gotp

Live preview and render Go templates with custom data, in the browser or on the command line.

• Single binary, self-contained
• Web UI with live reload
• Browse and render templates in a directory
• Automatic field discovery

Usage

# Live preview in the browser, reloading on save
gotp serve page.tmpl partials/*.tmpl -d @data.json --open

# Serve a whole directory and switch templates in the UI
gotp serve templates/ --addr 9000

# Render to stdout instead
gotp page.tmpl

# With its partials, and some data
gotp page.tmpl partials/*.tmpl -d '{"name": "Gopher"}'

# Data from a file, or from a pipe
gotp page.tmpl -d @data.json
kubectl get cm -o json | gotp page.tmpl -d @-

# List the fields the template reads, one path per line
gotp fields page.tmpl partials/*.tmpl

# Scaffold the data file out of those fields, then use it
gotp fields --json page.tmpl partials/*.tmpl > data.json
gotp page.tmpl partials/*.tmpl -d @data.json

# Not HTML: plain text mail, YAML, SQL. Values are not escaped
gotp mail.tmpl --text -d '{"name": "Tom & Jerry"}'

# Templates that call functions your application registers
gotp mail.tmpl --stub-funcs

# Fail on missing data instead of rendering <no value>
gotp page.tmpl -d @data.json --strict

# Read the template from stdin
cat page.tmpl | gotp -

# Check that a template still parses, for CI. No data needed
gotp fields page.tmpl partials/*.tmpl > /dev/null

Run gotp --help for all options, or gotp <command> --help for one command.

Install

uvx, pipx, or pip

Run without installing:

uvx gotp page.tmpl

Or install it:

pipx install gotp
uv tool install gotp
pip install gotp  # inside a virtual environment

npx or npm

npx @vividvilla/gotp page.tmpl   # run without installing
npm install --global @vividvilla/gotp

Homebrew

brew install vividvilla/tap/gotp

Standalone binary

Download an archive from GitHub Releases, verify it against SHA256SUMS, and put gotp (gotp.exe on Windows) on your PATH.

Prebuilt binaries cover Linux x86-64/ARM64/ARM, macOS x86-64/Apple Silicon, and Windows x86-64/ARM64. The web UI ships inside the binary, so there is nothing else to install.

From source

git clone https://github.com/vividvilla/gotp && cd gotp
make build

Commands

gotp [render] <template> [templates...]   Render a template to stdout
gotp serve <template|directory> [...]     Live preview in the browser
gotp fields <template> [templates...]     List the data fields a template reads

render is the default, so gotp page.tmpl and gotp render page.tmpl are the same thing.

Arguments

Every argument is a template, and they all get parsed together. Base templates and partials go in as arguments, not as repeated flags. Any argument can be a glob:

gotp page.tmpl partials/*.tmpl      # expanded by the shell
gotp page.tmpl 'partials/*.tmpl'    # expanded by gotp

Quote it under serve and gotp keeps the pattern, so a template you add to partials/ later triggers a reload too.

gotp renders the first argument, unless that file holds nothing but {{ define }} blocks. A file like that renders nothing by itself, so gotp moves on to the first argument that does, and prints on stderr which one it chose. Order does not matter:

gotp page.tmpl base-footer.tmpl     # both render page.tmpl
gotp base-footer.tmpl page.tmpl
gotp *.tmpl                         # order decided by the shell, still fine

When two arguments both render something, the first one wins. Name nothing but partials and you get an error saying so, not a blank page.

Use - as the template to read it from stdin.

Flags

These apply to every command:

Flag
-d, --data JSON	Template data: inline JSON, @file.json, or @- for stdin
--text	Render with text/template, so values are not HTML-escaped
--sprig	Register the Sprig function library
--stub-funcs	Stub out functions the template calls but nothing defines
--strict	Fail on data the template reads but does not have
--stdin	Read the template from stdin, the same as passing -

fields adds --json, which prints a JSON data skeleton instead of one path per line. serve adds:

Flag
-a, --addr address	Address to listen on, default 127.0.0.1:1111
--open	Open the preview in the default browser

--addr takes a bare port (--addr 9000) and defaults to loopback, so a preview is not exposed to the network. Pass --addr :9000 to listen on every interface.

The web UI

gotp serve opens two panes: the rendered template on the left, its data on the right.

The left toolbar holds the template picker, a Preview/Source pair, and a width toggle for checking a mobile layout. The size being rendered shows in the middle. The gear sets the width the toggle uses.

The right panel is the data, as a flat list of one row per field path or as raw JSON. gotp compares what the template reads against what the data has, and marks a field the template reads but the data lacks as unset.

Editing a value keeps its JSON type, because a template calling printf "%.2f" breaks on a string. Applying patches the data you gave instead of replacing it, so keys the template never reads survive. If there is no data at all, the panel offers to fill in the skeleton that gotp fields --json prints.

The theme follows the operating system, with a switch in the top bar.

Styling is oat (MIT), vendored into assets/ and embedded with the rest of the UI. The rendered template loads in an iframe, so the UI's own CSS cannot reach it.

Serving a directory

gotp serve <directory> parses every .tmpl, .tpl, .gohtml and .html file in it, so partials defined in sibling files resolve, and the UI gets a picker to switch between them.

The picker only offers templates that render something, so a file of {{ define }} blocks never shows up as a blank preview. It opens on an index.* template if there is one, otherwise the first that renders. gotp works this out from what a file contains, not from how it is named.

Template functions

A template that calls a function your application registers will not parse on its own, since nothing defines that function. --stub-funcs registers a placeholder for each one so the template still previews:

• if the data has a key matching the function name, the stub returns that value, so {{ nominee_name }} can be mocked with -d '{"nominee_name": "Gopher"}'
• inside a pipeline the stub passes its input through, so {{ .date | myformat }} renders .date
• otherwise it renders a visible «name» placeholder

For the functions from Sprig, --sprig registers the real implementations instead.

Discovered fields

gotp fields lists every field a template reads, including fields inside if, range and with, and fields inside the partials it calls. Paths are scoped to where they are read, so a field inside {{ range .posts }} reads as .posts[].title, and a partial called as {{ template "totals" .receipt }} reports its fields against .receipt.

The scoping catches a common slip: {{ .url }} written inside {{ with .request }} reads .request.url, and the field list shows it.

Two limits. Fields reached through a variable ({{ range $_, $post := .posts }}{{ $post.title }}) are not reported, since resolving those needs a symbol table. A scope gotp cannot resolve statically gets a * segment and stays out of the --json skeleton.

Example

example/ is two emails over one set of partials, the way real template sets are built:

example/receipt.tmpl       a receipt: range, a conditional inside it, printf
example/otp.tmpl           a sign-in code: with, a nested scope
example/base-header.tmpl   {{ define "header" }}, the logo and sign-in button
example/base-rows.tmpl     {{ define "details" }} and {{ define "totals" }}
example/base-footer.tmpl   {{ define "footer" }}
example/data.json          data for both

# Render one. The partials can be named in any order
gotp --text example/receipt.tmpl example/base-*.tmpl -d @example/data.json

# Preview live, with a picker to switch between the two emails
gotp serve example/ --text -d @example/data.json --open

# What data does each one need? Both are covered by example/data.json
gotp fields example/receipt.tmpl example/base-*.tmpl
gotp fields example/otp.tmpl example/base-*.tmpl

This set needs the --text. Email HTML reaches Outlook through conditional comments like <!--[if mso]>, and html/template strips comments, so the default mode drops them silently. The same applies to any HTML email.

Both images are inline SVG, so the example renders the same with no network.

Development

make check   # gofmt, go vet, go test -race
make build   # ./gotp

The web UI is assets/index.html, assets/app.js and a vendored copy of oat, embedded with go:embed. Rebuild after changing it.

License

MIT