# Contributing to Manifolds.jl

First, thanks for taking the time to contribute. Any contribution is appreciated and welcome.

The following is a set of guidelines to Manifolds.jl.

## I just have a question

The developers can most easily be reached in the Julia Slack channel #manifolds. You can apply for the Julia Slack workspace here if you haven't joined yet. You can also ask your question on discourse.julialang.org.

## How can I file an issue?

If you found a bug or want to propose a feature, we track our issues within the GitHub repository.

## How can I contribute?

### Add a missing method

Not all methods from our interface ManifoldsBase.jl have been implemented for every manifold. If you notice a method missing and can contribute an implementation, please do so! Even providing a single new method is a good contribution.

### Provide a new manifold

A main contribution you can provide is another manifold that is not yet included in the package. A manifold is a concrete type of AbstractManifold from ManifoldsBase.jl. This package also provides the main set of functions a manifold can/should implement. Don't worry if you can only implement some of the functions. If the application you have in mind only requires a subset of these functions, implement those. The ManifoldsBase.jl interface provides concrete error messages for the remaining unimplemented functions.

One important detail is that the interface usually provides an in-place as well as a non-mutating variant See for example exp! and exp. The non-mutating one (e.g. exp) always falls back to use the in-place one, so in most cases it should suffice to implement the in-place one (e.g. exp!).

Note that since the first argument is always the AbstractManifold, the mutated argument is always the second one in the signature. In the example we have exp(M, p, X, t) for the exponential map and exp!(M, q, p, X, t) for the in-place one, which stores the result in q.

On the other hand, the user will most likely look for the documentation of the non-mutating version, so we recommend adding the docstring for the non-mutating one, where all different signatures should be collected in one string when reasonable. This can best be achieved by adding a docstring to the method with a general signature with the first argument being your manifold:

struct MyManifold <: AbstractManifold end

@doc raw"""
exp(M::MyManifold, p, X)

Describe the function.
"""
exp(::MyManifold, ::Any...)

### Code style

We try to follow the documentation guidelines from the Julia documentation as well as Blue Style. We run JuliaFormatter.jl on the repo in the way set in the .JuliaFormatter.toml file, which enforces a number of conventions consistent with the Blue Style.

We also follow a few internal conventions:

• It is preferred that the AbstractManifold's struct contain a reference to the general theory.
• Any implemented function should be accompanied by its mathematical formulae if a closed form exists.
• Within the source code of one manifold, the type of the manifold should be the first element of the file, and an alphabetical order of the functions is preferable.
• The above implies that the in-place variant of a function follows the non-mutating variant.
• There should be no dangling = signs.
• Always add a newline between things of different types (struct/method/const).
• Always add a newline between methods for different functions (including in-place/nonmutating variants).
• Prefer to have no newline between methods for the same function; when reasonable, merge the docstrings.
• All import/using/include should be in the main module file.