“ghci” refers to the Glasgow Haskell Compiler’s interactive environment. It is a command-line tool that provides an interactive and dynamic environment for working with Haskell code. GHCi allows developers to load Haskell modules, experiment with code snippets, and interactively evaluate expressions.
Here are some key features and functionalities of GHCi:
- Interactive Evaluation: GHCi enables developers to enter Haskell expressions, which are immediately evaluated and the result is displayed. This interactive mode allows for rapid prototyping, exploration, and testing of Haskell code without the need to compile and run a complete program.
- Code Loading: GHCi provides the ability to load Haskell source files or modules directly into the interactive session. This allows developers to incrementally develop and test their code, reloading modules as changes are made. It supports both individual file loading and loading entire projects or libraries.
- Expression Evaluation: In GHCi, developers can enter Haskell expressions, including functions, data types, and values, and observe the computed results. This feature is particularly useful for debugging and understanding the behavior of specific code snippets.
- Type Inference: GHCi performs type inference on entered expressions and displays the inferred types. It helps developers catch type errors and provides valuable feedback on the types of expressions, assisting in the development process.
- Documentation and Help: GHCi provides access to Haskell documentation and help resources. Developers can query information about functions, types, and modules, getting detailed documentation and usage examples. This feature helps developers explore and understand the available Haskell libraries and APIs.
- Module Exploration: GHCi allows developers to explore loaded modules, inspect their contents, and access their definitions. It provides a way to navigate through the imported modules, view function signatures, and explore the available functions and data types.
- Debugging Support: GHCi includes debugging capabilities, such as setting breakpoints, stepping through code, and inspecting variable values during execution. This facilitates the debugging process and helps developers identify and fix issues in their Haskell programs.
GHCi is a powerful tool for Haskell development, providing an interactive and flexible environment for writing, testing, and exploring Haskell code. Its ability to quickly evaluate expressions, load modules, and provide type information makes it invaluable for learning, experimenting, and debugging Haskell programs.
GHCi is bundled with the Glasgow Haskell Compiler (GHC) and is available on multiple platforms, including Windows, macOS, and various Unix-like systems. It is widely used by Haskell developers, both beginners and experienced, as an essential tool in their development workflow.
ghci Command Examples
1. Start a REPL (interactive shell):
# ghci
2. Start a REPL and load the specified Haskell source file:
# ghci source_file.hs
3. Start a REPL and enable a language option:
# ghci -Xlanguage_option
4. Start a REPL and enable some level of compiler warnings (e.g. all or compact):
# ghci -Wwarning_level
5. Start a REPL with a colon-separated list of directories for finding source files:
# ghci -i /path/to/directory1:/path/to/directory2