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3.2. THE METALANGUAGE generate, create barcode data matrix none for .net projects console app Hyphenated variable names ha VS .NET 2d Data Matrix barcode ve existed in COBOL from the beginning. When COBOL was extended at a later date to permit the use of arithmetic expressions an ambiguity arose: the hyphen character and the subtraction operator were the same character.

One way to avoid this problem is to use di erent characters for the two purposes. Modern languages use the - for subtraction and the underbar, _ , which has no other function in the language, to achieve readability. As you can see, the rules for delimiting tokens can be complex, and they do have varied repercussions.

The three important issues here are: Code should be readable. The language must be translatable and, preferably, easy to lex. It is preferable to use the same conventions as are used in English and/or mathematical notation.

The examples given show that a familiar, readable language may contain an ambiguous use of symbols. A few language designers have chosen to sacri ce familiarity and readability altogether in order to achieve lexical simplicity. LISP, APL, and FORTH all have simpler lexical and syntactic rules, and all are considered unreadable by some programmers because of the con ict between their prior experience and the lexical and syntactic forms of the language.

Let us examine the simple lexical rule in FORTH and its e ects. In other languages the decision was made to permit arithmetic expressions to be written without delimiters between the variable names and the operators. A direct consequence is that special symbols (nonalphabetic, nonnumeric, and nonunderbar) must be prohibited in variable names.

It may seem natural to prohibit the use of characters like + and ( in a name, but it is not at all necessary. FORTH requires one or more space characters or carriage returns between every pair of tokens, and because of this rule, it can permit special characters to be used in identi ers. It makes no distinction between user-de ned names and prede ned tokens: either may contain any character that can be typed and displayed.

The string #$% could be used as a variable or function name if the programmer so desired. The token ab* could never be confused with an arithmetic problem because the corresponding arithmetic problem, a b * , contains three tokens separated by spaces. Thus the programmer, having a much larger alphabet to use, is far freer to invent brief, meaningful names.

For example, one might use a+ to name a function that increments its argument (a variable) by the value of a. Lexical analysis is trivially easy in FORTH. Since its lexical rules treat all printing characters the same way and do not distinguish between alphabetic characters and punctuation marks, FORTH needs only three classes of lexical tokens: Names (prede ned or user-de ned).

Numeric literals. String literals. These can appear only after the string output command, which is .

" (pronounced dot-quote ). A string literal is terminated by the next " (pronounced quote )..

CHAPTER 3. ELEMENTS OF LANGUAGE These three token types corr visual .net Data Matrix 2d barcode espond to semantically distinct classes of objects that the interpreter handles in distinct ways. Names are to be looked up in the dictionary and executed.

Numeric literals are to be converted to binary and put on the stack. String literals are to be copied to the output stream. The lexical rules of the language thus correspond directly to its semantics, and the interpreter is very short and simple.

The e ect of these lexical rules on people should also be noted. Although the rules are simple and easy to learn, a programmer accustomed to the conventions in other languages has a hard time learning to treat the space character as important..

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