CSE 3341 Principles of Programming Languages

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CSE 3341Principles of Programming Languages

• Neelam Soundarajan
• Computer Sc. Eng.
• Dreese Labs 579
• e-mail neelam_at_cse

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Goals of the Course

• Main Goal
• Discuss key concepts underlying PLs
• Sub-Goals
• Alternative programming paradigms
• Implementation issues
• At end of course Given a feature, you should
  be able to
• Decide whether you like it or not, and why
• Have an idea how to implement it
• Decide what kinds of problems it is suited for.

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How do we study a language?

• Syntax What do legal programs in L look like?
• Semantics What do the various instructions of L
  do (when exec.)?
• Programming Methodology How are you supposed to
  use the features of L? For solving what kinds of
  problems?

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Compilers
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Compilers (contd.)
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Compilers Intermediate Langs.

• Identify a language I
• For each L, write
• For each M, write
• New machines are easy to handle
• New languages are easy to handle
• Common intermediate language C

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Interpreters
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Compilers Interpreters

• What is an assembler?A simulator?The JVM?
• JIT Just-in-time compilation
• Running theme for the course Runtime versus
  compile-time
• "Interpreter interprets each line into binary
  code which can be run on different platforms"
  Not true!

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Syntax

• BNF (Backus Normal Form) Notation for
  describing syntax of languages precisely.
• Example Set of all non-negative
  integers ltnogt ltdigitgt ltdigitgt
  ltnogt ltdigitgt 0 1 2 3 4 5 6 7
  8 9
• Set of all non-neg. nos. not starting with
  0ltnlznogt ltnlzdigitgt ltnlzdigitgt
  ltnlznogtltnlzdigitgt 1 2 3 4 5 6 7
  8 9 ??
• lt, gt, , are reserved (meta)
  symbolsltdigitgt, ltnogt Non-terminals 0, 1, 2,
  ... Terminals
• To define a BNF grammar Specify terminal and
  non-termnial symbols Define the production for
  each non-terminal.

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Derivation trees/Parse trees
How do you derive 655 from this grammar?
ltnogt
(There is a bug in this tree!)
The string derived (or parsed) by the tree
Append together the labels at the leaves in
left-to- right order.
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Example Grammar of expressions
ltexpgt ltnogt ltidgt ltexpgt ltexpgt ltexpgt
ltexpgt ltidgt X Y Z
Parse tree for X Y
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Grammar of expressions (contd.)
Parse tree for X Y Z
ltexpgt
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Grammar of expressions (contd.)
Another tree for X Y Z
Which is the right tree? The grammar is
ambiguous.
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Another grammar for expressions

• ltexpgt ltfacgt ltfacgt ltexpgtltfacgt ltnogt
  ltidgt ltnogt ltfacgt ltidgt ltfacgt
• This grammar is not ambiguous
• Reintroduce ambiguity among s and s (but
  not between and )
• Parenthesized expressions?

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Grammars (contd.)

• Exercise Make precedence left to right or
  right to left
• Ambiguous grammar for numbers ltnogt ltdigitgt
  ltnogt ltnogt
• Even the following is not a good grammar ltnogt
  ltdigitgt ltdigitgt ltnogtProblem Wrong
  semantics.Q Can you fix it?

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Grammars (contd.)

• Syntax graphs Pictorial representation of BNF
  grammars.
• Extended BNF (see Ch. 2.1.2 of book) ...
  optional item ... repetition (0 or more
  times) repetition (0 or more times)
  repetition (1 or more times)
• ltifgt if ltcondgt then ltstmtgt ltelse-ifgt
  else ltstmtsgt end-ifltelse-ifgt elseIf
  ltcondgt then ltstmtsgt