The `stack ghci` command does not load extra packages such as those installed "globally." This is a good thing, IMHO. You can instruct the command to load specific packages using the `--package` option. Example: stack repl --package safe Example using your file: stack repl --package safe GalaxyBeing.hs Note that your email indicates usage of the `Safe` package (uppercase), but that package is deprecated in favor of the `safe` package (lowercase). I mention it in case it is not intentional. * https://hackage.haskell.org/package/safe * https://hackage.haskell.org/package/Safe You may want to create a project that is just for testing. You can then specify which Stack snapshot to use in `stack.yaml` and which packages to use in `package.yaml`. stack new experiment Travis