Partial values

It can often make sense to "partial" a prompt template - passing in a subset of the required values to create a new prompt template which expects only the remaining subset of values.

LangChain supports this in two ways:

1. Partial formatting with string values
2. Partial formatting with functions that return string values

Partial with strings

One common use case is when you get some variables before others. For example, if you have a prompt template requiring two variables, foo and baz, but you get foo early in the chain and baz later, you can partial the prompt template with the foo value:

prompt := prompts.NewPromptTemplate(
    "{{.foo}}{{.baz}}",
    []string{"foo", "baz"},
)
prompt.PartialVariables = map[string]any{
    "foo": "foo",
}

result, _ := prompt.Format(map[string]any{
    "baz": "baz",
})
// Output: foobaz

Partial with functions

The other common use is to partial with a function. This is useful when you have a variable you always want to fetch in a common way, like the current date:

templateWithPartials := prompts.PromptTemplate{
    Template:       "Daily Report for {{.user}}\nGenerated on: {{.timestamp}}\n\nSummary: {{.summary}}",
    InputVariables: []string{"user", "summary"},
    TemplateFormat: prompts.TemplateFormatGoTemplate,
    PartialVariables: map[string]any{
        "timestamp": func() string {
            return time.Now().Format("2006-01-02 15:04:05")
        },
    },
}

report, _ := templateWithPartials.Format(map[string]any{
    "user":    "Alice",
    "summary": "All systems operational",
})
// Output: Daily Report for Alice
//         Generated on: 2023-06-28 14:30:45
//         Summary: All systems operational

For more examples, see the comprehensive prompt templates example.