Basic Stencil Module

NOTE

This quick start assumes you're familiar with stencil usage already. If you aren't be sure to go through the reference documentation or the quick start.

Step 1: Create a module

Using the stencil create command we're able to quickly create a module, let's start with a simple hello world module.

mkdir helloworld; cd helloworld
stencil create module github.com/yourorg/helloworld

You'll notice this created a stencil.yaml file and ran stencil on itself. This isn't used now, but will be in the future.

Not created for you is the templates/ directory. This is where your templates will live.

Step 2: Creating a Template

Let's create a template that creates a simple hello world message in Go. We'll start by creating a hello.go.tpl in the templates/ directory.

package main

func main() {
  fmt.Println("Hello, world!")
}

Step 3: Consuming the Module in an Application

Now that we've done that, how do we test it locally without CI'ing up a full build? This is super easy with the replacements map in a stencil.yaml.

Let's quickly create a test application:

mkdir testapp; cd testapp
cat >stencil.yaml <<EOF
name: testapp
modules:
  - name: github.com/yourorg/helloworld

replacements:
  # Replace ../helloworld with the path to your module.
  github.com/yourorg/helloworld: ../helloworld
EOF

Now if we run stencil on the test application, we should see the following:

testapp ❯ stencil
INFO stencil 0.9.0
INFO Fetching dependencies
INFO  -> github.com/yourorg/helloworld local
INFO Loading native extensions
INFO Rendering templates
INFO Writing template(s) to disk
INFO   -> Created hello.go
INFO Running post-run command(s)

It looks like it created hello.go for us! Let's validate:

testapp ❯ cat hello.go
package main

func main() {
  fmt.Println("Hello, world!")
}

🎉 We have a hello world application!

Step 4: Using a Block

Blocks are incredibly easy to use in Stencil.

NOTE

In case you don't remember, blocks are areas in your generated code that you'd like to persist across runs.

Let's create our own block in the hello.go template from earlier:

package main

func main() {
  fmt.Println("Hello, world!")

  // <<Stencil::Block(additionalMessage)>>
  {{- /* It's important to not indent the file.Block to prevent the indentation from being copied over and.. over again. */ }}
{{ file.Block "additionalMessage" }}
  // <</Stencil::Block>>
}

If we re-run stencil and look at hello.go we should see the following:

package main

func main() {
  fmt.Println("Hello, world!")
  // <<Stencil::Block(additionalMessage)>>

  // <</Stencil::Block>>
}

If we add contents to the block and re-run stencil they'll be persisted across the run!

Step 5: (Optional/Advanced) Creating Multiple Files

One of the powerful parts of stencil is the ability to create an arbitrary number of files with a single template. This is done with the file.Create function. Let's create a greeter.go.tpl template in the templates/ directory that'll create <greeting>.go based on the greetings argument.

# This is important! We do not want to create a greeter.go file
{{- $_ := file.Skip "Generates multiple files" }}

{{- define "greeter" -}}
{{- $greeting := .greeting }}
package main

func main() {
  fmt.Println("$greeting, world!")
}

{{- end -}}

{{- range $_, $greeting := stencil.Arg "greetings" }}

# Create a new $greeting.go file

{{- file.Create (printf "%s.go" $greeting) 0600 now }}

# We'll render the template greeter with $greeting as the values being passed to it

# Once we've done that we'll use the output to set the contents of the file we just created.

{{- stencil.ApplyTemplate "greeter" $greeting | file.SetContents }}
{{- end }}

NOTE

Blocks are supported in multiple files! When file.SetPath is called the host is searched to see if a file already exists at that path, if it does it is searched to see if it contains any blocks, if it does they are loaded and accessible via file.Block as normal

Now let's modify the manifest.yaml to accept the argument greetings:

arguments:
  greetings:
    description: A list of greetings to use
    required: true
    default: ["hello", "goodbye"]
    schema:
      type: arrary

If we now run stencil on the test application, we should see the following:

testapp ❯ stencil
INFO[0000] stencil v1.14.2
INFO[0000] Fetching dependencies
INFO[0002]  -> github.com/yourorg/helloworld local
INFO[0002] Loading native extensions
INFO[0002] Rendering templates
INFO[0002] Writing template(s) to disk
INFO[0002]   -> Created hello.go
INFO[0002]   -> Created goodbye.go

> `hello` and `goodbye` came from the default list of

greetings that was set in the manifest.yaml file. Setting arguments.greetings on the test application and see it change!

If we look at the files, we should see the following:

testapp ❯ cat hello.go
package main

func main() {
  fmt.Println("hello, world!")
}

testapp ❯ cat goodbye.go
package main

func main() {
  fmt.Println("goodbye, world!")
}

Reflection

We've created a module, used it in a test application via the replacements map in the stencil.yaml and used a block. Optionally we've also created multiple files with a template. This is just the beginning of what you can do with modules. Modules have a rich amount of functions available to them. Check out the reference for more information about modules and how to use them.