Kalker is a multi-platform mathematical calculator implemented primarily in Rust. It provides a CLI with a REPL, a web component compiled to WebAssembly, and an Android mobile app via Capacitor. The core library (`kalk`) supports user-defined variables and functions, units, differentiation, integration, root finding, equation systems, matrices/vectors, complex numbers, and multiple number bases. The codebase is well-structured with a clear separation between parsing (lexer/parser), evaluation (interpreter), and a custom value system. It features extensive test files and integration tests for numerical methods. While the core numerical routines (tanh-sinh quadrature, Newton's method, Gaussian elimination) are tightly coupled to the interpreter, they are implemented with care and demonstrate non-trivial numerical analysis. The repository also includes build scripts for multiple targets and CI/CD pipelines.