# Overview of `erg

This section introduces the function of each layer and especially important functions and methods.

## 1. Lexical analysis

* `Lexer` performs lexical analysis. `Lexer::next` (`Lexer` is implemented as an iterator) handles the main logic of lexical analysis. `Token` is output as a result of parsing.

## 2. Parsing

* `Parser` performs the parsing. Especially important is `Parser::parse_expr`. The result of parsing is `AST`, a collection of `ast::Expr`.

## 3. Desugaring

* `Desugarer` performs desugaring. An `AST` is output.

## 4. Type checking/type inference

* `ASTLowerer` performs typing. Type checking is mainly done by `Context`. Of particular importance are `Context::supertype_of` (to determine subtype relationships), `Context::unify/sub_unify` (to unify/semi-unify type variables) and `Context::init_builtin_*` (to define built-in API). The `HIR` is output as a result of the analysis.

## 5. Side Effect Check

* `SideEffectChecker` does this.

## 6. Ownership check

* Performed by `OwnershipChecker`.

## 7. Bytecode generation

* `CodeGenerator` converts `HIR` to `CodeObj`. The `CodeObj` holds the bytecode and execution settings. Especially important is `CodeGenerator::compile_expr`.

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* All the above processes are put together by `Compiler` as a facade.
* Execution of the generated bytecode is of course done by Python, which is called by `DummyVM`.