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Published: Jun 26, 2017 License: Apache-2.0 Imports: 5 Imported by: 0



Command stencil generates specialized versions of Go packages by replacing types.


Given a package with an interface "A", stencil can generate a new package with all uses of "A" replaced by "int" (or any other type). The generated package is stored in the topmost vendor directory of the repo. If no such directory exists, one is created.

As a trivial example, consider a package "" with a function Max that computes the maximum value of a list of numbers.

func Max( int {
	// compute max

This only works for int, but we need a version of Max that works on float32. stencil can automatically generate an float32 version by reading an import path with the substitution.

Import the float32 version

import (
	float32_num ""

and use it in your code

func PrintMax(values []float32) {
	fmt.Println("Max of", values, "=", float32_num.Max(values...))

This will not compile, since the "" package doesn't exist yet. So in your package directory run


This generates a "stencilled" version of the package having int substituted with float32. You can now use it in your code If your repo has a vendor directory, this will generate the float32 stencilled version in that vendor directory. If not, a vendor directory will be created in your package directory and the stencilled version is generated there.

Supported Types

The set of types that can be replaced are currently restricted to the following:

* Named types in the stencil
* Boolean types
* Numeric types
* String types
* interface{} - Use interface in the import path

These types can be replaced with the same set of types as above, with the exception of interface{}.

With go generate

Add the below line to any package that imports a stencilled package.

//go:generate stencil

and run

go generate

in the package directory. You only need one go generate directive per package.

Generate on save

The process of generating stencilled packages can be further streamlined by using stencil as a replacement for goimports. Running

stencil -w <path/to/file>

will also run goimports on your code, while generating any needed stencilled packages. You can add this as a separate command to run on save in your editor.

Source Files

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