1. Bison
Marcin Zubrowski

2. What is it YACC – Yet Another Compiler Compiler Parser Generator
Define the BNF
Context-free grammar
By default generates an LALR parser, but you can generate a GLR parser
For C, C++, and Java
Can be used in conjunction with Flex
Need some sort of lexical analyzer for token input

3. Bison Grammar File
To define the language for Bison you need to write a Grammar File

4. Bison Grammar File General Structure C declarations Header files
Can omit this section

5. Bison Grammar File – Bison Declarations
Declare tokens
“%type” specifies your nonterminals
“%Union” specifies the different types that the lexical analysis can return
“%Start” defines the starting rule
Can be omitted, will see the first rule defined as start

6. Bison Grammar File – Grammar Rules
General rule format
‘|’ for multiple rules that have the same result
Actions follow the rule

7. Show Example Calc.y

8. The Bison Parser Algorithm
Bison reads tokens and pushes them onto the parser stack
Bison looks at the stack and decides the reduction based on the rules you set
Tries to reduce the stack to a single grouping, which is the start symbol
Reductions are based on the look-ahead token

9. The Bison Parser Algorithm (cont.)
Can handle shift/reduce conflicts if the number of shift/reduce conflicts is exactly n
Use the “%expect n” declaration to avoid warnings
Else you may need to specify operator precedence with “%left” and “%right”

10. The Bison Parser Algorithm (cont.)
Imagine we have “1 – 2 * 3”
‘1’, ‘-’, and ‘2’ are on the stack, do we reduce or shift?
Each will produce different results
“1 – (2 op 3)” vs. “(1 – 2) op 3”
The first is right precedence which is preferable for assignment operators
The second is left precedence which is preferable for most operators
Declare the operator whose precedence is lower first
A third alternative is “%nonassoc”, which declares that it is a syntax error to find the same operator twice in a row

11. The Bison Parser Algorithm (cont.)
Bison handles reduce/reduce conflicts by choosing the first rule that appears in the grammar which is risky
Just rewrite your rules