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GHCi

stack ghci allows you to load components and files of your project into ghci. It uses the same TARGET syntax as stack build, and can also take options like --test, --bench, and --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.

Selecting Main module

When loading multiple packages, there may be multiple definitions for the Main module. You can specify which Main module to load by passing in the --main-is TARGET flag. 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:

  • --no-build, to skip an initial build step. This only works if the dependencies have already been built.

  • --no-load, to skip loading all defined modules into ghci. You can then directly use :load MyModule 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 --only-main 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 directly run 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 / 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 flag. For example, if I want to experiment with the lens library, I can run 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 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 (#2790) or internal libraries (#4148). It is recommended to always run a stack build before running stack ghci, until these two issues are closed.