When it starts, unless the -ignore-dot-ghci flag is given, GHCi reads and executes commands from the following files, in this order, if they exist:

The ghci.conf file 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 every time 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.)

Once you have a library of GHCi macros, you may want to source them from separate files, or you may want to source your .ghci file into your running GHCi session while debugging it

:def source readFile

With this macro defined in your .ghci file, you can use :source file to read GHCi commands from file. You can find (and contribute!-) other suggestions for .ghci files on this Haskell wiki page: GHC/GHCi

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

Additional .ghci files can be added through the -ghci-script option. These are loaded after the normal .ghci files.