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Episode: 1107
Title: HPR1107: Compilers Part 3
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1107/hpr1107.mp3
Transcribed: 2025-10-17 19:03:52
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I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do
to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know what to do, I don't know
is what lexical analysis does. It generates tokens based on its input stream.
The input stream to lexical analyzer is typically raw bytes from a source code file.
From that comes things like keyword tokens, string tokens, integer tokens,
flooding quick tokens, operator tokens, and so on.
The mechanism that implements lexical analyzer is called a finite state machine.
But as a conceptual machine, that only can be in one state at a time.
Its current state is changed by the roles of the machine versus the input stream.
As it transitions from one state to another, it eventually settles on a state
that outputs a token indicating that a token was found.
Let's take searching for the keyword break, for instance.
The same that the input stream is BJ-BR-E-A-K.
Once the B flows by the stream, we might transition from our initial state to one where they are.
The state is adjacent. We find that's a J flows by the stream and we go back to the initial state.
Once again, we find that a B flows by and we transition to the B state.
Then in our flows by, we transition to the R states and so on until we reach the K state,
at which point we output a token.
Typically, we don't build these machines directly, rather they're built by a generator,
which takes it into a description of each token.
One of the more popular lexical analyzer generators is called Lex.
Let's review.
Our source flows through a lexical analyzer,
which matches its input stream with token descriptions using a finite state machine generator.
Then outputs tokens to a parser, which then uses ship-reduced parsing to build parse trees
and tell us significant match occurs in the global parse tree.
It performs actions to stitch together a syntax tree at each of these match points
and continues until the entire global parse tree is matched.
The theoretical machine that implements a parser is a pushdown finite automata,
which is just a finite state machine with a stack attached to it to handle the inherent stacking procedure of building trees.
Like a lexical analysis, we typically don't build these machines directly, rather they're generated.
In the case of parsing, one of the more common generators is called Yak.
Yak takes the language grammar and automatically generates a parser for us,
which is damn convenient.
So we have a syntax tree, what next?
Well, semantic analysis.
We've already validated input stream against context-free grammar.
Now we've validated against a context-aware grammar called a natural-beaded grammar.
What we're going to be doing in semantic analysis is validating that the symbols are declared before they are used.
At the same time, we're going to generate a symbol table and then we'll perform type checking.
There are any number of semantic checks you can do based on the nature of your language,
but we're just going to focus on these two as they're applicable to most languages,
and actually are two major classes.
In the process of validating semantics of a syntax tree, two things are generated,
an attributed syntax tree and a symbol table.
These two things are used in the next step of a compiler code generation.
There are two primary types of attribute grammar.
I'll attribute a grammar and that's attribute grammar.
I'll attribute a grammar table with carries information from parent to child,
whereas as attribute a grammar carries information from child to parent.
These two directions produce, on a syntax tree, what are called inherited and synthesized attributes.
I'll attribute a grammar being associated with inherited attributes and as attribute a grammar being associated with synthesized attributes.
The production of synthesized attributes is what type checking will do.
And the production of inherited attributes is what checking to see if a symbol is declared before it's used.
We'll call this scope checking, we'll do.
An attribute a grammar, it's just a grammar with attributes associated with it.
Typically this is something like the name, value rules for generating these things.
All right, blah, blah, blah, blah, blah, blah, blah, blah, blah.
Whatever, attribute a grammar is so our convenient way to tell us what to do with each step.
So let's do scope checking.
First, we use a data structure called a symbol table.
A symbol table is a map of symbol entries within a hierarchical scope or the basic scope model of your language,
which is typically hierarchical.
We start scope checking.
We're going down top-down in the syntax tree, so naturally we're going to start the global scope.
When a symbol is declared, we save it in a symbol table within the current scope.
It's already declared within the current scope we throw in error.
We encounter a function or something else that has a nested scope to make this our current scope.
Now whenever we encounter a symbol being used, we walk up the symbol table scope hierarchy in search of its declaration.
This will tell us if it's been declared, if it hasn't, we'll throw an error.
And also tell us what scope the symbol tables used in.
A part of the table entry is being used at each use of a symbol would make a fine, a fine contribution to the attributed syntax tree.
See what I'm getting at here? See what I'm saying?
Other information might trickle down the syntax tree like function return type, for instance.
And everything else in your L attribute is grammar.
That's enough of that.
Now let's talk synthesized attributes.
So we have a context-free grammar, e produces e times e, or e plus e, or id.
Let's take the following source code, 3 times 2 plus 5.
Imagine the tree that this produces, e on the top branching to e plus e on the left branching to e times e and so on.
Now let's take the s attribute grammar, e value equals e one value times e two value, whereas e produces e times e.
We do the same thing for e plus e, but we add values together.
And finally, we have e value equals id value, where e produces id.
First thing we do is check our frontier for syntax tree, which is id times e, id plus id.
We expand on that using e value equals id value, and we find that e value 3 times e value 2 plus e value 5.
We expand e value equals e one value times e two value, and find that e value equals 6 plus e value equals 5.
Finally, we expand on e value equals e one value plus e two value, and we find that at the very top e value equals 11.
We usually throw a type checker if the left node type of a parent operator does not match the right type.
That isn't always the case, of course.
Depending on your language, there might be room to add another node in the tree or type conversion between parent and child, for instance.
The funny thing about synthesized attributes is we can actually compute them at the same time as we do parsing, since we're building the syntax tree bottom up.
Just thought I'd throw that in there.
There's a logical separation between parsing and semantic analysis, and that's important to conceptualize, but practice the lines may be blurred just a little bit.
The goal of everything we talk about so far is to verify that source code matches our language description.
Almost as a byproduct of this verification, we generate attribute syntax tree and symbol tables.
Everything we've done so far is what's called front end of the compiler.
In the next episode, we're going to start talking about the backend of a compiler.
So, that's it. Thanks for listening to this episode, and I look forward to talking with you a bit later about the backend of a compiler.
Take care. Bye-bye.
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