The Bison package contains the bison program.
Bison is a parser generator, a replacement for YACC. YACC stands for Yet Another Compiler Compiler. What is Bison then? It is a program that generates a program that analyses the structure of a textfile. Instead of writing the actual program you specify how things should be connected and with those rules a program is constructed that analyses the textfile.
There are alot of examples where structure is needed and one of them is the calculator.
Given the string :
1 + 2 * 3
You can easily come to the result 7. Why ? Because of the structure. You know how to interpretet the string. The computer doesn't know that and Bison is a tool to help it understand by presenting the string in the following way to the compiler:
+
/ \
* 1
/ \
2 3
You start at the bottom of a tree and you come across the numbers 2 and 3 which are joined by the multiplication symbol, so the computers multiplies 2 and 3. The result of that multiplication is remembered and the next thing that the computer sees is the result of 2*3 and the number 1 which are joined by the add symbol. Adding 1 to the previous result makes 7. In calculating the most complex calculations can be broken down in this tree format and the computer just starts at the bottom and works it's way up to the top and comes with the correct answer. Of course, Bison isn't only used for calculators alone.