The simplest valid ALGOL program is a BEGIN/END pair. The BEGIN/END pair can enclose a list of declarations, a list of modules, or a list of statements. If the BEGIN/END pair is preceded by a procedure heading, the entire program is a procedure, which can be typed or untyped and can have one or more parameters.

Program units can be blocks, compound statements, level 2 procedures, or separate procedures that have a lexical (lex) level of three or greater and that can have global declarations.

A block is a statement that groups one or more declarations and statements into a logical unit by using a BEGIN/END pair. A compound statement is a statement that groups one or more statements into a logical unit by using a BEGIN/END pair. A compound statement is a block without any declarations.

The definitions of a compound statement and a block are recursive: both compound statements and blocks are made, in part, of statements. A statement can itself be a compound statement or a block, as you can see in the following sample:

Compound Statements

   BEGIN                                   BEGIN
      <statement>;                             <statement>;
      <statement>;                             <statement>;
         .                                     BEGIN
         .                                        <declaration>;
         .                                        BEGIN
      <statement>;                                   <statement>;
   END                                               <statement>;
                                                  END;
                                               END;
                                               <statement>;
                                            END

Blocks

   BEGIN                                     BEGIN
      <declaration>;                            <declaration>;
      <declaration>;                            <declaration>;
          .                                     <statement>;
          .                                     BEGIN
          .                                        <declaration>;
      <declaration>;                               <statement>;
      <statement>;                              END;
      <statement>;                              BEGIN
          .                                        <statement>;
          .                                        <statement>;
          .                                        <statement>;
      <statement>;                              END;
   END                                       END

A program unit that is a separate procedure is typically bound to a host program to produce a more complete program.

The <global part> construct allows global identifiers to be referenced within a separate procedure. Any program unit that has a global part is valid only for binding to a host.

A program unit can be preceded, but not followed, by a remark.

A compound statement is executed in-line and does not require a procedure entrance and exit. A block, however, is executed like a procedure and requires a procedure entrance and exit. Entering a block requires extra processor resources; entering a compound statement does not.

Examples

Compound Statement

BEGIN
   DISPLAY("HI THERE");
   DISPLAY("THAT̍S ALL FOLKS");
END.

Block

BEGIN
   REAL X;
   X := 100;
END.

Level 2 Procedure

PROCEDURE S;
BEGIN
   REAL X;
   X := SQRT(4956);
END.

Separate Procedure with Global Part

[REAL S;
 ARRAY B[0:255];
 FILE LINE;]
REAL PROCEDURE Q;
BEGIN
   Q := S*B[4];
   WRITE(LINE,/,"DONE");
END.

According to the syntax, the last statement of a block or compound statement is not followed by a semicolon (;). However, in the above examples (and throughout this manual), the last statement is always followed by a semicolon. This is valid because the statement before the END is the null statement.