Lecture 2: Mini Language Interpreter

Background Material

• Lecture 1

Reading

• Chapter 5 (sec. 1-5) of the text (Basic Semantics)
• Chapter 7 (sec. 1-3) of the text (Control I - Expressions and Statements)
• Sections 13.1-2 of the text (Operational Semantics)

Theme

Topics

1. Mini language from chapter 13
2. Operational semantics for mini language
   • Syntax
     1. < program > &rarr < stmt-list>
     2. < stmt-list> &rarr < stmt > ; < stmt-list > | < stmt >
     3. < stmt > &rarr < assign-stmt > | < if-stmt > | < while-stmt >
     4. < assign-stmt > &rarr < identifier > := < expr >
     5. < if-stmt > &rarr if < expr > then < stmt-list > else < stmt-list > fi
     6. < while-stmt > &rarr while < expr > do < stmt-list > od
     7. < expr > &rarr < expr > + < term > | < expr > - < term > | < term >
     8. < term > &rarr < term > * < factor > | < factor >
     9. < factor > &rarr ( < expr > ) | < number > | < identifier >
     10. < number > &rarr < number > < digit > | < digit >
     11. < digit > &rarr 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
     12. < identifier > &rarr < identifier > < letter > | < letter >
     13. < letter > &rarr a | b | c | ... | z
   • Semantics
     1. Env: Identifier &rarr Integer Union {undef}
     2. (Env and {I = n})(J) = n if J=I, Env(J) otherwise
     3. Env_0 = undef for all I
     4. for if-stmt, if expr evaluates to value greater than 0, then evaluate stmt-list after then, else evaluate stmt-list after else
     5. for while-stmt, as long as expr evaluates to a value greater than 0, stmt-list is repeatedly executed and expr evaluated.
   • Example
     1. n := 0 - 5;
     2. if n then i := n else i := 0 - n fi;
     3. fact := 1;
     4. while i do fact := fact * i; i := i - 1 od
   • Lecture 2a - Mini Language Interpreter Part I: parser and expression trees
   • Lecture 2b - Mini Language Interpreter Part II: adding statements
   • Lecture 2c - Mini Language Interpreter Part III: adding procedures

References, programs, and lecture notes

• Lecture summary slides (lec2summary.ppt, (lec2summary.pdf)

Exercises

• Assignment 2