Template String

Writing prompts requires a lot of string manipulation. BAML has a template_string to let you combine different string templates together. Under-the-hood they use jinja to evaluate the string and its inputs.

Think of template strings as functions that have variables, and return a string. They can be used to define reusable parts of a prompt, or to make the prompt more readable by breaking it into smaller parts.

Example

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// Inject a list of "system" or "user" messages into the prompt.
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template_string PrintMessages(messages: Message[]) #"
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  {% for m in messages %}
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    {{ _.role(m.role) }}
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    {{ m.message }}
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  {% endfor %}
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"#
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function ClassifyConversation(messages: Message[]) -> Category[] {
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  client GPT4Turbo
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  prompt #"
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    Classify this conversation:
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    {{ PrintMessages(messages) }}
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    Use the following categories:
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    {{ ctx.output_format}}
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  "#
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}

In this example we can call the template_string PrintMessages to subdivide the prompt into “user” or “system” messages using _.role() (see message roles). This allows us to reuse the logic for printing messages in multiple prompts.

You can nest as many template strings inside each other and call them however many times you want.

Use the playground preview to ensure your template string is being evaluated correctly!