Miller (mlr) is a command-line data processing tool for structured text formats including CSV, TSV, JSON, JSON Lines, YAML, DKVP, and others. It operates as a streaming pipeline: input records are read from files or stdin, processed through a chain of transformer verbs (e.g., cut, sort, join, stats1, put, filter), and written to output in any supported format. The tool includes a domain-specific language (DSL) for record-level transformations, with support for user-defined functions, subroutines, loops, and higher-order functions. Miller is designed for performance, using Go channels for concurrent streaming, batch processing, and a slab allocator for efficient memory use. The architecture is a monolithic CLI application written in Go with no external runtime dependencies. Key entry points include the main binary (cmd/mlr/main.go), the entrypoint dispatcher (pkg/entrypoint), and the terminal subcommands (help, repl, script, regtest, mcp). The codebase is organized into packages for input readers, output writers, transformers (verbs), DSL parsing and execution, CLI flag handling, and a core value type system (Mlrval). Miller also provides an MCP (Model Context Protocol) server for AI agent integration, shell completion scripts, and a REPL for interactive use. Miller is not a research project; it is a production-grade data munging tool comparable to awk, sed, cut, join, and sort but for name-indexed data. Its primary data flows involve reading heterogeneous records, applying transformations via verbs or DSL expressions, and writing results. The tool handles compressed files, in-place editing, and output redirection to files or pipes.
VisiData is a terminal-based interactive spreadsheet application for exploring, cleaning, restructuring, and analyzing tabular data. It provides a real-time curses interface for loading data from diverse sources (CSV, JSON, Excel, SQL databases, APIs, etc.), applying transformations via hundreds of keyboard-driven commands (sort, filter, pivot, melt, unfurl, etc.), and saving results. The architecture centers around a Sheet/Column abstraction with a plugin system for loaders, features, and specialized applications (vgit for Git, vdsql for SQL). Key strengths include its extensible command system, dynamic type inference, and reversible operations via undo/redo. While the core is cohesive, the extensive optional dependencies and experimental features introduce some maintenance complexity.