Course Notes on Formal Languages and Compilers

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9. PROGRAMMING LANGUAGE DESIGN AIMS
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1) It should not be difficult to write programs which are easy for anyone
to understand.
2) It should not be difficult to write programs to solve the problems you
need to solve.
3) Effective methods should be available for defining the syntax and
semantics of programming languages.
4) Efficient algorithms should be available for recognizing legal
programs.
5) Programs should be easy to translate to any machine language.
6) The translated programs should run efficiently.
7) Preferably programming errors should be prevented or easily
detected and corrected.
However, programming is hard and designing and implementing
programming languages is very much harder. Nonetheless, there has been
progress in achieving these aims.
Class discussion: Which of these aims are important to the programmer, his
supervisor, the language designer, the writer of a compiler or interpreter, a
computer manufacturer, a software producer Give examples illustrating
some design aims.
Exercise: In ADA the index of a FOR loop is a local variable of the loop.
What advantages does this give to the compiler writer
The definition of programming languages
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Syntax: The syntax of assembly languages (except for expressions) can be
defined by a regular expression or grammar. However, for high level
languages context free grammars are needed, but are not fully adequate.
So additional restrictions concerning the form of a legal program are defined
in natural language (English).
Semantics: In practice the meaning of the program is defined in natural
language (English). The following formal (mathematical) methods have
been proposed but are rarely used in practice.
1) Translational Semantics 2) Operational semantics
3) Axiomatic definition 4) Extensible definition
5) Mathematical or Denotational semantics