PROMPT_TEMPLATE_LANGUAGE.md

Prompts are the inputs or queries that a user or a program gives to an LLM AI, in order to elicit a specific response from the model.

Prompts can be natural language sentences or questions, or code snippets or commands, or any combination of text or code, depending on the domain and the task.

Prompts can also be nested or chained, meaning that the output of one prompt can be used as the input of another prompt, creating more complex and dynamic interactions with the model.

SK Prompt Template Syntax

The Semantic Kernel prompt template language is a simple and powerful way to define and compose AI functions using plain text. You can use it to create natural language prompts, generate responses, extract information, invoke other prompts or perform any other task that can be expressed with text.

The language supports three basic features that allow you to (#1) include variables, (#2) call external functions, and (#3) pass parameters to functions.

You don't need to write any code or import any external libraries, just use the curly braces {{...}} to embed expressions in your prompts. Semantic Kernel will parse your template and execute the logic behind it. This way, you can easily integrate AI into your apps with minimal effort and maximum flexibility.

Variables

To include a variable value in your text, use the {{$variableName}} syntax. For example, if you have a variable called name that holds the user's name, you can write:

Hello {{$name}}, welcome to Semantic Kernel!

This will produce a greeting with the user's name.

Spaces are ignored, so if you find it more readable, you can also write:

Hello {{ $name }}, welcome to Semantic Kernel!

Function calls

To call an external function and embed the result in your text, use the {{namespace.functionName}} syntax. For example, if you have a function called weather.getForecast that returns the weather forecast for a given location, you can write:

The weather today is {{weather.getForecast}}.

This will produce a sentence with the weather forecast for the default location stored in the input variable. The input variable is set automatically by the kernel when invoking a function. For instance, the code above is equivalent to:

The weather today is {{weather.getForecast $input}}.

Function parameters

To call an external function and pass a parameter to it, use the {{namespace.functionName $varName}} and {{namespace.functionName "value"}} syntax. For example, if you want to pass a different input to the weather forecast function, you can write:

The weather today in {{$city}} is {{weather.getForecast $city}}.
The weather today in Schio is {{weather.getForecast "Schio"}}.

This will produce two sentences with the weather forecast for two different locations, using the city stored in the city variable and the "Schio" location value hardcoded in the prompt template.

Design Principles

The template language is designed to be simple and fast to render, allowing to create functions with a simple text editor, using natural language, reducing special syntax to a minimum, and minimizing edge cases.

The template language uses the «$» symbol on purpose, to clearly distinguish between function calls that retrieve content executing some code, from variables, which are replaced with data from the local temporary memory.

Branching features such as "if", "for", and code blocks are not part of SK's template language. This reflects SK's design principle of using natural language as much as possible, with a clear separation from traditional programming code.

By using a simple language, the kernel can also avoid complex parsing and external dependencies, resulting in a fast and memory efficient processing.

Semantic function example

Here's a very simple example of a semantic function defined with a prompt template, using the syntax described.

== File: skprompt.txt ==

My name: {{msgraph.GetMyName}}
My email: {{msgraph.GetMyEmailAddress}}
My hobbies: {{memory.recall "my hobbies"}}
Recipient: {{$recipient}}
Email to reply to:
=========
{{$sourceEmail}}
=========
Generate a response to the email, to say: {{$input}}

Include the original email quoted after the response.

If we were to write that function in C#, it would look something like:

async Task<string> GenResponseToEmailAsync(
    string whatToSay,
    string recipient,
    string sourceEmail)
{
    try {
        string name = await this._msgraph.GetMyName();
    } catch {
        ...
    }

    try {
        string email = await this._msgraph.GetMyEmailAddress();
    } catch {
        ...
    }

    try {
        // Use AI to generate an email using the 5 given variables
        // Take care of retry logic, tracking AI costs, etc.
        string response = await ...

        return response;
    } catch {
        ...
    }
}

Notes about special chars

Semantic function templates are text files, so there is no need to escape special chars like new lines and tabs. However, there are two cases that require a special syntax:

1. Including double curly braces in the prompt templates
2. Passing to functions hardcoded values that include quotes

Prompts needing double curly braces

Double curly braces have a special use case, they are used to inject variables, values, and functions into templates.

If you need to include the {{ and }} sequences in your prompts, which could trigger special rendering logic, the best solution is to use string values enclosed in quotes, like {{ "{{" }} and {{ "}}" }}

For example:

{{ "{{" }} and {{ "}}" }} are special SK sequences.

will render to:

{{ and }} are special SK sequences.

Values that include quotes, and escaping

Values can be enclosed using single quotes and double quotes.

To avoid the need for special syntax, when working with a value that contains single quotes, we recommend wrapping the value with double quotes. Similarly, when using a value that contains double quotes, wrap the value with single quotes.

For example:

...text... {{ functionName "one 'quoted' word" }} ...text...
...text... {{ functionName 'one "quoted" word' }} ...text...

For those cases where the value contains both single and double quotes, you will need escaping, using the special «\» symbol.

When using double quotes around a value, use «\"» to include a double quote symbol inside the value:

... {{ "quotes' \"escaping\" example" }} ...

and similarly, when using single quotes, use «\'» to include a single quote inside the value:

... {{ 'quotes\' "escaping" example' }} ...

Both are rendered to:

... quotes' "escaping" example ...

Note that for consistency, the sequences «\'» and «\"» do always render to «'» and «"», even when escaping might not be required.

For instance:

... {{ 'no need to \"escape" ' }} ...

is equivalent to:

... {{ 'no need to "escape" ' }} ...

and both render to:

... no need to "escape"  ...

In case you may need to render a backslash in front of a quote, since «\» is a special char, you will need to escape it too, and use the special sequences «\\\'» and «\\\"».

For example:

{{ 'two special chars \\\' here' }}

is rendered to:

two special chars \' here

Similarly to single and double quotes, the symbol «\» doesn't always need to be escaped. However, for consistency, it can be escaped even when not required.

For instance:

... {{ 'c:\\documents\\ai' }} ...

is equivalent to:

... {{ 'c:\documents\ai' }} ...

and both are rendered to:

... c:\documents\ai ...

Lastly, backslashes have a special meaning only when used in front of «'», «"» and «\».

In all other cases, the backslash character has no impact and is rendered as is. For example:

{{ "nothing special about these sequences: \0 \n \t \r \foo" }}

is rendered to:

nothing special about these sequences: \0 \n \t \r \foo