mkast 2.0.1

AST generator

mkast

What is it?

mkast is a CLI tool that generates an AST data structure from a language-agnostic description expressed in YAML.

Usage

Latest schema URLs:

Input schema

Config schema

The schemas are also shipped in the package and can be imported from there

Installing

The easiest way is to install mkast is from PyPI using pip:

pip install mkast

Running

./ast.py < input.yml

First, import the library.

import mkast

Input

The input is a YAML file containing one or two documents.

If the input contains two documents, the first document is considered configuration; the second document is considered as the actual input.

An AST is expressed as a tree of nodes. There are two kinds of nodes: product nodes, having properties, are product types, while union nodes represent the sum type of other nodes.

Configuration

Configuration can be specified at four levels.

1. Individual options
2. First document of a bi-document input
3. -c option
4. Defaults

Each level takes precedence over the ones below.

Configuration options:

name	type	default value	description
known_types	array of identifier	[]	Types to always consider defined
root	identifier		If defined, adds an outer union node wrapping everything
common_props	map of identifier → identifier	{}	Common properties present in every product node
target	"csharp" or "agnostic"	(value is required)	Output languages.
namespace	identifier		Namespace or package name
assert	code snippet		Expands to an assertion statement. $1 is replaced by the boolean expression to assert
imports	array of identifier	[]	Importations to add to the top of the file
union	snippet	depends on target language	Expands to the declaration of an union node. $1 is replaced by the name of the node.
product	snippet	depends on the target language	Expands to the declaration of an product node. $1 is replaced by the name of the node.
modifiers	map of modifier char → modifier (see dedicated section)	{}	Modifiers are used to hook into the type names and expressions emitted.

Modifiers:

name	char	description
one	(empty)	Implicit. Used to wrap every type or apply an invariant everywhere.
optional	?	Optional element
one or more	+	Non-empty list of elements
zero or more	*	List of elements

Modifier code snippets (all optional):

name	expands to	arguments
type	The type name	$1 is replaced by the payload type.
must	A boolean expression that validates the value. Used in combination with assertions.	$1 is replaced by the name of the variable to check.
none_when	A boolean expression that indicates when it is invalid to unwrap the value.	$1 is replaced by name of the variable to check.
unwrap	An expression that yields the payload value. Or, if there are multiple payload values (such as for lists), a boolean expression that is true when all the values satisfy $2.	$1 is replaced by the name of the variable to unwrap. $2, if present, is replaced by the payload validation boolean expression (based on $1).

Features

Properties

Properties have a type which is checked to exist.

Types marked as Do Not Touch (by prefixing them with an equal sign =) are not checked and their casing is not altered.

Multiple ?, +, and * suffixes can be appended for optionals, non-empty lists and lists respectively.

Casing

The casing of identifiers is altered to match the conventions of the target language.

In the agnostic language, snake_case is used and expected as input.

TODO

• merge complex configs values from different sources

  • imports
  • modifiers

• instead of visiting on the fly, build a data structure and revisit. this means we'll be able to query the properties and subnodes of a node when generating it, which will allow for:

  • smarter code generation (semi-colon body)
  • mermaid class diagram target
  • csharp: only strictly necessary interfaces in base type list (currently all parents are added)

• union node properties: currently, we decide to make a product node if it contains properties (which is why we need root in config to wrap everything in an interface). this means we cannot have interfaces with properties. solution: use '*' for products and '+' for sum types

• account for the empty modifier in csharp.py (currently it seems to be ignored)

• Modifier stack-ability (C# nullable reference types cannot be stacked)

• Multi-document input support

• Optional root

• Customize target (current config attr is not read)

• Target-specific options

• Configure modifiers (config is read (untested), but not used)

• read from config file so options can be persisted on a per-project basis

• C# target:

  • Custom namespace
  • Custom common properties
  • Custom root type

• Known type option

• Use argparse