Frame

From Free Pascal wiki

A frame (frequently also referred to as block) is a language construct grouping a (possibly empty) sequence of statements (or instructions in the case of asm frames).

available types

All frames but repeat  until are terminated by the word end. Frame types are distinguished by their corresponding opening words.

  • Pascal: These frames expect Pascal statements or may contain other frames.
    begin
    This frame begins a (possibly empty) sequence of statements. In the context of routine definitions or a program it can delimit a scope.
    else/otherwise
    This frame surrounds a “catch-all”-alternative as part of a case statement.
    repeat  until
    repeat in conjunction with until is used to surround the loop body of a tail-controlled loop. It is the only frame type not ending with an end.
    exception treatment
    If exceptions are supported in the current compiler mode, the following frames are available as well. These frames are in fact “double”-frames: They group two sequences at once. Neither of them can be used independently (e. g. writing finally  end; without a proper try is illegal).
    tryexcept
    Use this to install exception handlers.
    tryfinally
    Use this to ensure a certain code fragment is executed despite any thrown exceptions.
    unit overhead
    initializationfinalization
    This double-frame designates code being executed when the corresponding unit is loaded or unloaded. Either part of this frame is optional. This frame may also delimit a scope.
    begin
    If there is no need for a finalization part, initialization can be replaced by begin.
  • Assembly language: Frames beginning with asm expect assembly language. In pure assembly routines, this kind of frame may delimit a scope, too. Note, you cannot nest other frames in asm frames.

style

Although not mandatory, it is customary to indent all code surrounded by frame markers by one level.

try
	openJar;
except
	throwATantrum;
end;

Some styles add another indentation level for nested or subordinate frame markers per se.

if apples = oranges then
  begin
    protest;
    halt(123);
  end;

As you can see from the examples above, it is customary to put frame delimiters isolated in their own line. Exempt of this guideline are of course frames accepting or requiring additional clauses to be syntactically complete:

repeat
	write('Enter a non-negative integer: ');
	readLn(i);
until i >= 0;
asm
	rdrand eax
	mov n, eax
end ['eax'];

technical background

Frames frequently, but not always, turn up to be (conditional) jmp targets. Some compile-time optimizations require code to be structured in a certain way, frames setting boundaries for that.