Lexical Structure¶
This chapter describes several of the basic building blocks of Modelica such as characters and lexical units including identifiers and literals. Without question, the smallest building blocks in Modelica are single characters belonging to a character set. Characters are combined to form lexical units, also called tokens. These tokens are detected by the lexical analysis part of the Modelica translator. Examples of tokens are literal constants, identifiers, and operators. Comments are not really lexical units since they are eventually discarded. On the other hand, comments are detected by the lexical analyzer before being thrown away.
The information presented here is derived from the more formal specification in Appendix B.
Character Set¶
The character set of the Modelica language is Unicode, but restricted to the Unicode characters corresponding to 7-bit ASCII characters in several places; for details see Appendix B.1.
Comments¶
There are two kinds of comments in Modelica which are not lexical units in the language and therefore are treated as whitespace by a Modelica translator. The whitespace characters are space, tabulator, and line separators (carriage return and line feed); and whitespace cannot occur inside tokens, e.g., <= must be written as two characters without space or comments between them. [The comment syntax is identical to that of C++]. The following comment variants are available:
// comment |
Characters from // to the end of the line are ignored. |
/* comment */ |
Characters between /* and */ are ignored, including line terminators. |
Modelica comments do not nest, i.e., /* */ cannot be embedded within
/* */. The following is invalid:
/* Commented out - erroneous comment, invalid nesting of comments!
/* This is an interesting model */
model interesting
// ...
end interesting;
*/
There is also a kind of “documentation comment,” really a documentation string that is part of the Modelica language and therefore not ignored by the Modelica translator. Such “comments” may occur at the ends of declarations, equations, or statements or at the beginning of class definitions. For example:
model TempResistor "Temperature dependent resistor"
// ...
parameter Real R "Resistance for reference temp.";
// ...
end TempResistor;
Identifiers, Names, and Keywords¶
Identifiers are sequences of letters, digits, and other characters such as underscore, which are used for naming various items in the language. Certain combinations of letters are keywords represented as reserved words in the Modelica grammar and are therefore not available as identifiers.
Identifiers¶
Modelica identifiers, used for naming classes, variables, constants, and other items, are of two forms. The first form always start with a letter or underscore (_), followed by any number of letters, digits, or underscores. Case is significant, i.e., the names Inductor and inductor are different. The second form (Q-IDENT) starts with a single quote, followed by a sequence of any printable ASCII character, where single-quote must be preceded by backslash, and terminated by a single quote, e.g. ‘12H’, ‘13\’H’, ‘+foo’. Control characters in quoted identifiers have to use string escapes. The single quotes are part of the identifier, i.e., ‘x’ and x are distinct identifiers. The following BNF-like rules define Modelica identifiers, where curly brackets {} indicate repetition zero or more times, and vertical bar | indicates alternatives. A full BNF definition of the Modelica syntax and lexical units is available in old Appendix B.
IDENT = NONDIGIT { DIGIT | NONDIGIT } | Q-IDENT ;
Q-IDENT = "'" { Q-CHAR | S-ESCAPE } "'" ;
NONDIGIT = "_" | letters "a" to "z" | letters "A" to "Z" ;
DIGIT = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
Q-CHAR = NONDIGIT | DIGIT | "!" | "#" | "$" | "%" | "&" | "(" | ")" | "\*" | "+" |
"," | "-" | "." | "/" | ":" | ";" | "<" | ">" | "=" | "?" | "@" | "[" | "]" |
"^" | "{" | "}" | "|" | "~" | " " | "_" ;
S-ESCAPE = "\" ( "'" """" "?" "\" "a" "b" "f" "n" "r" "t" "v" ) ;
Names¶
A name is an identifier with a certain interpretation or meaning. For example, a name may denote an Integer variable, a Real variable, a function, a type, etc. A name may have different meanings in different parts of the code, i.e., different scopes. The interpretation of identifiers as names is described in more detail in todo: Chapter 5. The meaning of package names is described in more detail in todo: Chapter 13.
Modelica Keywords¶
The following Modelica keywords are reserved words and may not be used as identifiers, except as listed in TODO: Appendix B.1:
algorithm, and, annotation, block, break, class, connect, connector,
constant, constrainedby, der, discrete, each, else, elseif, elsewhen,
encapsulated, end, enumeration, equation, expandable, extends, external,
false, final, flow, for, function, if, import, impure, in, initial,
inner, input, loop, model, not, operator, or, outer, output, package,
parameter, partial, protected, public, pure, record, redeclare,
replaceable, return, stream, then, true, type, when, while, within
Literal Constants¶
Literal constants are unnamed constants that have different forms depending on their type. Each of the predefined types in Modelica has a way of expressing unnamed constants of the corresponding type, which is presented in the ensuing subsections. Additionally, array literals and record literals can be expressed.
Floating Point Numbers¶
A floating point number is expressed as a decimal number in the form of
a sequence of decimal digits optionally followed by a decimal point,
optionally followed by an exponent. At least one digit must be present.
The exponent is indicated by an E or e, followed by an optional sign (+
or -) and one or more decimal digits. The minimal recommended range is
that of IEEE double precision floating point numbers, for which the
largest representable positive number is 1.7976931348623157E+308 and the
smallest positive number is 2.2250738585072014E-308. For example, the
following are floating point number literal constants:
22.5, 3.141592653589793, 1.2E-35
The same floating point number can be represented by different literals. For example, all of the following literals denote the same number:
13., 13E0, 1.3e1, 0.13E2
Integer Literals¶
Literals of type Integer are sequences of decimal digits, e.g. as in the
integer numbers 33, 0, 100, 30030044. [Negative numbers are formed by
unary minus followed by an integer literal]. The minimal recommended
number range is from -2147483648 to +2147483647 for a two’s-complement
32-bit integer implementation.
Boolean Literals¶
The two Boolean literal values are true and false.
Strings¶
String literals appear between double quotes as in “between”. Any character in the Modelica language character set (see appendix B.1 for allowed characters) apart from double quote (”) and backslash (\), including new-line, can be directly included in a string without using an escape code. Certain characters in string literals can be represented using escape codes, i.e., the character is preceded by a backslash (\) within the string. Those characters are:
"\'" |
single quote may also appear without backslash in string constants. |
"\"" |
double quote |
"\?" |
question-mark may also appear without backslash in string constants. |
"\\" |
backslash itself |
"\a" |
alert (bell, code 7, ctrl-G) |
"\b" |
backspace (code 8, ctrl-H) |
"\f" |
form feed (code 12, ctrl-L) |
"\n" |
new-line (code 10, ctrl-J) |
"\r" |
return (code 13, ctrl-M) |
"\t" |
horizontal tab (code 9, ctrl-I) |
"\v" |
vertical tab (code 11, ctrl-K) |
For example, a string literal containing a tab, the words: This is, double quote, space, the word: between, double quote, space, the word: us, and new-line, would appear as follows:
"\tThis is\" between\" us\n"
Concatenation of string literals in certain situations (see the Modelica
grammar) is denoted by the + operator in Modelica, e.g. "a" + "b"
becomes "ab". This is useful for expressing long string literals that
need to be written on several lines.
[Note, if the contents of a file is read into a Modelica string, it is assumed that the reading function is responsible to handle the different line ending symbols on file (e.g. on Linux systems to have a “newline” character at the end of a line and on Windows systems to have a “newline” and a “carriage return” character. As usual in programming languages, the content of a file in a Modelica string only contains the “newline” character.
For long string comments, e.g., the “info” annotation to store the documentation of a model, it would be very inconvenient, if the string concatenation operator would have to be used for every line of documentation. It is assumed that a Modelica tool supports the non-printable “newline” character when browsing or editing a string literal. For example, the following statement defines one string that contains (non-printable) newline characters:
assert(noEvent(length > s_small), "
The distance between the origin of frame_a and the origin of frame_b of a
LineForceWithMass component became smaller as parameter s_small
(= a small number, defined in the \"Advanced\" menu). The distance is
set to s_small, although it is smaller, to avoid a division by zero
when computing the direction of the line force.",
level = AssertionLevel.warning);
]