Go asset embedding for use with `go generate`
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tv42 Enforce variables to be valid Go identifiers
Had to update `bad-no-wrap.t` to avoid triggering error too early.

fixes #3
Latest commit 018eb7d Dec 14, 2016


Becky -- Go asset embedding for use with go generate

Becky embeds assets as string literals in Go source.


Just copy asset.go in your source tree, and in your Go source do

//go:generate -command asset go run asset.go
//go:generate asset index.html

and run

$ go generate

This will create new files, named *.gen.go. You should add those into your version control system, to ensure go get works for others.

You can pass multiple asset files at once, or repeat the go:generate line.

Variable name

The generated files declare variables that now contain your assets. Given the above index.html, the variable will be named index.

You can override the name with -var=NAME, or skip it with -var=_ and use side effects in your wrapper function (discussed later).

The asset will be an value of type asset (this code is autogenerated, you don't need to type it in):

type asset struct {
	Name    string
	Content string

Name has the original filename, as a hint for Content-Type selection.


For most uses, an asset value needs to be given application or file type specific functionality. To make this easy, the asset value will be passed to a function caller wrapper. You need to write these wrapper functions.

The name of the default wrapper is the (final) extension of the asset filename. For index.html, that's html. You can override the wrapper with -wrap=NAME.

In your application, you'd do something like

func html(a asset) http.Handler {
	return a


func txt(a asset) string {
	return a.Content


func tmpl(a asset) *template.Template {
	return template.Must(template.New(a.Name).Parse(a.Content))

to smartly handle *.html, *.txt and *.tmpl assets. Feel free to pass the fields of asset to a factory function or type that matches what you need, or use the asset, whatever suits your project.


Type asset implements http.Handler, including ETag cache validation. It uses http.ServeContent which will set Content-Type from the file name or content, and handle Range requests.

Build speed

gc, the Go compiler, can slow down with large source files. As e.g. image assets can get big, this can start to slow down your builds. The mechanism used for embedding has been chosen to be the most efficient available.

Embedding a 10MB asset (creating a 28MB Go source file) takes <1 second to generate the code and about 1 second for every compilation.

You can minimize the number of times assets need to be compiled by putting them in a different package that updates less often than most of your source.

Development mode

If you build your application with -tags dev, asset.ServeHTTP will reload the asset from disk on every request, and not use the bundled copy. This makes editing HTML, CSS and such more convenient.


By including asset.go in your source tree, you're isolating yourself from changes to the upstream project. Given the input-driven nature of asset embedding, if it works once, it'll probably keep working for you.

Feel free to grab a newer asset.go every now and then, or not; if a new version for some reason doesn't work, just don't commit it to your repo and keep using the old version. Bug reports are welcome.

If you want, you can also just install the executable with go get github.com/tv42/becky, and use it like becky index.html.

Go source as asset

If you are embedding Go source files as assets, and are using go run asset.go, note that go run needs to be told where source files to run end and where arguments start. Example:

//go:generate -command asset go run asset.go
//go:generate asset -- example.go