# Contributing to DSGE.jl

## Notes for DSGE.jl Contributors

As these steps are under development, we would welcome improvements to the existing code from the community. Some examples could be:

• Performance improvements
• Alternatives to algorithms used here (optimization, hessian, etc.)
• Extension of the DSGE-VAR, DSGE-VECM, and VAR code a. Allowing measurement error uncorrelated with the exogenous structural shocks. b. Implementing a test DSGE model with cointegrating relationships in the measurement equation so that the DSGEVECM object and functions defined for it can be tested, e.g. impulse_responses. Currently, most of the tested DSGE-VECM code are internal functions which are not intended to be the user-level interface. For example, the impulse responses of a DSGE-VECM can be calculated if the state space system matrices are given as inputs, but we do not have tests for the wrapper impulse_responses(m, . . .), where m <: AbstractDSGEVECMModel'.
• Extension of regime-switching code to include regime-switching beyond changes in parameter values and in policy rules a. Example "toy" extension: A second sector is introduced at some point in the history, so the final consumption good is now an aggregate of intermediate goods from two sectors. This example would require extending solution functions (e.g. solve and compute_system) to be robust to regime switches in the equilibrium conditions of a DSGE model.
• Extension of model averaging techniques beyond PoolModel
• Other general improvements a. Adding documentation/test coverage b. Adding existing notable DSGE models into the models/ directory

### Git Recommendations For Pull Requests

• Avoid working from the main branch of your fork, creating a new branch will make it easier if DSGE's main changes and you need to update your pull request.
• Try to squash together small commits that make repeated changes to the same section of code so your pull request is easier to review, and Julia's history won't have any broken intermediate commits. A reasonable number of separate well-factored commits is fine, especially for larger changes.
• If any conflicts arise due to changes in DSGE's main, prefer updating your pull request branch with git rebase versus git merge or git pull, since the latter will introduce merge commits that clutter the git history with noise that makes your changes more difficult to review.
• Descriptive commit messages are good.
• Using git add -p or git add -i can be useful to avoid accidentally committing unrelated changes.
• GitHub does not send notifications when you push a new commit to a pull request, so please add a comment to the pull request thread to let reviewers know when you've made changes.
• When linking to specific lines of code in discussion of an issue or pull request, hit the y key while viewing code on GitHub to reload the page with a URL that includes the specific version that you're viewing. That way any lines of code that you refer to will still make sense in the future, even if the content of the file changes.
• Whitespace can be automatically removed from existing commits with git rebase.
• To remove whitespace for the previous commit, run git rebase --whitespace=fix HEAD~1.
• To remove whitespace relative to the main branch, run git rebase --whitespace=fix main.

## DSGE Julia Style Guide

### Intro

This document lists Julia coding recommendations consistent with best practices in the software development community. The recommendations are based on guidelines for other languages collected from a number of sources and on personal experience. These guidelines are written with the New York Fed DSGE code in mind. All pull requests submitted should follow these general style guidelines.

### Naming conventions

Emphasize readability! Our goal is for the code to mimic the mathematical notation used in New York Fed DSGE papers as closely as possible.

• The names of variables should document their meaning or

use. Variables with a large scope should have especially meaningful names. Variables with a small scope can have short names.

• Exhibit consistency with the existing codebase.

• Modules and type names in UpperCamelCase

• Variable names in snake_case.

Variable names should be in lower case, using underscores to separate parts of a compound variable name. For example, steady_state and equilibrium_conditions are two fields in the Model990() object that follow this convention. Also notice that, though the words could be shortened, they are spelled out for maximum clarity.

• The prefix n_ should be used for variables representing the

number of objects (e.g. n_parameters or n_anticipated_shocks) use the suffix s as is natural in spoken language.

• Negative Boolean variable names should be avoided. A problem arises

when such a name is used in conjunction with the logical negation operator as this results in a double negative. It is not immediately apparent what !isNotFound means. Use isFound. Avoid isNotFound.

• Named constants can be all uppercase using underscore to separate words:

MAX_ITERATIONS

• Naming mathematical objects

Variables with mathematical significance should use unicode characters and imitate LaTeX syntax. For example, ρ should be used to name the autocorrelation coefficient in an AR(1) process, and σ should be used to name standard deviation. Parameters in the text should keep the same symbol in the code (e.g. α in the code is the same α as in this paper, and takes on it usual significance as the capital share in a Cobb-Douglas output function.

• General conventions
• Underscores can and should be used when the variable refers to a mathematical object that has a subscript. (In this case, we are imitating LaTeX syntax.) For example, $r_m$ in LaTeX should be represented by the variable r_m.
• If the mathematical object has multiple subscripts, for example $x_{i,j}$, simply concatenate the subscripts: x_ij.
• If the object has superscripts as well as subscripts, for example $y^f_t$, separate the superscripts with an underscore and place them first: y_f_t.
• For compatibility with existing code, variables with numeric subscripts should exclude the underscore: G0, ψ1.
• Matrices that have mathematical significance (e.g. the matrices of the transition and measurement equations) should be upper case, as they are in mathematical notation, and can repeat the letter to avoid collisions: TTT or QQ.
• Symbols such as overbars (which indicate mean values) and tildes (which indicate log-deviations from the steady state) are written using a 3- or 4-letter abbreviation immediately after the variable they modify: kbar_t, ztil (z tilde).
• Stars indicating steady-state variables are included as subscripts: π_star_t
• Suffixes
• Time: Consistent with the previous bullet points, the suffix _t as in x_t signifies the value of x at time t. The suffix _t1 signifies the value of x at time t-1.
• Shocks: The suffix _sh refers to a model shock.
• Prefixes
• The prefix eq_ refers to an equilibrium condition.
• The prefix obs_ refers to an observable.
• The prefix E refers to the expectation operator.
• The prefix I refers to the indicator operator.
• Observables with the prefix g` refer to growth rates.

### Code Formatting Guidelines

• Indent 4 spaces
• Wrap lines at 92 characters
• Use whitespace to enhance readability
• No trailing whitespace