Synthesized Translation

The term synthesized translation includes two ideas:

• A complex expression is constructed from subexpressions
• The translation of a complex expression is constructed in a mechanical way form the translations of its subexpressions
  if x > max then max := x
  is constructed of (x > max) and (max := x), therefore, ( if (> x max) (:= max x) )

Controllability and Observability

These are central concepts from control theory. We will define them as :

• Controllability: the ability to chance the behavior of a system by changing its parameters.
• Observability: the ability to observe the behavior of a system well enough to control it.

In order to control a system, both controllability and observability are required.

The implications For Large Software Systems

• Aspects of software that cannot easily be observed will never be debugged
• All Large Software Systems must have observability built in
• Observability is a requirement, not a luxury. The time spent building in observability will be well repaid.
• In-process traces can be turned on by setting bits in a bit vector.

Comparison of Parsing Techniques

Recursive Descent and Operator Precedence

Advantages:

• easy to control, understand
• Good error messages can be produced

Disadvantages:

• More code to write
• Grammar not as clear

Parser Generator (yacc)

Advantages

• Less code to write
• Grammar is clear and easy to change

Disadvantages:

• The programmer may get stuck on grammar errors that are hard to understand
• Error messages during compilation may be unhelpful and far from the actual error

Lexical Scoping

Scope is the region of program text over which a symbol can be referenced.

With lexical scoping, a name is defined when it can be looked up in the lexicon at the point of use.

Structure References

• Every expression has type
• Types from a tree structure (a graph when pointers to types are included, but still treated as a tree)
• The structure references in source code specify a traversal down the type tree
• A reference to a part of a structure depends on the type of the structure; the results of the reference are:
  • An address expression for the substructure
  • A new type for the substructure

Citation:

• Aho, Lam, Sethi, & Ullman, Compilers: Principles, Techniques, and Tools
• UT Austin CS375: Compilers by G.Novak.