GHCi is the interactive GHC environment, a.k.a. the REPL. You could access it with command:
stack exec ghci
But that won't load up locally written modules for access. For that, use the
stack ghci or stack repl commands, which are equivalent.
To then load
modules from your project in GHCi, use the :module command (:m for short)
followed by the module name.
!!! note
If you have added packages to your project please make sure to mark them as
extra-deps for faster and reliable usage of `stack ghci`. Otherwise GHCi may
have trouble due to conflicts of compilation flags or having to
unnecessarily interpret too many modules. See Stack's project-level
[configuration](../configure/yaml/project.md#extra-deps) to learn how to
configure a package as an extra-dep.
For further information, see the REPL environment documentation.