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Lightning uses Go’s standard text/template package for server-side rendering. Load templates once at startup with app.LoadHTMLGlob(), then render them from any handler with ctx.HTML().

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Loading Templates

Call app.LoadHTMLGlob(pattern) before starting the server. The pattern follows the same glob syntax as filepath.Glob. 
app.LoadHTMLGlob("templates/*.html")
LoadHTMLGlob panics if the pattern matches no files or if any template fails to parse. Always verify your template paths before deploying.

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Custom Template Functions

Register custom functions with app.SetFuncMap() before calling app.LoadHTMLGlob(). The function map is applied when templates are parsed. 
import (
    "fmt"
    "text/template"
    "time"

    "github.com/go-labx/lightning"
)

func formatDate(t time.Time) string {
    year, month, day := t.Date()
    return fmt.Sprintf("%d-%02d-%02d", year, month, day)
}

func main() {
    app := lightning.DefaultApp()

    // Register custom functions first
    app.SetFuncMap(template.FuncMap{
        "formatDate": formatDate,
    })

    // Then load templates (so they can use the functions)
    app.LoadHTMLGlob("templates/*.tmpl")
}

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Rendering a Template

Call ctx.HTML(code, name, data) from a handler. name is the template file name as registered by the glob, and data is any value passed as the template’s dot (.). 
app.Get("/", func(ctx *lightning.Context) {
    ctx.HTML(lightning.StatusOK, "index.tmpl", lightning.Map{
        "title":       "My App",
        "description": "Built with Lightning",
        "now":         time.Now(),
    })
})
If template execution fails, Lightning responds with 500 and the error message as plain text.

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Complete Example

1

Create the template file

templates/index.tmpl:
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>{{.title}}</title>
    <link rel="stylesheet" href="/static/css/style.css" />
</head>
<body>
    <h1>{{.title}}</h1>
    <p>{{.description}}</p>
    <p>Today is {{.now | formatDate}}</p>
</body>
</html>
2

Write the server code

main.go:
package main

import (
    "fmt"
    "text/template"
    "time"

    "github.com/go-labx/lightning"
)

func formatDate(t time.Time) string {
    year, month, day := t.Date()
    return fmt.Sprintf("%d-%02d-%02d", year, month, day)
}

func main() {
    app := lightning.DefaultApp()

    // Serve CSS/JS from ./public at /static
    app.Static("./public", "/static")

    // Register custom template functions before loading
    app.SetFuncMap(template.FuncMap{
        "formatDate": formatDate,
    })

    // Load all templates from the templates directory
    app.LoadHTMLGlob("templates/*.tmpl")

    app.Get("/", func(ctx *lightning.Context) {
        ctx.HTML(lightning.StatusOK, "index.tmpl", lightning.Map{
            "title":       "Lightning",
            "description": "A fast web framework for Go.",
            "now":         time.Now(),
        })
    })

    app.Run(":8080")
}
3

Run and visit the page

go run main.go
# open http://localhost:8080
Use lightning.Map (a map[string]any alias) to pass ad-hoc template data without defining a dedicated struct.