lfs-book/appendixa/bison-desc.xml

<sect2><title>Contents of Bison</title>

<para>Last checked against version &bison-contversion;.</para>

<sect3><title>Program Files</title>

<para>bison and yacc</para></sect3>

<sect3><title>Descriptions</title>

<sect4><title>bison</title>

<para>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 analyzes the structure of a text file. Instead of
writing the actual program a user specifies how things should be connected
and with those rules a program is constructed that analyzes the 
text file. There are a lot of examples where structure is needed and 
one of them is the calculator.</para>

<para>Given the string :</para>

<blockquote><literallayout>        1 + 2 * 3</literallayout></blockquote>

<para>A human can easily come to the result 7. Why? Because of the structure. 
Our brain knows
how to interpret 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:</para>

<blockquote><literallayout>            +
           / \
          *   1
         / \
        2   3</literallayout></blockquote>

<para>Starting at the bottom of a tree and coming across the numbers 2 and
3 which are joined by the multiplication symbol, the computer
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 its way up to the top and comes with the correct
answer. Of course, bison isn't only used for calculators
alone.</para></sect4>

<sect4><title>yacc</title>

<para>We create a bash script called yacc which calls bison using the -y
option. This is for compatibility purposes for programs which use yacc
instead of bison.</para></sect4>

</sect3>

</sect2>