9.2. Using GHC as a Library
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## 9.2. Using GHC as a Library

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.

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