Prompt templates: v2 migration

The prompt-template engine used by render(...) / render_prompt(...) now supports else/elif, loops, includes, filters, comments, raw blocks, and whitespace trim markers. Existing templates keep rendering unchanged — this is a strict superset. But many pre-v2 workarounds can now be simplified.

If / else

Before — mutually-exclusive {{ if }} blocks with inverted flags:

{{if expected_output}}
Expected: {{expected_output}}
{{end}}{{if no_expected_output}}
(no expected output provided)
{{end}}

After:

{{if expected_output}}
Expected: {{expected_output}}
{{else}}
(no expected output provided)
{{end}}

Loops instead of hand-rolled list concatenation

Before — build a string in .harn and inject it as a single variable:

let block = ""
for sample in samples {
  block = "${block}### ${sample.path}\n\`\`\`\n${sample.content}\n\`\`\`\n\n"
}
let prompt = render("enrichment.prompt", {block: block, ...})

# enrichment.prompt
## Samples
{{block}}

After — iterate in the template:

let prompt = render("enrichment.prompt", {samples: samples, ...})

# enrichment.prompt
## Samples
{{for s in samples}}
### {{s.path}}
```
{{s.content}}
```
{{end}}

Shared prose → {{ include }}

When multiple repair-stage prompts share the same boilerplate (“self-verification instructions”, system rules, etc.), extract the shared text into a partial:

# lib/partials/self-verify.harn.prompt
Before responding, verify your answer against: {{verification_hint}}

Call it from each repair stage:

{{include "partials/self-verify.harn.prompt"}}
...stage-specific instructions...

Pass stage-specific overrides with with:

{{include "partials/self-verify.harn.prompt" with { verification_hint: "compile output" }}}

Filters instead of pre-processing

Before — uppercase, join lists, JSON-stringify in .harn before rendering:

let tags_str = join(map(tags, fn(t) { return uppercase(t) }), ", ")
render("x.prompt", {tags: tags_str})

After:

Tags: {{tags | join: ", " | upper}}

Comments and raw blocks

Add {{# authoring notes #}} to document a template without leaking the note into the final prompt. Wrap literal {{ / }} (e.g. examples of another template language embedded in a prompt) in a {{ raw }} ... {{ endraw }} block.

Whitespace trim

{{- ... -}} markers strip whitespace and one newline on the respective side. Use them to keep source templates readable without introducing blank lines in the rendered output:

Items:
{{- for x in xs -}}
  {{ x }},
{{- end -}}
DONE

See Prompt templating for the full reference.