add codegen post

_posts/2023/2023-10-02-exploring-codegen.md

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+---
+title: "Exploring Code Generation with JsonSchema.Net.CodeGeneration"
+date: 2023-10-02 09:00:00 +1200
+tags: [json-schema, codegen]
+toc: true
+pin: false
+---
+
+About a month ago, my first foray into the world of code generation was published with the extension library JsonSchema.Net.CodeGeneration.  For this post, I'd like to dive into the process a little to show how it works.  Hopefully, this will give better insight on how to use it as well.
+
+This library currently serves as an exploration platform for the [JSON Scheam IDL Vocab](https://github.com/json-schema-org/vocab-idl/issues/47) work, which aims to create a new vocabulary designed to help support translating between code and schemas (both ways).
+
+## Extracting type information
+
+The first step in the code generation process is determining what the schema is trying to model.  This library uses a complex set of mini-meta-schemas to identify supported patterns.
+
+> A meta-schema is just a schema that validates another schema.
+{: .prompt-tip }
+
+For example, [in most languages](https://github.com/json-schema-org/vocab-idl/issues/43), enumerations are basically just named constants.  The ideal JSON Schema representation of this would be a schema with an `enum`.  So .Net's `System.DayOfWeek` enum could be modelled like this:
+
+```json
+{ 
+  "title": "DayOfWeek",
+  "enum": [ "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" ]
+}
+```
+
+To identify this schema as defining an enumeration, we'd need a meta-schema that looks like this:
+
+```json
+{
+  "type": "object",
+  "title": "MyEnum",
+  "properties": {
+    "enum": {
+      "type": "array"
+    }
+  },
+  "required": [ "enum" ]
+}
+```
+
+However, in JSON Schema, an `enum` item can be _any_ JSON value, whereas most languages require strings.  So, we also want to ensure that the values of that `enum` are strings.
+
+```json
+{
+  "type": "object",
+  "title": "MyEnum",
+  "properties": {
+    "enum": {
+      "items": { "type": "string" }
+    }
+  },
+  "required": [ "enum" ]
+}
+```
+
+> We don't need to include `type` or `uniqueItems` because we know the data is a schema, and its meta-schema (e.g. Draft 2020-12) already has those constraints.  We only need to define constraints _on top of_ what the schema's meta-schema defines.
+{: .prompt-info }
+
+Now that we have the idea, we can expand this by defining mini-meta-schemas for all of the patterns we want to support.  There are some that are pretty easy, only needing the `type` keyword:
+
+- string
+- number
+- integer
+- boolean
+
+And there are some that are a bit more complex:
+
+- arrays
+- dictionaries
+- custom objects (inheritable and non-inheritable)
+
+And we also want to be able to handle references.
+
+The actual schemas that were used are listed in the [docs](https://docs.json-everything.net/schema/codegen/mini-meta-schemas/).  As with any documentation, I hope to keep these up-to-date, but short of that, you can always look at the [source](https://github.com/gregsdennis/json-everything/blob/master/JsonSchema.CodeGeneration/Model/ModelGenerator.cs).
+
+## Building type models
+
+Now that we have the different kinds of schemas that we want to support, we need to represent them in a sort of type model from which we can generate code.
+
+The idea behind the library was to be able to generate multiple code writers that could support just about any language, so .Net's type system (i.e. `System.Type`) isn't quite the right model.
+
+The type model as it stands has the following:
+
+- `TypeModel` - Serves as a base class for the others while also supporting our simple types.  This basically just exposes a type name property.
+- `EnumModel` - Each value has a name and an integer value derived from the item's index.
+- `ArrayModel` - Exposes a property to track the item type.
+- `DictionaryModel` - Exposes properties to track key and item types.
+- `ObjectModel` - Handles both open and closed varieties.  Each property has a name, a type, and whether it can read/write.
+
+Whenever we encounter a subschema or a reference, that represents a new type for us to generate.
+
+Lastly, in order to avoid duplication, we set up some equality for type models.
+
+With this all of the types supported by this library can be modelled.  As more patterns are identified, this modelling system can be expanded as needed.
+
+## Writing code
+
+The final step for code generation is the part everyone cares about: actually writing code.
+
+The library defines `ICodeWriter` which exposes two methods:
+
+- `TransformName()` - Takes a JSON string and transforms it into a language-compatible nme.
+- `Write()` - Renders a type model into a type declaration in the language.
+
+There's really quite a bit of freedom in how this is implemented.  The [built-in C# writer](https://github.com/gregsdennis/json-everything/blob/master/JsonSchema.CodeGeneration/Language/CSharpCodeWriter.cs) branches on the different model types and has private methods to handle each one.
+
+One aspect to writing types that I hadn't thought about when I started writing the library was that there's a difference between writing the usage of a type and writing the declaration of a type.  Before, when I thought about code generation, I typically thought it was about writing type declarations: you have a schema, and you generate a class for it.  But what I found was that if the properties of an object also use any of the generated types, only the type name needs to be written.
+
+For example, for the `DayOfWeek` enumeration we discussed before, the declaration is
+
+```c#
+public enum DayOfWeek
+{
+    Sunday,
+    Monday,
+    Tuesday,
+    Wednesday,
+    Thursday,
+    Friday,
+    Saturday
+}
+```
+
+But if I have an array of them, I need to generate `DayOfWeek[]`, which only really needs the type name.  So my writer needs to be smart enough to write the declaration once and write just the name any time it's used.
+
+There are a couple of other little nuance behaviors that I added in, and I encourage you to read the [docs](https://docs.json-everything.net/schema/codegen/schema-codegen/) on the capabilities.
+
+## Generating a conclusion
+
+Overall, writing this was an enjoyable experience.  I found a simple architecture that seems to work well and is also extensible.
+
+My hope is that this library will help inform the IDL Vocab effort back in JSON Schema Land.  It's useful having a place to test things.