When it starts, unless the -ignore-dot-ghci flag is given, GHCi reads and executes commands from ./.ghci, followed by $HOME/.ghci.

The .ghci in your home directory is most useful for turning on favourite options (eg. :set +s), and defining useful macros. Placing a .ghci file in a directory with a Haskell project is a useful way to set certain project-wide options so you don't have to type them everytime you start GHCi: eg. if your project uses GHC extensions and CPP, and has source files in three subdirectories A, B and C, you might put the following lines in .ghci:

:set -fglasgow-exts -cpp
:set -iA:B:C

(Note that strictly speaking the -i flag is a static one, but in fact it works to set it using :set like this. The changes won't take effect until the next :load, though.)

Two command-line options control whether the .ghci files are read: