2.6. Invoking GHCi

GHCi is invoked with the command ghci or ghc ––interactive. One or more modules or filenames can also be specified on the command line; this instructs GHCi to load the specified modules or filenames (and all the modules they depend on), just as if you had said :load modules at the GHCi prompt (see Section 2.7, “GHCi commands”). For example, to start GHCi and load the program whose topmost module is in the file Main.hs, we could say:

$ ghci Main.hs

Most of the command-line options accepted by GHC (see Chapter 4, Using GHC) also make sense in interactive mode. The ones that don't make sense are mostly obvious.

2.6.1. Packages

Most packages (see Section 4.9.1, “Using Packages ”) are available without needing to specify any extra flags at all: they will be automatically loaded the first time they are needed.

For hidden packages, however, you need to request the package be loaded by using the -package flag:

$ ghci -package readline
GHCi, version 6.8.1: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Loading package readline-1.0 ... linking ... done.

The following command works to load new packages into a running GHCi:

Prelude> :set -package name

But note that doing this will cause all currently loaded modules to be unloaded, and you'll be dumped back into the Prelude.

2.6.2. Extra libraries

Extra libraries may be specified on the command line using the normal -llib option. (The term library here refers to libraries of foreign object code; for using libraries of Haskell source code, see Section 2.2.1, “Modules vs. filenames”.) For example, to load the “m” library:

$ ghci -lm

On systems with .so-style shared libraries, the actual library loaded will the liblib.so. GHCi searches the following places for libraries, in this order:

  • Paths specified using the -Lpath command-line option,

  • the standard library search path for your system, which on some systems may be overridden by setting the LD_LIBRARY_PATH environment variable.

On systems with .dll-style shared libraries, the actual library loaded will be lib.dll. Again, GHCi will signal an error if it can't find the library.

GHCi can also load plain object files (.o or .obj depending on your platform) from the command-line. Just add the name the object file to the command line.

Ordering of -l options matters: a library should be mentioned before the libraries it depends on (see Section 4.11.6, “Options affecting linking”).