go-generator-cli

A golang command line utility for generating files from templates. Can be used to scaffold application code.

Command line parameters

--generator=<path> to set the path of the generator base directory.
--target=<path> to set the path of the target directory.
--create[=<generator name>] to write a specfile with defaults. If the generator name is omitted, it defaults to main, and the file written will be called generated-main.yaml.
--render[=<specfile>] to use a specfile to render. If the filename is omitted, it defaults to generated-main.yaml.

You will need to specify exactly one of --create and --render, while the other two arguments are mandatory.

Generators

A generator is a directory that contains one or more generator-*.yaml files, called generator specification files, plus a number of golang text/templates. The main generator spec is usually called generator-main.yaml.

Example:

templates:
  - source: 'src/sub/sub.go.tmpl'
    target: 'sub/sub.go'
  - source: 'src/main.go.tmpl'
    target: 'main.go'
  - source: 'src/some.text.file.copied.verbatim'
    target: 'text.txt'
    just_copy: true
  - source: 'src/web/controller.go.tmpl'
    target: 'web/controller/{{ .item }}.go'
    condition: '{{ if eq .item "skipped" }}false{{ end }}'
    with_items:
     - health
     - reservations
     - skipped
variables:
  serviceUrl:
    description: 'The URL of the service repository, to be used in imports etc.'
    default: 'github.com/StephanHCB/temp'
  serviceName:
    description: 'The name of the service to be rendered.'
    pattern: '^[a-z-]+$'
  helloMessage:
    description: 'A message to be inserted in the code.'
    default: 'hello {{ "world" }}'

This defines templates like src/sub/sub.go.tmpl and src/main.go.tmpl and what target path they'll be rendered to in the target directory. It also specifies which parameter variables will be available during rendering.

If a variable does not have a default value, it is a required parameter.
default values are evaluated as templates, too, but you will not be able to refer to other variables
if a variable has a pattern set, the parameter value must regex-match that pattern. Please be advised that you must enclose the pattern with ^...$ if you want to force the whole value to match, otherwise it's enough for part of the value to match the pattern.
variables are assumed to be string-valued by default, but the template generator actually allows any valid yaml structure (lists and maps, even nested) both as default values and as variable values. There is no type checking whatsoever, parsing templates that access missing fields or list items will fail, so it is not recommended to overuse this feature. Also, you should definitely provide a default value for any list or map typed variable, for else how will your users know what structure you are assuming?

The idea is that you keep your generators under version control.

Note how you can create ansible-style loops using the same template to generate multiple output files using with_items. In fact, the output file name is always parsed using the same template engine as the actual templates, so you could also use other variables in it.

If you set with_items, the template is used multiple times with the item variable set to the value you provided under with_items. These values can also be a whole yaml data structure, you simply access it as {{ .item.some.field }}.

At this time, it is not possible to dynamically assign the full list in with_items from a variable, so you can not dynamically determine the number of render runs.

Note how you can add a condition that will be evaluated for the template. Inside it, you can use variables, or even item. If the condition evaluates to any one of 0, false, skip, no the template will not be rendered. Note that the empty string counts as true, that means that if you do not specify a condition, the template is rendered.

You can disable the template parse run for a file by setting just_copy to true. This will allow you to copy a file verbatim, useful for files that contain lots of {{ }}. But then you cannot use any template commands in those files. You can still use variables in the source and target, and you can still use condition or with_items.

Also note how output directories are created for you on the fly if they don't exist.

Template Language

The golang template language is pretty versatile, vaguely similar to the .j2 templates used by ansible. Here's a very simple example of how to include one of the parameters in your template output:

fmt.Println("{{ .helloMessage }}")

Assuming someList is a list variable, access the second entry as follows:

{{ index .someList 1 }}

Assuming someMap is a map variable with a field message, access it as follows:

{{ .someList.message }}

You can combine the two for structures with nested lists: {{ (index .someList 0).someField }}.

Additional Template Functions

We include Masterminds/sprig when parsing any template, which offers a collection of useful template functions, so you can do stuff like

{{ .helloMessage | upper }}

Read the sprig documentation, it adds much of what you would otherwise miss compared to ansible j2 templates.

Render Targets

A render target is a directory that contains a yaml file which records the name of the generator used and all parameter values. We call this a render specification file. If you do not specify anything, the generator expects the file to be called generated-main.yaml.

Example:

generator: main
parameters:
  helloMessage: hello world
  serviceName: 'my-service'
  serviceUrl: github.com/StephanHCB/temp

Build and test

This program uses go modules. If cloned outside your GOPATH, you can build and test it using go build main.go and go test ./.... This will also download all required dependencies.

Acceptance Tests (give you examples)

go-generator-lib has almost complete coverage with BDD-style acceptance tests.

In the course of the test runs, several generator specs and templates are read from the test resources, and a number of render specs are written to the test output directory.

Silthus / go-generator-cli