Add a way to partially render a template (ignore missing variables in context)

[minev-dev]

Problem

There is no way to partially render a template with the only variables available in a context and ignore the template variables that are not presented in the context.

Currently the Processor::render returns an error if at least one variable is failed to render

tera/src/renderer/processor.rs

Lines 1058 to 1062 in ae13d7c

	pub fn render(&mut self, write: &mut impl Write) -> Result<()> {
	for node in &self.template_root.ast {
	self.render_node(node, write)
	.map_err(|e| Error::chain(self.get_error_location(), e))?;
	}

Context

We have a multi-step rendering where not all the variables are available at the same time. There is a config file which should support system env variables injection (first step) and then cross-services variables injection in runtime (second step).

Here is a draft PR with the implementation and failing unit test opencloudtool/opencloudtool@20ae6de

Expected behavior

Initial template

[project]
name = "{{ env.PROJECT_NAME }}"
description = "{{ runtime.GOOD_DESCRIPTION }}"

First step

Context

{
  "env": {
    "PROJECT_NAME": "Project Name"
  }
}

Rendered template

[project]
name = "Project Name"
description = "{{ runtime.GOOD_DESCRIPTION }}"

Second step

Context

{
  "runtime": {
    "GOOD_DESCRIPTION": "Good Description"
  }
}

Rendered template

[project]
name = "Project Name"
description = "Good Description"

[Keats]

That's not going to be built in but you can probably write your own filter to handle that

[minev-dev]

That's not going to be built in

Is there a reason for this? Do you need help with the implementation?

you can probably write your own filter to handle that

I suppose it'll require the following syntax {{ runtime.GOOD_DESCRIPTION | some_filter }} which is not a pretty syntax if we want end user to use that. Or is there a way to have a default filter?

[Keats]

Is there a reason for this? Do you need help with the implementation?

It's an important feature of Tera to error if it's missing things rather than silently ignoring it.

I suppose it'll require the following syntax {{ runtime.GOOD_DESCRIPTION | some_filter }} which is not a pretty syntax if we want end user to use that. Or is there a way to have a default filter?

You can also have it as a function eg, {{ get_runtime(name=...) }}

[minev-dev]

It's an important feature of Tera to error if it's missing things rather than silently ignoring it.

I didn't mean that the current behavior should be replaced. There should be a way to optionally ignore or tolerate the errors

You can also have it as a function eg, {{ get_runtime(name=...) }}

Will try it, thanks. But anyway it doesn't look like an easy-to-use format for the end user who just wants to use a variable

[minev-dev]

@Keats Added a Proof-of-Concept implementation of what we approximately need master...minev-dev:tera:master
It helps to tolerate the template variables that are not presented in a context.

What do you think about investing some time into a well-shaped implementation? Or can you please give me recommendations on how I can properly implement it so I'll submit a PR?

[Keats]

No that won't be added