render.md

Render Trait

The Render trait controls how cell outputs are displayed. Venus provides default implementations and allows custom rendering.

Default Rendering

By default, outputs use Debug formatting:

#[venus::cell]
pub fn numbers() -> Vec<i32> {
    vec![1, 2, 3, 4, 5]
}
// Output: [1, 2, 3, 4, 5]

Built-in Implementations

Venus provides Render for common types:

Type	Rendering
String	Plain text
i32, i64, f32, f64	Formatted number
bool	"true" / "false"
Vec<T>	Debug list
Option<T>	"Some(x)" / "None"
serde_json::Value	Pretty JSON

Custom Rendering

Implement Render for custom types:

use venus::Render;

pub struct Report {
    pub title: String,
    pub data: Vec<f64>,
}

impl Render for Report {
    fn render_text(&self) -> String {
        format!("{}: {:?}", self.title, self.data)
    }

    fn render_html(&self) -> Option<String> {
        Some(format!(
            "<div class='report'>
                <h2>{}</h2>
                <pre>{:?}</pre>
            </div>",
            self.title, self.data
        ))
    }
}

Render Methods

The trait provides multiple output formats:

pub trait Render {
    /// Plain text representation
    fn render_text(&self) -> String;

    /// HTML representation (optional)
    fn render_html(&self) -> Option<String> { None }

    /// Image bytes (optional, PNG/SVG)
    fn render_image(&self) -> Option<Vec<u8>> { None }

    /// Structured data (optional, JSON)
    fn render_data(&self) -> Option<serde_json::Value> { None }
}

JSON Wrapper

Use Json<T> for pretty-printed JSON:

use venus::Json;

#[venus::cell]
pub fn config() -> Json<Config> {
    Json(Config {
        name: "Analysis".to_string(),
        threshold: 0.5,
    })
}

Feature Flags

polars

Enable DataFrame rendering:

[dependencies]
venus = { version = "0.1", features = ["polars"] }

use polars::prelude::*;

#[venus::cell]
pub fn data() -> DataFrame {
    df! {
        "name" => ["Alice", "Bob"],
        "score" => [95, 87]
    }.unwrap()
}
// Renders as HTML table

image

Enable image rendering:

[dependencies]
venus = { version = "0.1", features = ["image"] }

use image::DynamicImage;

#[venus::cell]
pub fn chart() -> DynamicImage {
    // Create or load image
    DynamicImage::new_rgb8(100, 100)
}
// Renders as PNG in output

Display Priority

Venus uses outputs in this order:

1. render_html() - Rich web display
2. render_image() - Visual content
3. render_text() - Fallback text

The web UI automatically selects the best available format.