gen: type writers

gen is driven by “type writers” – packages which are responsible for interpreting the annotated tags and turning them into generated code.

gen includes two built-in TypeWriters:


The slice typewriter generates functional convenience methods that will look familiar to users of C#’s LINQ or JavaScript’s Array methods. It is intended to save you some loops, using a “pass a function” pattern. It offers grouping, filtering, ad-hoc sorts and projections. Details and docs…


The stringer typewriter is a fork of Rob Pike’s tool of the same name, which generates readable strings for consts. Details and docs…

Listing typewriters

To view the currently-available typewriters, cd into your package and type:

gen list

Adding third-party TypeWriters

TypeWriters can be implemented by third-parties and used at “gen time”. To use a third-party typewriter, cd into the root of your package and type (for example):

gen add

This will create a _gen.go file. Have a look at it – it should contain imports for the built-in slice (above) and your new typewriter.

Confirm it by typing:

gen list

To ensure you’ve got your third-party packages locally, type:

gen get

Now use the third-party tag (set in this case) to your type annotation:

// +gen set slice:"Where,Count,GroupBy[string]"
type MyType struct {}

And run gen on your package:


You should have a new file mytype_set.go. Third-parties are responsible for the quality and documentation of their typewriters, of course.

To remove a typewriter, simply remove the import from the _gen.go file.

Implementing TypeWriters

You can create your own typewriter by implementing typewriter.Interface.

Typewriters follow the pattern of formats in the image package of Go’s standard library. They are registered via an init() method.

The best thing to do is have a look at an existing implementation, Set is straightforward.

By convention, the name of the typewriter is the name of the tag that it uses. Type gen list to see the name of the typewriters available in your current package.

The typewriter package handles tag parsing and type evaluation, and passes this information to your typewriter. It offers some conveniences for text templating, as well.

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Published by Matt Sherman on January 1, 0001