kompact-io-landing/content/drafts/unpicking-aiken-air.md

Aims:

• Describe the pipeline, and components getting from aiken to uplc.

Preface

Aiken is undergoing active development. This post was started Aiken ~v1.14. With Aiken v1.15, there were already reasonably significant changes to the compilation pipeline. The word is that there aren't as big changes in the near future, but this article will undoubtably begin to diverge from the current codebase even before publishing.

Aiken build

Tracing aiken build, the pipeline is roughly something like:

  .               -> Project::read_source_files -> 
  Vec<Source>     -> Project::parse_sources ->
  ParsedModules   -> Project::type_check ->
  CheckedModules  -> CodeGenerator::build ->  
  AirTree         -> AirTree::to_vec -> 
  Vec<Air>        -> CodeGenerator::uplc_code_gen -> 
  Program / Term<Name> -> serialize -> 
  .

We'll pick our way through these steps

At a high level we are trying to do something straightforward: reformulate aiken code as uplc. Some aiken expressions are relatively easy to handle for example an aiken Int goes to an Int in uplc. Some aiken expressions require more involved handling, for example an aiken If... If Else... Else must have the branches "nested" in uplc.

The Preamble

cli handling

The cli enters at aiken/src/cmd/mod.rs which parses the command. With some establishing of context, the program enters Project::build (crates/aiken-project/src/lib.rs), which in turn calls Project::compile.

File crawl

The program looks for aiken files in both ./lib and ./validator subdirs. For each it walks over all contents (recursively) looking for .ak extensions. It treats these two sets of files a little differently. Only validator files can contain the special validator functions.

Parse and Type check

Project::parse_sources parses the module source code. The heavy lifting is done by aiken_lang::parser::module, which is evaluated on each file. It produces a Module containing a list of parsed definitions of the file: functions, types etc, together with "metadata" like docstrings and the file path.

Project::type_check inspects the parsed modules and, as the name implies, checks the types. It flags type level warnings and errors. It constructs a hash map of CheckedModules.

Code generator

The code generator CodeGenerator (aiken-lang/src/gen_uplc.rs) is given the definitions found from the previous step, together with the plutus builtins. It has additional fields for things like debugging.

This is handed over to a Blueprint (aiken-project/src/blueprint/mod.rs). A blueprint does little more than find the validators on which to run the code gen. The heavy lifting is done by CodeGenerator::generate.

We are now ready to take the source code and create plutus.

Up in the air

Things become a bit intimidating at this point in terms of sheer lines of code: gen_uplc.rs and three modules in gen_uplc/ totals > 8500 LoC.

Aiken has its own intermediate representation called air (as in Aiken Intermediate Representation). These are common in compiled languages. Air is defined in aiken-lang/src/gen_uplc/air.rs. Unsurprisingly, it looks little bit like a language between aiken and plutus.

In fact, Aiken has another intermediate representation: AirTree. This is constructed between the TypedExpr and Vec<Air> ie between parsed aiken and air.

AirTree

Within CodeGenerator::generate, CodeGenerator::build is called on the function body. This constructs and returns an AirTree. More on what an airtree is and its construction below. At the same time self is treated as mut, so we need to keep an eye on this too. The method which is called and uses this mutability of self is self.assignment. It does so by

  self.assignment >> self.expect_type_assign >> self.code_gen_functions.insert

and thus is creating a hashmap of all the functions that appear in the definition. (self.handle_each_clause is also called with mut which in turn calls self.build for which mut it is needed. self.clause_pattern is called with mut but it isn't used.)

Codegen assignment

~200 LoC

Codegen expect type assign

~400 LoC

... Back to build

Validators in aiken are boolean functions while in uplc they are unit-valued (aka void-valued) functions. Thus the airtree is wrapped such that false results in an error (wrap_validator_condition). (Ed: I don't know why there is a prevailing thought that boolean functions are preferable than functions that simply error if anything is wrong.)

check_validator_args again extends the airtree from the previous step, and again calls self.assignment mutating self. Something interesting is happening here. Script context is the final argument of a validator - for any script purpose. check_validator_args treats the script context like it is an unused argument. We'll circle back to how this works later on.

Next we encounter

  AirTree::no_op().hoist_over(validator_args_tree);

Its not very apparent why we need to do this. Let's look ahead and consider this later.

The final airtree to step(s) are in self.hoist_functions_to_validator. TODO: What happens here?!

Note that AirTree and its methods aren't fully typesafe. For example hoist_over will throw an error if called on an Expression. As AirTree is for internal use only, the scope for potential problems is reasonably contained.

The AirTree has the following definition

pub enum AirTree {
    Statement {
        statement: AirStatement,
        hoisted_over: Option<Box<AirTree>>,
    },
    Expression(AirExpression),
    UnhoistedSequence(Vec<AirTree>),
}

We can see it has a tree-like structure, as the name suggests.

AirExpression has multiple constructors. These include (non-exhaustive)

• air primitives (including all the ones that appear in plutus)
• constructors Call and Fn to handle functions
• binary and unary operators
• handling when and if
• error and tracing

AirStatement also has multiple constructors.

for handling functions, plutus primitives, along with An AirStatement`

Down to uplc

Air

Aiken compiles aiken code to uplc via air: Aiken Intermediate Representation.

Trace

Running aiken build...

The cli (See aiken/src/cmd/mod.rs) parses the command, finds the context and calls Project::build (crates/aiken-project/src/lib.rs), which in turn calls Project::compile.

Project::compile

1. Check dependencies are available eg aiken stdlib.
2. Read source files.
3. Walk over ./lib and ./validators and push aiken modules onto Project.sources.
4. Parse each source in sources:
5. Generate a ParsedModule containing the ast, docs, etc. The ast here is an UntypedModule, which contains untyped definitions.
6. Type check each parsed module.
7. For each untyped module, create a CheckedModule. This includes typed definitions.
8. compile forks into two depending on whether it's been called with build or check.
9. From CheckModules construct a CodeGenerator
10. Pass the generator to construct a new Blueprints.
11. Blueprints finds validators from checked modules.
12. From each it constructs a Validator with the constructor Validator::from_checked_module (which returns a vector of validators)
    1. Its here that the magic happens: The method generator.generate(def) is called, where def is the typed validator(s). This method outputs a Program<Name> which contains the UPLC.
    2. These are collected together.
13. The rest is collecting and handling the errors and warnings and writing the blueprint.

CodeGenerator::generate

1. Create a new AirStack.

AirStack

Consists of:

1. An Id
2. A Scope
3. A vector of Air

The Scope keeps track of ... [TODO]

Air

Air is a typed language... [TODO]