default" .
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</pre>
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</pre>
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<p>
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<p>
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As a special case, the type in the type switch case may be an
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The TypeSwitchGuard may include a
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identifier denoting the predeclared constant <code>nil</code>
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<a href="#Short_variable_declarations">short variable declaration</a>.
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(§<a href="#Predeclared_identifiers">Predeclared identifiers</a>).
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When that form is used, the variable is declared in each clause.
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If the interface value equals <code>nil</code>,
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In clauses with a case listing exactly one type, the variable
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only an explict <code>nil</code> case or "default"
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has that type; otherwise, the variable has the type of the expression
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case will execute.
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in the TypeSwitchGuard.
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</p>
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</p>
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<p>
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<p>
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Given a function <code>f</code>
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The type in a case may be <code>nil</code>
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that returns a value of interface type,
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(§<a href="#Predeclared_identifiers">Predeclared identifiers</a>);
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that case is used when the expression in the TypeSwitchGuard
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is a nil interface value.
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</p>
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<p>
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Given a function <code>f</code> that returns
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a value of type <code>interface{}</code>,
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the following type switch:
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the following type switch:
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</p>
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</p>
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@ -3318,11 +3324,13 @@ switch i := f().(type) {
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case nil:
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case nil:
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printString("f() returns nil");
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printString("f() returns nil");
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case int:
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case int:
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printInt(i); // i is an int
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printInt(i); // i is an int
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case float:
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case float:
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printFloat(i); // i is a float
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printFloat(i); // i is a float
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case func(int) float:
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case func(int) float:
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printFunction(i); // i is a function
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printFunction(i); // i is a function
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case bool, string:
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printString("type is bool or string"); // i is an interface{}
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default:
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default:
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printString("don't know the type");
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printString("don't know the type");
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}
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}
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@ -3337,25 +3345,31 @@ v := f();
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if v == nil {
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if v == nil {
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printString("f() returns nil");
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printString("f() returns nil");
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} else if i, is_int := v.(int); is_int {
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} else if i, is_int := v.(int); is_int {
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printInt(i); // i is an int
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printInt(i); // i is an int
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} else if i, is_float := v.(float); is_float {
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} else if i, is_float := v.(float); is_float {
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printFloat(i); // i is a float
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printFloat(i); // i is a float
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} else if i, is_func := v.(func(int) float); is_func {
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} else if i, is_func := v.(func(int) float); is_func {
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printFunction(i); // i is a function
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printFunction(i); // i is a function
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} else {
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} else {
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printString("don't know the type");
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i1, is_bool := v.(bool);
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i2, is_string := v.(string);
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if is_bool || is_string {
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i := v;
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printString("type is bool or string"); // i is an interface{}
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} else {
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i := v;
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printString("don't know the type"); // i is an interface{}
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}
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}
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}
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</pre>
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</pre>
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<p>
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<p>
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In a type switch, the guard is mandatory,
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The type switch guard may be preceded by a simple statement, which
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there can be only one type per "case", and
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executes before the guard is evaluated.
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the "fallthrough" statement is not allowed.
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</p>
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</p>
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<p>
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<p>
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The type switch guard may be preceded by a simple statement, which
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The "fallthrough" statement is not permitted in a type switch.
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executes before the guard is evaluated.
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</p>
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</p>
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<h3 id="For_statements">For statements</h3>
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<h3 id="For_statements">For statements</h3>
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