# The `stack ghci` and `stack repl` commands ~~~text stack ghci [TARGET/FILE] [--pedantic] [--ghci-options OPTIONS] [--ghc-options OPTIONS] [--flag PACKAGE:[-]FLAG] [--with-ghc GHC] [--[no-]load] [--package PACKAGE] [--main-is TARGET] [--load-local-deps] [--[no-]package-hiding] [--only-main] [--trace] [--profile] [--no-strip] [--[no-]test] [--[no-]bench] ~~~ A read–evaluate–print loop (REPL) environment takes single user inputs, executes them, and returns the result to the user. GHCi is GHC's interactive environment. The `stack ghci` or `stack repl` commands, which are equivalent, allow you to load components and files of your project into GHCi. The command uses the same TARGET syntax as `stack build`. It can also take flags like `--test` and `--bench` and options like `--flag`. Similarly to `stack build`, the default is to load up GHCi with all libraries and executables in the project. In order to load multiple components, `stack ghci` combines all of the GHC options together. This doesn't work in the general case, but when the packages being loaded share similar conventions, it should work out. A common source of issues is when one component defines default extensions which aren't assumed by another component. For example, specifying `NoImplicitPrelude` in one component but not another is quite likely to cause failures. GHCi will be run with `-XNoImplicitPrelude`, but it is likely that modules in the other component assume that the `Prelude` is implicitly imported. `stack ghci` configures GHCi by using a GHCi script file. Such files are located in subdirectories of `/stack/ghci-script`, where `` refers to the [XDG Base Directory Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html) for user-specific non-essential (cached) data. On Unix-like operating systems, the default for `` is `$HOME/.cache`. On Windows, the default is `%LOCALAPPDATA%`. ## Selecting Main module When loading multiple packages, there may be multiple definitions for the `Main` module. You can specify which `Main` module to load with the `--main-is ` option. If no selection is made and there are multiple `Main` modules, you will be asked to select from a list of options. ## Speeding up initial load There are two ways to speed up the initial startup of GHCi: 1. Pass the `--no-build` flag, to skip an initial build step. This works only if the dependencies have already been built. 2. Pass the `--no-load` flag, to skip loading all defined modules into GHCi. You can then directly use `:load MyModule` in GHCi to load a specific module in your project. ## Loading just the main module By default, `stack ghci` loads and imports all of the modules in the package. This allows you to easily use anything exported by your package. This is usually quite convenient, but in some cases it makes sense to only load one module, or no modules at all. The `--only-main` flag allows this. It specifies that only the main module will be loaded, if any. This is particularly useful in the following circumstances: 1. You're loading the project in order to run it in GHCi (e.g. via `main`), and you intend to reload while developing. Without the `--only-main` flag, you will need to quit and restart GHCi whenever a module gets deleted. With the flag, reloading should work fine in this case. 2. If many of your modules have exports named the same thing, then you'll need to refer to them using qualified names. To avoid this, it may be easier to use the `--only-main` flag to start with a blank slate and just import the modules you are interested in. ## Loading a filepath directly Instead of the `TARGET` syntax, it is also possible to command directly, for example: ~~~text stack ghci src/MyFile.hs ~~~ This will figure out which component the file is associated with, and use the options from that component. ## Specifying extra packages to build or depend on Sometimes you want to load GHCi with an additional package, that isn't a direct dependency of your components. This can be achieved by using the `--package` option. For example, if I want to experiment with the `lens` library, I can command: ~~~text stack ghci --package lens ~~~ ## Running plain GHCi `stack ghci` always runs GHCi configured to load code from packages in your project. In particular, this means it passes in flags like `-hide-all-packages` and `-package-id=` in order to configure which packages are visible to GHCi. For doing experiments which just involve packages installed in your databases, it may be useful to run GHCi plainly like: ~~~text stack exec ghci ~~~ This will run a plain GHCi in an environment which includes `GHC_PACKAGE_PATH`, and so will have access to your databases. !!! note Running `stack ghci` on a pristine copy of the code doesn't currently build libraries (issue [#2790](https://github.com/commercialhaskell/stack/issues/2790)) or internal libraries (issue [#4148](https://github.com/commercialhaskell/stack/issues/4148)). It is recommended to always use `stack build` before using `stack ghci`, until these two issues are closed.