Dynamic Data In Windmill: Nested Template Strings For Flows

Hey guys, let's talk about something super important for anyone building powerful automations with Windmill's Flow editor: the magic of dynamic data and how it interacts with complex structures. We’re diving deep into an area where Windmill, an incredibly versatile low-code platform, has a bit of a bottleneck, especially when you’re dealing with nested objects and arrays. Imagine building a complex workflow, and you need to pull a piece of information from a previous step – like an API key or a user ID – and insert it into a deeply nested part of your script’s input. Currently, this can be a real pain, forcing you out of the intuitive visual editor and into raw JavaScript. This article aims to explore this problem, highlight the current limitations, and paint a picture of a much smoother, more powerful future for template strings in nested object/array fields within the Flow editor. We'll break down why this feature is a game-changer for enhancing productivity, streamlining development, and making your flows even more robust and adaptable. Stick around, because if you're serious about leveraging Windmill to its fullest, this is a discussion you won't want to miss. We’re looking at how to make dynamic data insertion as seamless as possible, ensuring that every part of your workflow, no matter how intricate, can easily access and utilize information from previous steps without unnecessary friction. This isn't just about a small tweak; it's about unlocking a whole new level of flexibility and efficiency in how you build and manage your automated processes.

The Frustration with Static Inputs: Why Dynamic Data Matters So Much

Let’s get real, folks. The core of any powerful automation platform like Windmill lies in its ability to handle dynamic data. We're talking about information that isn't fixed, but changes based on previous steps, user inputs, or external events. In the Windmill Flow editor, template strings are your best friend for this. They allow you to easily inject values from previous steps into your current one, using a super intuitive syntax like ${results.previous_step.value}. It's brilliant for top-level string parameters – you get a neat text input with a handy ${} button, and you can just type or select your dynamic content. This makes building flows incredibly efficient and user-friendly. However, here’s where the frustration creeps in: this seamless experience doesn't extend to nested fields within objects or arrays. Imagine you’re setting up an HTTP request. You have headers, which is typically an array of objects, each with a key and a value. While most headers might be static (like Content-Type: application/json), one crucial header, say Authorization, needs a dynamic token fetched from a previous authentication step. In the current Windmill Flow editor, you can't just slap a template string into that value field of your Authorization header object if it's nested within an array. This limitation forces you to abandon the visual, form-based editing experience entirely. You have to switch to JavaScript Expression mode, and suddenly, what was a simple task becomes a mini-coding session. This isn’t just an inconvenience; it’s a significant productivity killer, especially when your flows involve complex data structures. The beauty of a low-code tool is its visual nature, and being pulled out of that environment for common dynamic data scenarios defeats a major part of its purpose. It makes the platform feel less intuitive for power users who are constantly dealing with varied, complex data, and it adds an unnecessary layer of complexity for newcomers. Our goal with automation is to reduce manual coding and increase clarity, not the other way around. Thus, supporting template strings in nested object/array fields is not just a nice-to-have; it's a fundamental improvement that would align Windmill even more closely with the best practices of low-code automation and significantly enhance the user experience for everyone, from casual users to seasoned developers. It would empower users to build more sophisticated and adaptable flows without constantly wrestling with mode switches and manual code writing, ensuring that the visual flow remains the primary interface for crafting logic, regardless of data structure complexity.

Why Template Strings are a Game-Changer

Guys, let’s be clear: template strings are a fundamental feature for any modern automation tool, and in Windmill, they represent the bridge between static configurations and dynamic, intelligent workflows. They allow your flows to be responsive and adaptable, reacting to the outcomes of prior steps. Imagine you have a sequence where step A fetches an api_key, and step B needs to use that api_key in an HTTP header. Without template strings, you'd be hard-pressed to make this connection without some seriously clunky workarounds. With them, it's as simple as ${results.A.api_key}. This elegant syntax makes your flows not only more powerful but also significantly more readable and maintainable. You can instantly see where dynamic data is being injected, which is crucial for debugging and understanding complex automations. The problem arises when this game-changing capability is arbitrarily restricted to only top-level string parameters. This limitation creates a disjointed experience, where some parts of your flow feel wonderfully fluid, while others feel like pulling teeth. It means that while you can easily fetch a simple string value, if that string needs to go into a property of an object that's itself part of an array, suddenly, you're back to square one, manually manipulating JSON or JavaScript. This isn’t just about convenience; it’s about consistency and empowerment. Users should feel empowered to leverage dynamic data wherever it's logically required, without hitting artificial barriers imposed by the editor's UI. The current situation forces users to write boilerplate JavaScript for common tasks like dynamic HTTP headers or nested API request bodies, negating the very benefits that a visual, low-code editor like Windmill strives to provide. Ultimately, extending comprehensive template string support to nested object/array fields isn’t just an enhancement; it’s a necessary evolution for Windmill to truly shine as a top-tier automation platform, allowing users to build truly dynamic, adaptable, and robust workflows with unparalleled ease and clarity. It enables more complex integrations and data transformations to be handled entirely within the visual editor, accelerating development and reducing the cognitive load on the user. This is about making Windmill more Windmill.

The Current Roadblock: A Deep Dive into Windmill's Editor

Alright, team, let's get into the nitty-gritty of why this is a problem in Windmill right now. The current behavior in the Flow editor, while great for simple string inputs, creates significant friction when dealing with more complex, nested data structures. We're talking about scenarios where your script parameters are objects or arrays, and you need to insert dynamic values into their inner fields. This is where the visual editing experience, which is otherwise fantastic, breaks down. The core issue is that only top-level string parameters have that glorious ${} button and the direct template string support. Anything nested, and you're essentially out of luck, forced to resort to less intuitive methods. Imagine you have a script defined with a Header interface, where key and value are both strings. If you want to use a dynamic value for value when Header is a parameter, or even worse, when you have an array of Header objects, the editor doesn't offer the same direct template string functionality. This means if you need to pass an Authorization header with a Bearer token that comes from a previous step, you can't just type ${results.b.api_key} into the value field within the visual form. Instead, you're immediately pushed into the JavaScript Expression mode, which, while powerful, completely bypasses the visual, form-driven input that Windmill excels at. This significantly slows down development, especially for users who prefer visual building over writing raw code. For developers, it's an unnecessary context switch and an increase in boilerplate code. For non-developers, it's a major barrier that makes powerful integrations feel inaccessible. The system should ideally abstract away this complexity, allowing users to focus on the logic and flow of their automation rather than the syntax of nested dynamic inputs. This isn't a small edge case; dynamic values in nested structures are a common requirement for almost any integration involving APIs, databases, or configuration management. The current limitation forces a choice between using static, inflexible inputs or losing the benefits of the visual editor, which is a compromise no user should have to make in a modern low-code platform. Addressing this roadblock would not only improve the user experience but also significantly broaden the types of complex workflows that can be built and maintained entirely within Windmill's intuitive interface, solidifying its position as a leading automation tool.

Code Example Walkthrough

Let’s dissect the provided code example, guys, to really hammer home this point about the current behavior in Windmill's Flow editor. Consider this simple TypeScript script that defines a Header interface and takes three parameters: a value (plain string), a header (a single Header object), and headers (an array of Header objects).

interface Header {
    key: string;
    value: string;
}

export async function main(
    value: string,        // ✅ Has template string support
    header: Header,       // ❌ No template support for nested fields
    headers: Header[],    // ❌ Must edit entire array as JS code
) {
    return
}

When you interact with this script in the Windmill Flow editor, here’s what you experience:

1. The value parameter: This is a top-level string. Excellent! You get a standard text input field, complete with the beloved ${} button. You can easily type something like Hello ${results.previous_step.name} or select a variable from the context. This works exactly as you'd expect and is super intuitive.

2. The header parameter: Now, things get a bit trickier. Since header is an object of type Header, the editor presents you with form fields for key and value. Visually, this is great! You can easily input static key and value pairs. However, if you need the value field to be dynamic – say, Bearer ${results.b.api_key} – you'll notice a crucial omission: there’s no ${} button on that nested value field. To inject a dynamic template string here, you are forced to switch the entire header object into JavaScript Expression mode. This means you lose the nice, structured form fields and have to write the whole object as JavaScript: `({ key: