COMP3610 Principles of Programming Languages

Flex and Bison

Manuals

• Bison manual
• Flex manual

Examples & Exercises

• The following three list grammars demonstrate the effect of left and right recursion.
  • list1.y, list1.l
  • list2.y, list2.l
  • list3.y, list3.l
• The following Flex specification demonstrates simple recognition (of a collection of verbs).
  • verb1.l

The following examples of Bison and Flex specifications are all various calculators. They form the basis of some of the tutorial exercises.

• The first example is a calculator for arithmetic expressions in reverse-polish notation. It is a simple Bison specification with a hand-coded lexer.
  • calc1.y
• Here is another Bison specification based on the first, but there is a problem with the grammar.
  • calc1a.y
• The second example is a calculator for arithmetic expressions in standard infix notation. There is a separate Flex specification for the lexer.
  The demonstration of %prec to specify context-dependent precedence is not a very clear one. As it stands, unary negation is given higher precedence than all other operators, so expressions like -3-4 and -3*4 are parsed as (-3)-4 and (-3)*4 respectively. This is as we would wish. Without the %prec UMINUS, unary negation is given the same precedence as subtraction. so the expressions are parsed as (-3)-4 and -(3*4) respectively. Unfortunately, this change to the parse is not reflected by a change to the value of the expressions.
  • calc2.y
  • calc2.l
• The third example allows single-letter variables to be declared.
  • calc3.y
  • calc3.l
• The fourth example allows typical identifiers to be declared. There is a separate implementation of a simple symbol table.
  • calc4.y
  • calc4.l
  • symtab4.c
  • symtab4.h