How to configure GHCi to automatically import modules

Question

When I use GHCi, I almost always end up importing Control.Applicative, Data.List, etc. . Is there a way to configure GHCi to automatically import those modules.

Also, after importing them, how do I keep the prompt from being insanely long?

Prelude Control.Applicative Data.List Database.HDBC Database.HDBC.Sqlite3 System.Directory>

Answer 1

GHCi looks for its configuration file at ~/.ghc/ghci.conf on Unix-like systems. The configuration file syntax is simple: it's a list of GHCi commands to execute on startup.

For example, your ghci.conf could contain:

import Control.Applicative
import Data.Char
import Data.List

:set prompt "> "

The last line sets the prompt to "> " so it won't show all the modules you imported on the command line.

Now you can get to work right away:

GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
> toLower <$> "Hello, world!"
"hello, world!"
> 

Also, if you decide you don't want Data.Char in the middle of a GHCi session, you can remove it with:

:m -Data.Char

and if you decide you don't want anything but Prelude during a session:

:m

Answer 2

GHC will also load any .ghci file it finds in the current directory. It's very useful to do per-project configuration of GHCi.

This is an example from a project I work on:

:set -isrc:dist/build/autogen
:set -hide-package mtl

The first is there to make sure that the modules generated by Cabal are easy to import. The second hides mtl since this particular project uses transformers.