5.1 Blocks

A block in Haxe starts with an opening curly brace { and ends with a closing curly brace }. A block may contain several expressions, each of which is followed by a semicolon ;. The general syntax is thus:

{
    expr1;
    expr2;
    ...
    exprN;
}

The value and by extension the type of a block-expression is equal to the value and the type of the last sub-expression.

Blocks can contain local variables declared by var expression, as well as local functions declared by function expressions. These are available within the block and within sub-blocks, but not outside the block. Also, they are available only after their declaration. The following example uses var, but the same rules apply to function usage:

{
    a; // error, a is not declared yet
    var a = 1; // declare a
    a; // ok, a was declared
    {
        a; // ok, a is available in sub-blocks
    }
  // ok, a is still available after
    // sub-blocks    
    a;
}
a; // error, a is not available outside

At runtime, blocks are evaluated from top to bottom. Control flow (e.g. exceptions or return expressions) may leave a block before all expressions are evaluated.

Variable Shadowing

Haxe allows local variable shadowing within the same block. This means that a var or function can be declared with the same name that was previously available in a block, effectively hiding it from the further code:

{
    var v = 42; // declare v
    $type(v); // Int
    var v = "hi"; // declare a new v
    $type(v); // String, previous declaration is not available
}

It might come as a surprise that this is allowed, but it's useful to avoid pollution of local name space and thus prevent accidental usage of a wrong variable.

Note, that the shadowing strictly follows syntax, so if a variable was captured in a closure before it was shadowed, that closure would still reference the original declaration:

{
    var a = 1;
    function f() {
        trace(a);
    }
    var a = 2;
    f(); // traces 1
}