7-alternative-delimiters.md

Alternative delimiters

In previous cases, harp template engine was rendering a template to create data. {{ and }} are default template delimiters and interpreted by template engine during template evaluation.

Sometimes, you might want to create a more complex scenario where harp engine is used to render a meta template to create a template (and then use harp a second time to render the generated template to create the final data)

For example, secrets construction based on application configuration files, an external list of resources, etc.

In this case, you can use alternative delimiters [[ and ]] and let harp know using --alt-delims ({{ and }} won't be interpreted)

Let's create a default template, we want to generate a paranoidPassword :

echo "{{ paranoidPassword }}" | harp template
EOF

For example, if we want to generate multiple passwords, i can create a simple template:

cat << EOF | harp template
{{ paranoidPassword }}
{{ paranoidPassword }}
{{ paranoidPassword }}
EOF

With alternative delimiters, we can create a meta template:

cat << EOF | harp template --alt-delims | harp template
[[ range until 3 -]]
{{ paranoidPassword }}
[[ end -]]
EOF

We can add an external value count to limit password generation count :

cat << EOF | harp template --alt-delims --set count=3 | harp template
[[ range until (.Values.count | int) -]]
{{ paranoidPassword }}
[[ end -]]
EOF

Note the int conversion, by default all integers are int64, you need to truncate them to int to be able to use the until function.

We could set a default count, if the external value is not set :

cat << EOF | harp template --alt-delims --set count=50 | harp template
[[ range until (default 5 (.Values.count | int)) -]]
{{ paranoidPassword }}
[[ end -]]
EOF

The interest of meta template is that we can create dynamic template with external values:

[[ $values := .Values ]]
[[ range $item := .Values.list ]]
[[ $item ]] : {{ paranoidPassword }}
[[ end ]]

and then invoke harp :

harp template --alt-delims --values value.yaml --in template.tmpl | harp template

You can compose a complete transformation pipeline for values and template files. So that you can prepare, clean, organize data before the final template interpretation.

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