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Write a high-level introduction for the SPA series

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Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>
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Danila Fedorin 2024-05-12 13:49:21 -07:00
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---
title: "Implementing and Verifying \"Static Program Analysis\" in Agda, Part 1: Lattices"
title: "Implementing and Verifying \"Static Program Analysis\" in Agda, Part 0: Intro"
series: "Static Program Analysis in Agda"
date: 2024-03-11T14:23:03-07:00
date: 2024-04-12T14:23:03-07:00
draft: true
---
@ -22,13 +22,70 @@ theory were formalizing their results in
Having [played]({{< relref "meaningfully-typechecking-a-language-in-idris" >}})
[with]({{< relref "advent-of-code-in-coq" >}}) [dependent]({{< relref "coq_dawn_eval" >}})
[types]({{< relref "coq_palindrome" >}}) before, I was excited to try it out.
Thus began my journey to formalize the (first few chapters of) _Static Program Analysis_
Thus began my journey to formalize (the first few chapters of) _Static Program Analysis_
in Agda.
In this post (and subsequent follow-ups), I hope to describe what I have so far.
Although not everything works, formalizing fixed-point algorithms (I'll get back
to this) and getting even the most straightforward analysis working was no
joke, and I think is worthy of discussion. So, here goes.
In all, I built a framework for static analyses, based on a tool
called _motone functions_. This framework can be used to implement and
reason about many different analyses (currently only a certain class called
_forward analyses_, but that's not hard limitation). Recently, I've proven
the _correctness_ of the algorithms my framework produces. Having reached
this milestone, I'd like to pause and talk about what I've done.
In subsequent posts in this series, will describe what I have so far.
It's not perfect, and some work is yet to be done; however, getting to
this point was no joke, and I think it's worth discussing. In all,
I envision three major topics to cover, each of which is likely going to make
for a post or two:
* __Lattices__: the analyses I'm reasoning about use an algebraic structure
called a _lattice_. This structure has certain properties that make it
amenable to describing degrees of "knowledge" about a program. In
lattice-based static program analysis, the various elements of the
lattice represent different facts or properties that we know about the
program in question; operations on the lattice help us combine these facts
and reason about them.
Interestingly, lattices can be made by combining other lattices in certain
ways. We can therefore use simpler lattices as building blocks to create
more complex ones, all while preserving the algebraic structure that
we need for program analysis.
* __The Fixed-Point Algorithm__: to analyze a program, we use information
that we already know to compute additional information. For instance,
we might use the fact that `1` is positive to compute the fact that
`1+1` is positive as well. Using that information, we can determine the
sign of `(1+1)+1`, and so on. In practice, this is often done by calling
some kind of "analyze" function over and over, each time getting closer to an
accurate characterization of the program's behavior. When the output of "analyze"
stops changing, we know we've found as much as we can find, and stop.
What does it mean for the output to stop changing? Roughly, that's when
the following equation holds: `knownInfo = analyze(knownInfo)`. In mathematics,
this is known as a [fixed point](https://mathworld.wolfram.com/FixedPoint.html).
Crucially, not all functions have fixed points; however, certain types of
functions on lattices do. The fixed-point algorithm is a way to compute these
points, and we will use this to drive our analyses.
* __Correctness__: putting together the work on lattices and the fixed-point
algorithm, we can implement a static program analyzer in Agda. However,
it's not hard to write an "analyze" function that has a fixed point but
produces an incorrect result. Thus, the next step is to prove that the results
of our analyzer accurately describe the program in question.
The interesting aspect of this step is that our program analyzer works
on [control-flow graphs](https://en.wikipedia.org/wiki/Control-flow_graph) (CFGs),
which are a relatively compiler-centric representation of programs. On the
other hand, what the language _actually does_ is defined by its
[semantics](https://en.wikipedia.org/wiki/Semantics_(computer_science)),
which is not at all compiler-centric. We need to connect these two, showing
that the CFGs we produce "make sense" for our language, and that given
CFGs that make sense, our analysis produces results that match the language's
execution.
{{< todo >}}
Once the posts are ready, link them here to add some kind of navigation.
{{< /todo >}}
### Monotone Frameworks