Cecil

An experimental templating library designed specifically for generating source code (especially for languages that aren’t as meta-programmable as Ruby).

Cecil templates closely resemble the target source code, making templates easier to write, read, and maintain.

Features

Write templates in plain Ruby

Call Cecil::Code.generate_string and pass it a block. Inside the block, add lines of code via backticks (or use src if you prefer). Cecil returns your generated source code as a string.

Example

model_code = Cecil::Code.generate_string do
  # Use backticks to add lines of code
  `import Model from '../model'`

  # Multi-line strings work, too.
  # Cecil preserves indentation.
  `class User extends Model {
    id: number
    name: string
    companyId: number | undefined
  }`

  # use #src if you prefer to avoid backticks
  src "export type Username = User['name']"
end

puts model_code

Returns:

import Model from '../model'
class User extends Model {
  id: number
  name: string
  companyId: number | undefined
}
export type Username = User['name']

Interpolate values with Cecil's low-noise syntax

Use #[] on the backticks to replace placeholders with actual values.

By default, placeholders start with $ and are followed by an identifier.

Positional arguments match up with placeholders in order. Named arguments match placeholders by name.

Example

field = "user"
types = ["string", "string[]"]
default_value = ["SilentHaiku", "DriftingSnowfall"]
field_class = "Model"

Cecil::Code.generate_string do
  # positional arguments match placeholders by position
  `let $field: $FieldType = $default`[field, types.join('|'), default_value.sort.to_json]

  # named arguments match placeholders by name
  `let $field: $FieldClass<$Types> = new $FieldClass($default)`[
    field: field,
    FieldClass: field_class,
    Types: types.join('|'),
    default: default_value.sort.to_json
  ]
end

Returns:

let user: string|string[] = ["DriftingSnowfall","SilentHaiku"]
let user: Model<string|string[]> = new Model(["DriftingSnowfall","SilentHaiku"])

"Doesn't Ruby already have string interpolation?"

Yes, but compare the readability of these two approaches:

`let $field: $FieldClass<$Types> = new $FieldClass($default)`[
  field: field,
  FieldClass: field_class,
  Types: types.join('|'),
  default: default_value.sort.to_json
]

# vs

field_types = types.join('|'),
default_json = default_value.sort.to_json
"let #{field}: #{field_class}<#{field_types}> = new #{field_class}(#{default_json})"

Indents code blocks & closes brackets automatically

A block passed to #[] gets indented and open brackets get closed automatically.

Example

model = "User"
field_name = "name"
field_default = "Unnamed"

Cecil::Code.generate_string do
  `class $Class extends Model {`[model] do
    # indentation is preserved
    `id: number`

    `override get $field() {`[field_name] do
      `return super.$field ?? $defaultValue`[field_name, field_default.to_json]
    end
  end # the open bracket from `... Model {` gets closed with "}"
end

Returns:

class User extends Model {
    id: number
    override get name() {
        return super.name ?? "Unnamed"
    }
}

Emit source code to other locations

When generating source code, things like functions, parameters, classes, etc, often need to be declared, imported, or otherwise setup before being used.

content_for can be used to add content to a different location of your file.

Call content_for(some_key) { ... } with key and a block to store content under the key you provide. Call content_for(some_key) with the key and no block to insert your stored content at that location.

Example

models = [
  { name: 'User', inherits: 'AuthModel' },
  { name: 'Company', inherits: 'Model' },
]

Cecil::Code.generate_string do
  # insert content collected for :imports
  content_for :imports

  models.each do |model|
    ``
    `class $Class extends $SuperClass {`[model[:name], model[:inherits]] do
      `id: number`
    end

    content_for :imports do
      # this gets inserted above
      `import $SuperClass from '../models/$SuperClass'`[SuperClass: model[:inherits]]
    end

    content_for :registrations do
      # this gets inserted below
      `$SuperClass.registerAncestor($Class)`[model[:inherits], model[:name]]
    end
  end

  ``
  # insert content collected for :registrations
  content_for :registrations
end

Returns:

import AuthModel from '../models/AuthModel'
import Model from '../models/Model'

class User extends AuthModel {
    id: number
}

class Company extends Model {
    id: number
}

AuthModel.registerAncestor(User)
Model.registerAncestor(Company)

Collect data as you go then use it earlier in the document

The #defer method takes a block and waits to call it until the rest of the template is evaluated. The block's result is inserted at the location where #defer was called.

This gives a similar ability to #content_for, but is more flexible because you can collect any kind of data, not just source code.

Example

models = [
  { name: 'User', inherits: 'AuthModel' },
  { name: 'Company', inherits: 'Model' },
  { name: 'Candidate', inherits: 'AuthModel' },
]

Cecil::Code.generate_string do
  superclasses = []

  defer do
    # This block gets called after the rest of the parent block is finished.
    #
    # By the time this block is called, the `superclasses` array is full of data
    #
    # Even though this block is called later, the output is added at the location where `defer` was called
    `import { $SuperClasses } from '../models'`[superclasses.uniq.sort.join(', ')]
    ``
  end

  models.each do |model|
    superclasses << model[:inherits] # add more strings to `superclasses`, which is used in the block above

    `class $Class extends $SuperClass {}`[model[:name], model[:inherits]]
  end
end

Returns:

import { AuthModel, Model } from '../models'

class User extends AuthModel {}
class Company extends Model {}
class Candidate extends AuthModel {}

Customizable syntax and behaviors

Easily customize the following features to make Cecil suit your needs/preferences:

• placeholder syntax
• auto-closing brackets
• indentation

Customizations are performed by subclassing Cecil::Code and overriding the relevant methods.

For example, Cecil comes with Cecil::Lang::TypeScript that you can use instead of of Cecil::Code. It has a few JavaScript/TypeScript-specific customizations. It's a subclass of Cecil::Code so it can be used the same way:

Cecil::Lang::TypeScript.generate_string do
  # ...
end

Use cases

Things I've personally used Cecil to generate:

• serialization/deserialization code generated from from specs (e.g. OpenAPI)
• diagrams (e.g. Mermaid, PlantUML, Dot/Graphviz)
  • ERDs/schemas
  • state machine diagrams
  • graphs
  • data visualizations
• state machines generated from a list of states and transitions
• test cases generated from data that describes inputs/setup and expected outputs; because parameterized tests can be very hard to debug
• complex types because meta-programming in TypeScript can get complex quickly

Quick Reference

Reference documentation is on RubyDoc.info: gem | repo

Calling Cecil

Call Cecil::Code.generate / generate_string with a block and inside the block, use backticks or #src to emit lines of source code. E.g.

# returns a string
Cecil::Code.generate_string do
  `function greet() {}`
  `function respond() {}`
end

# outputs to $stdout
Cecil::Code.generate do
  `function greet() {}`
  `function respond() {}`
end

See: Methods available inside a Cecil block

Emitting source code

• backticks/#``/#src emit source code. E.g.:

  Cecil::Code.generate_string do
    `function greet() {}`
    `function respond() {}`
    src "function ask() {}"
  end
  # outputs:
  # function greet() {}
  # function respond() {}
  # function ask() {}
  

• #[] interpolates data into placeholders. E.g.

  Cecil::Code.generate_string do
    `function $fn() {}`["greet"]
    `function $fn() {}`[fn: "respond"]
  end
  # outputs:
  # function greet() {}
  # function respond() {}
  

• #[]{ ... } given a block, interpolates and indents the code emitted in its block. E.g.

  Cecil::Code.generate_string do
    `function $fn() {`["greet"] do
      `console.log("hello")`
    end
  end
  # outputs:
  # function greet() {
  #     console.log("hello")
  # }
  

• #<< adds code the last line of the block. E.g.

  Cecil::Code.generate_string do
    `(function ${fn}Now() {`["greet"] do
      `console.log("hello")`
    end << ')()'
  end
  # outputs:
  # (function greetNow() {
  #     console.log("hello")
  # })()
  

• #content_for emits source code to different locations

• #defer waits to emit the given source until after data has been gathered

Customizing behavior for the language of the source code you're generating

Many of Cecil's defaults can be customized by creating a subclass of Cecil::Code and overriding methods to customize syntax and behavior of:

• placeholder syntax
• indentation
• auto-closing brackets

Currently, Cecil comes with:

• Cecil::Code for generic code
• Cecil::Lang::TypeScript for JavaScript and TypeScript
• Cecil::Lang::Rust for Rust

Auto-closing brackets

Customize which opening brackets are auto-closed by overriding Cecil::Code#block_ending_pairs in a subclass.

When nesting code blocks with #[] { ... }, open brackets at the end of the string get closed automatically.

For example, notice how we don't have to manually provide a closing } in the following:

`$var = {`[var: "user"] do
  `id: 42`
end

becomes

user = {
    id: 42
}

Multiple brackets

Every consecutive closing bracket at the end of the string gets closed. E.g.

`$var = [{(`[var: "user"] do
  `id: 42`
end

becomes

user = ([{
    id: 42
}])

Currently, the algorithm is simplistic, so open brackets that aren't at the end of the string will not get closed.

In this example, the ( in test( needs to be closed manually:

`test("getter $fn", () => {`[fn: 'getUsername'] do
  `assert(false)`
end << `)`

test("getter getUsername", () => {
    assert(false)
})

Placeholder syntax

Default placeholder rules:

• start with $-- e.g. $foo
• named can contain alpha-numeric and underscore characters-- e.g. $foo_bar123
• names can optionally be surrounded by brackets -- e.g ${my_placeholder}, $[my_placeholder], $<my_placeholder>, or $(my_placeholder)

Surrounding with brackets can be useful to separate a placeholder from subsequent characters that would otherwise get parsed as a placeholder.

E.g. function ${fn}Sync()-- without curly brackets $fnSync would be the placeholder.

Customize placeholder syntax by subclassing Cecil::Code and overriding placeholder-related methods.

Helper methods

If you use your generator frequently it can be helpful to define reusable helper methods on a subclass of Cecil::Code.

For example, the Cecil::Lang::TypeScript]{Lang::TypeScript subclass defines several [helper methods} for generating TypeScript code.

Installation

From your shell:

bundle add cecil

In your Gemfile like:

gem 'cecil'

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/nicholaides/cecil.

License

The gem is available as open source under the terms of the MIT License.