Com S 342 meeting -*- Outline -*-

* Tools for symbolic programming (EOPL 1)

** Motivation

        In your career, you're likely to define some "extension language",
        such as macros for a spreadsheet, etc.
        How to describe such a thing?  Let's try it.
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         FOR YOU TO DO IN GROUPS

Write down on a piece of paper,
a description of the C (or C++)
"for" statement.

Make it clear and unambiguous.

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        I.e., someone who has never used it before
        should be able to write a program with it,
        different complier writers would all produce the same behavior from it.

        After this, have them describe theirs out loud.

        Then have groups exchange their solutions.

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        FOR YOU TO DO IN GROUPS

Take the description from another group.

1. Write down as many things about it
   as you can find that are unclear
   or ambiguous.

2. What makes it unclear or ambiguous?

3. What would make it more clear
   or unambiguous?
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** why use programs?

        In this course we'll use programs to describe
        the semantics of languages, because it's precise.
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         WHY USE PROGRAMS?

Why do we care about being unambiguous
when describing languages?




How can a program *not* be clear?






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        ... so user's don't have to flounder
            so we have portable programs
            it keeps us honest

        ... side-effects on globals, implicit arguments,
			prohibit these (mostly)
		wierd transfers of control
			don't do that
		too much detail
			use abstraction,
				hence want a language
				in which can abstract at will
	hence Scheme

** syntax and semantics

	The basic way we subdivide the study of languages is...

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     SYNTAX AND SEMANTICS

def: the *syntax* of a language
says


Usually given by a context-free grammar.

  <expr> ::= <literal> | <variable>
	     | <proc-call>
  <literal> ::= <numeral> | <boolean>
	     | <string> | <char>
  <proc-call> ::= ( {<expr>}+ )


def: the *semantics* of a language
tells 


Can be given by describing interpreter.
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	... what strings are well-formed.
	... the meaning of (syntactically legal) strings.