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Vocabulary and representation

The representation of symbols in terms of characters depends on the underlying character set. The ASCII set is used in this reference text, and the following lexical rules must be observed. Blanks must not occur within symbols (except in strings). Blanks and line breaks are ignored unless they are essential to separate two consecutive symbols.


Identifiers are sequences of letters and digits. The first character must be a letter.

ident = letter {letter | digit}

Examples: x scan Modula ETH GetSymbol firstLetter


Numbers are (unsigned) integers or real numbers. Integers are sequences of digits. If the number is followed by the letter B, it is taken as an octal number; if it is followed by the letter H, it is taken as a hexadecimal number; if it is followed by the letter C, it denotes the character with the given (octal) ordinal number (and is of type CHAR). An integer i in the range 0 <= i <= MaxInt can be considered as either of type INTEGER or CARDINAL; if it is in the range MaxInt < i <= MaxCard, it is of type CARDINAL. For 16-bit computers: MaxInt = 32767, MaxCard = 65535.

A real number always contains a decimal point. Optionally it may also contain a decimal scale factor. The letter E is pronounced as "ten to the power of". A real number is of type REAL.

   number = integer | real.
   integer = digit {digit} | octalDigit {octalDigit} ("B" | "C") | digit {hexDigit} "H".
   real = digit {digit} "." {digit} [ScaleFactor].
   ScaleFactor = "E" ["+"|"-"] digit {digit}.
   hexDigit = digit | "A" | "B" | "C" | "D" | "E" | "F".
   digit = octalDigit | "8" | "9".    |
   octalDigit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7".

Examples: 1980 3764B 7BCH 33C 12.3 45.67E 8


Strings are sequences of characters enclosed in quote marks. Both double quotes and single quotes (apostrophes) may be used as quote marks. However, the opening and closing marks must be the same character, and this character cannot occur within the string. A string must not extend over the end of a line.

string = '"' {character} '"' | "'" {character} "'"

A string consisting of n characters is of type ARRAY [0..n-1] OF CHAR

Examples: "MODULA" "Don't worry!" 'codeword"Barbarossa"'

Operators and delimiters

Operators and delimiters are the special characters, character pairs, or reserved words listed below. These reserved words consist exclusively of capital letters and must not be used in the role of identifiers. The symbols # and <> are synonyms, and so are &, AND, and ~, NOT.

   +     =      AND            FOR                QUALIFIED
   -     #      ARRAY          FORWARD            RECORD
   *     <      BEGIN          FROM               REPEAT
   /     >      BY             IF                 REM
   :=    <>     CASE           IMPLEMENTATION     RETRY
   &     <=     CONST          IMPORT             RETURN
   .     >=     DEFINITION     IN                 SET
   ,     ..     DIV            LOOP               THEN
   ;     :      DO             MOD                TO
   (     )      ELSE           MODULE             TYPE
   [     ]      ELSIF          NOT                UNTIL
   {     }      END            OF                 VAR
   ^     |      EXCEPT         OR                 WHILE
   ~            EXIT           PACKEDSET          WITH
                EXPORT         POINTER
                FINALLY        PROCEDURE

Additional reserved words defined by the ISO generics language extension


Additional reserved words defined by the ISO object oriented language extension

   AS         INHERIT     TRACED


Comments may be inserted between any two symbols in a program. They are arbitrary character sequences opened by the bracket (* and closed by *). Comments may be nested, and they do not affect the meaning of a program.


  • Wirth N: Programming in Modula-2, 3rd ed. Springer Verlag, Berlin, 1985.
  • Stony Brook Modula-2 documentation. Used with permission. Note: Stony-Brook M2 offers an extended syntax with features not described here. Stony Brook M2 users are encouraged to visit the Stony Brook website and to consult the Stony Brook help system.