How can I import a Haskell module in GHCi?

When you use import ModuleName in GHCi, it works (mostly) in the same way import Data.List works: GHC checks your local package database for the module, loads it, and brings its (exported) contents into scope.

However, Geometry isn’t a module of a package installed with ghc-pkg. Therefore, GHC doesn’t know that a module Geometry exists at all. Neither does it interactive variant GHCi.

But if you :load a program, things change. GHC will take its used modules into account:

-- Foo.hs
module Foo where

foo :: IO ()
foo = putStrLn "Hello from foo!"

-- Main.hs
module Main where
import Foo (foo)

main :: IO ()
main = foo

$ cd /path/to/your/files
$ ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> import Foo

<no location info>:
    Could not find module ‘Foo’
    It is not a module in the current program, or in any known package.

Prelude> :l Main.hs
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Main             ( Main.hs, interpreted )
Ok, modules loaded: Main, Foo.
*Main> :l Main.hs
*Main> foo
Hello from foo!
*Main> import Foo
*Main Foo> -- module now loaded

As you can see, importing Foo first failed. However, after we’ve actually loaded the program that uses Foo, we were able to use import Foo in GHCi.

So if you want to use import in GHCi, make sure that GHC can find your module, either by including it in a wrapper or installing it. If you just want to load the module itself, use :load.