Back to Vala Reference Manual

Statements

Statements define the path of execution within methods and similar constructions. They combine expressions together with structures for choosing between different code paths, repeating code sections, etc.

  • statement:

    • empty-statement
      simple-statement
      statement-block
      variable-declaration-statement
      if-statement
      switch-statement
      while-statement
      do-statement
      for-statement
      foreach-statement
      return-statement
      throw-statement
      try-statement
      lock-statement


    embedded-statement:

    • statement

Simple statements

The Empty Statement does nothing, but is a valid statement nonetheless, and so can be used wherever a statement is required.

  • empty-statement:

    • ;

A Simple Statement consists of one a subset of expressions that are considered free-standing. Not all expressions are allowed, only those that potentially have a useful side effect - for example, arithmetic expressions cannot form simple statements on their own, but are allowed as part of an assignement expressions, which has a useful side effect.

  • simple-statement:

    • statement-expression ;


    statement-expression:

    • assigment-expression
      class-instantiation-expression
      struct instantiation-expression
      invocation-expression

A Statement Block allows several statements to be used in a context that would otherwise only allow one.

  • statement-block:

    • { [ statement-list ] }


    statement-list:

    • statement [ statement-list ]

Blocks create anonymous, transient scopes. For more details about scopes, see Concepts/Scope and naming.

Variable declaration

Variable Declaration Statements define a local variable in current scope. The declaration includes a type, which signifies the variable will represent an instance of that type. Where the type can be inferred by the compiler, the type-name can be replaced with the literal "var"

  • variable-declaration-statement:

    • variable-declaration-with-explicit-type
      variable-declaration-with-explicit-type-and-initialiser
      variable-declaration-with-type-inference


    variable-declaration-with-explicit-type:

    • type-name identifier ;


    variable-declaration-with-explicit-type-and-initialiser:

    • type-name identifier = expression ;


    variable-declaration-with-type-inference:

    • var identifier = expression ;

Type inference is possible in any case where the variable is immediately assigned to. The type chosen will always be the type of the assigned expression, as decided by the rules described at Expressions. It is important to realise that the type of the variable will be fixed after the first assignment, and will not change on assigning another value to the variable. If the variable should be created with a type other than that of the assigned expression, the expression should be wrapped with a cast expression, provided that the cast is valid.

Selection statements

The If Statement decides whether to execute a given statement based on the value of a boolean expression. There are two possible extensions to this model:

An else clause declares that a given statement should be run if-and-only-if the condition in the the if statement fails.

Any number of "else if" clauses may appear between the "if" statement and its "else" clause (if there is one.) These are equivalent to:

FIXME: This doesn't work.

In simple terms, the program will test the conditions of the if statement and its "else if" clauses in turn, executing the statement belonging to the first that succeeds, or running the else clause if every condition fails.

  • if-statement:

    • if ( boolean-expression ) embedded-statement [ elseif-clauses ] [ else embedded-statement ]


    elseif-clauses:

    • elseif-clause
      [ elseif-clauses ]


    elseif-clause:

    • else if ( boolean-expression ) embedded-statement

The switch statement decides which of a set of statements to execute based on the value of an expression. A switch statement will lead to the execution of one or zero statements. The choice is made by:

  • switch-statement:

    • switch ( expression ) { [ case-clauses ] [ default-clause ] }


    case-clauses:

    • case-clause
      [ case-clauses ]


    case-clause:

    • case literal-expression : embedded-statement


    default-clause:

    • default : embedded-statement

Iteration statements

Iteration statements are used to execute statements multiple times based on certain conditions. Iteration Statements contain loop embedded statements - a superset of embedded statements which adds statements for manipulating the iteration.

  • loop-embedded-statement:

    • loop-embedded-statement-block
      embedded-statement
      break-statement
      continue-statement


    loop-embedded-statement-block:

    • { [ loop-embedded-statement-list ] }


    loop-embedded-statement-list:

    • loop-embedded-statement [ loop-embedded-statement-list ]

Both break and continue statement are types of jump statement, described in Statements/Jump Statements.

The While Statement

The while statement conditionally executes an embedded statement zero or more times. When the while statement is reached, the boolean expression is executed. If the boolean value is true, the embedded statement is executed and execution returns to the while statement. If the boolean value is false, execution continues after the while statement.

  • while-statement:

    • while ( boolean-expression ) loop-embedded-statement

The do statement conditionally executes an embedded statement one or more times. First the embedded statement is executed, and then the boolean expression is evaluated. If the boolean value is true, execution returns to the do statement. If the boolean value is false, execution continues after the do statement.

  • do-statement:

    • do loop-embedded-statement while ( boolean-expression ) ;

The For Statement

The for statement first evaluates a sequence of initialization expressions and then repeatedly executes an embedded statement. At the start of each iteration a boolean expression is evaluated, with a true value leading to the execution of the embedded statement, a false value leading to execution passing to the first statement following the for statement. After each iteration a sequence of iteration expressions are evaluated. Executing this type of statement creates a new transient scope, in which any variables declared in the initializer are created.

  • for-statement:

    • for ( [ for-initializer ] ; [ for-condition ] ; [ for-iterator ] ) loop-embedded-statement


    for-initializer:

    • variable-declaration [ , expression-list ]


    for-condition:

    • boolean-expression


    for-iterator:

    • expression-list

The Foreach Statement

The foreach statement enumerates the elements of a collection, executing an embedded statement for each element of the collection. Each element in turn is assigned to a variable with the given identifier and the embedded statement is executed. Executing this type of statement creates a new transient scope in which the variable representing the collection element exists.

  • foreach-statement:

    • foreach ( type identifier in expression ) loop-embedded-statement

Foreach Statements are able to iterate over arrays and any class that implements the Gee.Iterable interface. This may change in future if an Iterable interface is incorporated into GLib.

Jump Statements

Jump statements move execution to an arbitary point, dependent on the type of statement and its location. In any of these cases any transient scopes are ended appropriately: Concepts/Scope and naming and Statements/Simple statements.

A break statement moves execution to the first statement after the nearest enclosing while, do, for, or foreach statement.

  • break-statement:

    • break ;

A continue statement immediately moves execution the nearest enclosing while, do, for, or foreach statement.

  • continue-statement:

    • continue ;

The return statement ends the execution of a method, and therefore completes the invocation of the method. The invocation expression has then been fully evaluated, and takes on the value of the expression in the return statement if there is one.

  • return-statement:

    • return [ expression ] ;

The throw statement throws an exception.

  • throw-statement:

    • throw expression ;

Try Statement

The try statement provides a mechanism for catching exceptions that occur during execution of a block. Furthermore, the try statement provides the ability to specify a block of code that is always executed when control leaves the try statement.

For the syntax of the try statement, See Errors/Error catching.

Lock Statement

Lock statements are the main part of Vala's resource control mechanism.

FIXME: Haven't actually written anything here about resource control.

  • lock-statement:

    • lock ( identifier ) embedded-statement

Projects/Vala/Manual/Statements (last edited 2017-05-30 14:53:38 by AlThomas)