The ghc package exposes most of GHC's frontend to users, and thus allows you to write programs that leverage it. This library is actually the same library used by GHC's internal, frontend compilation driver, and thus allows you to write tools that programmatically compile source code and inspect it. Such functionality is useful in order to write things like IDE or refactoring tools. As a simple example, here's a program which compiles a module, much like ghc itself does by default when invoked:

import GHC
import GHC.Paths ( libdir )
import DynFlags ( defaultDynFlags )
 
main = 
    defaultErrorHandler defaultDynFlags $ do
      runGhc (Just libdir) $ do
        dflags <- getSessionDynFlags
        setSessionDynFlags dflags
        target <- guessTarget "test_main.hs" Nothing
        setTargets [target]
        load LoadAllTargets

The argument to runGhc is a bit tricky. GHC needs this to find its libraries, so the argument must refer to the directory that is printed by ghc --print-libdir for the same version of GHC that the program is being compiled with. Above we therefore use the ghc-paths package which provides this for us.

Compiling it results in:

$ cat test_main.hs
main = putStrLn "hi"
$ ghc -package ghc simple_ghc_api.hs
[1 of 1] Compiling Main             ( simple_ghc_api.hs, simple_ghc_api.o )
Linking simple_ghc_api ...
$ ./simple_ghc_api
$ ./test_main 
hi
$ 

For more information on using the API, as well as more samples and references, please see this Haskell.org wiki page.