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:
./.ghci
appdata/ghc/ghci.confappdata depends on your system,
but is usually something like C:/Documents and Settings/user/Application Data
On Unix: $HOME/.ghc/ghci.conf
$HOME/.ghci
The ghci.conf file is most useful for
turning on favourite options (eg. :set +s), and
defining useful macros. Note: when setting language options in
this file it is usually desirable to use :seti
rather than :set (see Section 2.8.3, “Setting options for interactive evaluation only”).
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 multi-parameter
type classes, scoped type variables,
and CPP, and has source files in three subdirectories A, B and C,
you might put the following lines in
.ghci:
:set -XMultiParamTypeClasses -XScopedTypeVariables -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
Additionally, any files specified with
-ghci-script flags will be read after the
standard files, allowing the use of custom .ghci files.
Two command-line options control whether the startup files files are read:
When defining GHCi macros, there is some important behavior you should be aware of when names may conflict with built-in commands, especially regarding tab completion.
For example, consider if you had a macro named
:time and in the shell, typed :t
3 - what should happen? The current algorithm we use
for completing commands is:
First, look up an exact match on the name from the defined macros.
Look for the exact match on the name in the built-in command list.
Do a prefix lookup on the list of built-in commands - if a built-in command matches, but a macro is defined with the same name as the built-in defined, pick the macro.
Do a prefix lookup on the list of built-in commands.
Do a prefix lookup on the list of defined macros.
Here are some examples:
You have a macro :time and enter :t 3
You get :type 3
You have a macro :type and enter :t 3
You get :type 3 with your defined macro, not the builtin.
You have a macro :time and a macro
:type, and enter :t
3
You get :type 3 with your defined macro.