easi is a library for the Easy Initialization of models in three (or less or more) dimensional domains. The purpose of easi is to evaluate functions f : R^m -> R^n, which are described in a YAML configuration file. In grid-based simulation software, such as SeisSol, easi may be used to define models. In SeisSol, the function f maps every point in space (say x,y,z) to a vector of parameters (e.g. ρ, μ, and λ), which define a rheological model. Here, over 5000 lines of model-specific Fortran code could be replaced with YAML files.
The reference documentation can be found on Read the Docs or be downloaded as PDF manual.