osh

osh is a javascript library that provides component-based model for generating text data.

Documentation

$3

In this tutorial we will create a code generator that generates type validation functions from simple schemas like this:

``js
const USER_SCHEMA = {
id: "number",
name: "string",
};
`

USER_SCHEMA is a schema that defines expected types for object properties. With this schema, we would like to
generate validation function that look like this:

`js
function validateUser(user) {
let errors;
let type;

type = typeof user.id;
if (type !== "number") {
if (errors === undefined) {
errors = [];
}
errors.push({ prop: "id", type: type });
}

type = typeof user.name;
if (type !== "string") {
if (errors === undefined) {
errors = [];
}
errors.push({ prop: "name", type: type });
}

return errors;
}
`

Full source for this tutorial is available here.

#### Install dependencies

To build codegenerator we will use four packages:

- osh package provides basic building blocks for text generation and a string renderer.
- osh-text package provides general purpose text utilities.
- osh-code package provides generic components that can be used in many different
programming languages (line(), indent(), comment() etc).
- osh-code-js package provides javascript specific components and a preset that
sets up osh-code environment.
- incode package will be used for injecting generated code.

`sh
$ npm i --save osh osh-text osh-code osh-code-js incode
`

#### Set up environment

First thing that we need to do is set up osh-code environment for javascript code generation.
osh-code-js provides a configurable jsCode preset that sets up codegen
environment for javascript.

`js
import { renderToString } from "osh";
import { jsCode } from "osh-code-js";

function emit(...children) {
return renderToString(
jsCode(
{
lang: "es2016",
},
children,
),
);
}
`

emit function will take any osh nodes and render them inside a javascript codegen environment.

#### Function declaration

Let's start from generating a function declaration for our validate function:

`js
function validateUser() {
return;
}
`

To generate this declaration we will use three different components: capitalize, line and indent.

line(...children) and indent(...children) are basic components and capitalize(...children) is a transformer. The
main difference between components and transformers is that transformers perform transformation step on a final
string, and components are producing another components and strings.

indent(...children) component increases indentation level.

line(...children) component wraps children into a line.

capitalize(...children) transformer converts first character to uppercase.

`js
import { capitalize } from "osh";
import { line, indent } from "osh-code";

function validateFunction(name, schema) {
return [
line("function validate", capitalize(name), "() {"),
indent(
line("return;"),
),
line("}"),
];
}
`

#### Function arguments

One of the most common problems when automatically generating code is preventing name collisions for symbols.

osh-code package has a scope component that solves this problem. With scope component we can define symbols with
automatic conflict resolution for colliding symbol names. jsCode() environment automatically registers all reserved
keywords as scope symbols, so symbols like for will be automatically renamed to prevent invalid javascript code
generation.

When defining a scope, there are three mandatory properties: type, symbols and children.

- type is a scope type, it is used for symbol lookups when there are different symbols associated with the same key.
- symbols is an array of symbols, all symbols should be created with declSymbol(key, symbol) factory.
- children is an array of osh nodes.

There are two ways to retrieve symbols from the current scope:

getSymbol(context, type, key) is a low-level function that retrieves symbol from the context provided as a first
argument.

sym(type, key) is a component that will retrieve symbol from the current context.

`js
import { scope, declSymbol, sym } from "osh-code";

const ARGUMENTS = Symbol("Arguments");

function arg(name) {
return sym(ARGUMENTS, name);
}

function declArg(arg, children) {
return scope({
type: ARGUMENTS,
symbols: [declSymbol("data", arg)],
children: children,
});
}

function validateFunction(name, schema) {
return (
declArg(
name,
[
line("function validate", capitalize(name), "(", arg("data"), ") {"),
indent(
line("return;"),
),
line("}"),
],
)
);
}
`

Here we defined two functions: arg() and declArg(). arg() is a helper function that will retrieve symbols from
scopes with ARGUMENTS type. And declArg() will declare a name symbol with "data" key.

#### Local variables

In our validation function we are using two local variables: errors and type, so we should create another scope
and declare this variables.

`js
const LOCAL_VARS = Symbol("LocalVars");

function lvar(name) {
return sym(LOCAL_VARS, name);
}

function declVars(vars, children) {
return scope({
type: LOCAL_VARS,
symbols: vars.map((name) => declSymbol(name, name)),
children: children,
});
}

function validateFunction(name, schema) {
return (
declArg(
name,
declVars(
["errors", "type"],
[
line("function validate", capitalize(name), "(", arg("data"), ") {"),
indent(
line("let ", lvar("errors"), ";"),
line("let ", lvar("type"), ";"),
line(),
line("return ", lvar("errors"), ";"),
),
line("}"),
],
),
)
);
}
`

#### Generating type checking code

Now we just need to generate type checking code for all fields.

`js
import { intersperse } from "osh-text";

function checkType(prop, type) {
return [
line(lvar("type"), " = typeof ", arg("data"), ".", prop),
line("if (", lvar("type"), " !== \"", type, "\") {"),
indent(
line("if (", lvar("errors"), " === undefined) {"),
indent(
line(lvar("errors"), " = [];"),
),
line("}"),
),
line("}"),
];
}

function validateFunction(name, schema) {
return (
declArg(
name,
declVars(
["errors", "type"],
[
line("function validate", capitalize(name), "(", arg("data"), ") {"),
indent(
line("let ", lvar("errors"), ";"),
line("let ", lvar("type"), ";"),
line(),
intersperse(
Object.keys(schema).map((prop) => checkType(prop, schema[prop])),
line(),
),
line(),
line("return ", lvar("errors"), ";"),
),
line("}"),
],
),
)
);
}
`

#### Injecting generated code into existing code

And the final step is to inject generated code into existing code. To inject generated code we will use incode
package, it is using different directives for defining injectable regions:

- chk:assign({ ... }) assigns a data in JSON format to the current scope.
- chk:emit(type) injects a code block into the region

`js
// chk:assign({ "schema": "user" })
// chk:emit("validate")
// chk:end

function getUser() {
const user = fetchUser();
const errors = validateUser(user);
if (errors !== undefined) {
throw new Error("Invalid user");
}
return user;
}
`

incode automatically detects line paddings for injectable regions, and we would like to use this information to indent
our generated code. To do this, we will need to change our emit() function and assign a padding value to a PADDING
symbol in the context.

`js
import * as fs from "fs";
import { context } from "osh";
import { PADDING } from "osh-code";
import { createDirectiveMatcher, inject } from "incode";
import { USER_SCHEMA } from "./schema";

const FILE = "./code.js";

const SCHEMAS = {
user: USER_SCHEMA,
};

function emit(padding, ...children) {
return renderToString(
context(
{
[PADDING]: padding === undefined ? "" : padding,
},
jsCode(
{
lang: "es2016",
},
children,
),
),
);
}

const result = inject(
fs.readFileSync(FILE).toString(),
createDirectiveMatcher("chk"),
(region) => {
if (region.type === "validate") {
const name = region.data.schema;
const schema = SCHEMAS[name];
return emit(region.padding, validateFunction(name, schema));
}

throw new Error(Invalid region type: ${region.type}.);
},
);

fs.writeFileSync(FILE, result);
`

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Components are declared with a simple functions that has two optional parameters ctx and props.

`js
function Component(ctx, props) {
return "text";
}
`

Component nodes are created with component function:

`ts
function component(fn: () => TChildren): ComponentNode;
function component(fn: (context: Context) => TChildren): ComponentNode;
function component(fn: (context: Context, props: T) => TChildren, props: T): ComponentNode;
function component(fn: (context: Context, props: any) => TChildren, props?: any): ComponentNode;
`

##### Example

`js
import { component, renderToString } from "osh";
import { line, indent } from "osh-code";

function Example(ctx, msg) {
return [
line("Example"),
indent(
line("Indented Text: ", msg),
),
];
}

console.log(renderToString(component(Example, "Hello")));
`

#### Context

Context is an immutable object that propagates contextual data to components.

Context nodes are created with context factory function:

`ts
function context(ctx: Context, children: TChildren): ContextNode;
`

##### Example

`js
import { component, context, renderToString } from "osh";

// Unique symbol to prevent name collisions
const VARS = Symbol("Vars");

// Def component will assign variables to the current context
function Def(ctx, props) {
return context(
{ [VARS]: { ...ctx[VARS], ...{ props.vars } } },
props.children,
);
}

function def(vars, ...children) {
return component(Def, { vars, children });
}

// V component will extract variable from the current context
function V(ctx, name) {
return ctx[VARS][name];
}

function v(name) {
return component(V, name);
}

function Example(ctx, msg) {
return line("Var: ", v("var"))];
}

console.log(
renderToString(
def(
{ "var": "Hello" },
component(Example),
),
),
);
`

#### Transformers

Transformer components perform transformations on rendered strings.

`ts
function transform(fn: (s: string) => string, ...children: TChildren[]): TransformNode;
function transform(fn: (s: string, context: Context) => string, ...children: TChildren[]): TransformNode;
``

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- osh-code provides a basic set of components for generating program code.
- osh-text provide general purpose text utilities.
- osh-code-go provides a basic set of components for generating Go program
code.
- osh-code-js provides a basic set of components for generating
Javascript(TypeScript) program code.
- osh-debug debug utilities.