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www.angelfire.com/dragon/letstry
cwave04 at yahoo dot com |
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Back to the virtual laboratory
To understand how we use actions to store information
in the intermediate form, let us go back to our virtual
laboratory. Last time the lab had three windows. Now it has
four. The new window is called the action window, and it
shows the action to be taken everytime a reduce step is
performed.
The main idea is this. When we use a rule in a reduce
step we must collect the attributes in the right hand side of
the rule, since once the step is over the righthand side will
be replaced by the lefthand side. In our case, only the tokens
have attributes, so we need actions for only the rules with tokens
in their righthand sides.
Here we have used the standard C function
memcpy to copy
the exits. The function is declared in string.h.
The next question is how to insert the above things inside the
bison input file. In particular, we have three kinds of
things to insert:
- the data structure and variables
- the actions
- the functions arrcpy and find
The data structure and variables all go in the first part of the
bison input file. The actions, as we have already mentioned,
are to be writen after the rules they are related to. The functions
are to be writen in the fourth part.
After inserting all these, the bison input file becomes
adv6.y, which is the final version.
Putting everything together
Recall that a compiler consists of two ends: the front end and the back
end. We had already implemented the back end earlier. Now we have finished making the front
end. The front end is invoked by calling the function yyparse(). The back
end is invoked similarly by calling the function emitCode(), which we
wrote earlier. So now the main function of the compiler takes the simple
form:
main() {
yyparse();
emitCode();
}
The complete system is given here.
And that's the end of this tutorial. Hope you enjoyed it!
In this tutorial we have introduced the basics of language
design and compiler construction using a carefully crafted
example language called AdvLang. The idea is to encourage the
reader to try to design similar languages on his or her own.
However, this tutorial has deliberately disregarded certain
pitfalls that the reader is bound to encounter when creating a new
language. In the second part
of this tutorial we shall take up those issues.
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