GHC/As a library (up to 6.8)

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Using GHC as a library

In GHC 6.5 and subsequently you can import GHC as a Haskell library, which lets you write a Haskell program that has access to all of GHC.

This page is a place for everyone to add

  • Notes about how to get it working
  • Comments about the API
  • Suggestions for improvement

and so on.

Getting started

You'll need a version of GHC (at least 6.5) that supports the GHC API. The GHC download page offers stable releases and development versions; you can also use CVS (instructions) or darcs (e.g., darcs get --partial

To use the GHC API you say

import GHC

Doing this imports the module GHC from the package ghc. This module exports the "GHC API", which is still in a state of flux. Currently it's not even Haddock-documented. You can see the source code (somewhat documented). There are also other modules of interest as you do more special things.

Here's an example main program that does it Media:Main.hs (good for GHC 6.6). You need to manually change the value of myGhcRoot to point to your GHC directory.

To compile Media:Main.hs, you have to turn on the flag "-package ghc", e.g.

  ghc -package ghc Main.hs

Common use cases and functions

Assumes GHC 6.6.

Default exception and cleanup handling



First create a session:

newSession :: GhcMode         -- BatchCompile | Interactive | MkDepend | ...
           -> Maybe FilePath  -- GHC installation directory
           -> IO Session      -- your seesion; you will need it

The path to your GHC installation directory (e.g., /usr/local/lib/ghc-6.6) is in practice mandatory, even though in theory marked as optional.

The session is configurable by dynamic flags (GHC dynamic flags plus session state; think -O2, -fvia-C, -fglasgow-exts, -package). This can be done with:

getSessionDynFlags :: Session -> IO DynFlags
setSessionDynFlags :: Session
                   -> DynFlags
                   -> IO [PackageId]  -- important iff dynamic-linking
parseDynamicFlags :: DynFlags  -- old flags
                  -> [String]  -- e.g., all or part of getArgs
                  -> IO (DynFlags, [String])  -- new flags, unknown args

The DynFlags record has a gazillion fields; ask ghci to show all of them. You can change them by hand, or use the parser (which implements the GHC command line format and does the Right Thing). But there is one you must note:

data DynFlags = DynFlags { ...,
    hscTarget :: HscTarget }    -- HscC | HscAsm | HscInterpreted | ...

This corresponds to -fvia-C, -fasm, or interpreting. When the session needs to re-compile a module, this field controls how. The default is HscAsm, even in the interactive mode, meaning the interactive mode may produce .hi and .o files too. If you want to follow GHCi in not doing that, you must set this field to HscInterpreted yourself. (On the other hand, it is fun to contemplate an interactive session that generates machine code upon your command.)

setSessionDynFlags also sets up your session's awareness of the package database (without which you can't even use the Prelude), so even if you like the defaults, you should still call it. (Older code called PackageConfig.initPackages for this.)


  • vanilla compiler, use all defaults (rare but good start)
session <- newSession BatchCompile (Just "/usr/local/lib/ghc-6.6")
getSessionDynFlags session >>= setSessionDynFlags session
  • compiler with custom flags, easy with parser
session <- newSession BatchCompile (Just "/usr/local/lib/ghc-6.6")
f0 <- getSessionDynFlags session
(f1,b) <- parseDynamicFlags f0 ["-fglasgow-exts", "-O", "-package", "ghc", "-package Cabal",
                                "foo", "-v", "bar"]
-- b = ["foo", "bar"]; the other args are recognized
-- in GHC 6.6 "-O" implies "-fvia-C", that kind of thing is automatic here too
setSessionDynFlags session f1
  • interactive session with interpreter
session <- newSession Interactive (Just "/usr/local/lib/ghc-6.6")
f0 <- getSessionDynFlags session
setSessionDynFlags session f0{hscTarget = HscInterpreted}

Load or compile modules

To compile code or load modules, first set one or more targets, then call the load function.

guessTarget :: String       -- "filename.hs" or "filename.lhs" or "MyModule"
            -> Maybe Phase  -- if not Nothing, specifies starting phase
            -> IO Target
addTarget :: Session -> Target -> IO ()
setTargets :: Session -> [Target] -> IO ()
getTargets :: Session -> IO [Target]
removeTarget :: Session -> TargetId -> IO ()
load :: Session -> LoadHowMuch -> IO SuccessFlag
data LoadHowMuch
  = LoadAllTargets
  | LoadUpTo ModuleName
  | LoadDependenciesOf ModuleName


t <- guessTarget "Main.hs" Nothing
addTarget session t    -- setTargets session [t] is also good
sf <- load session LoadAllTargets
case sf of Succeeded -> ...
           Failed -> ...

Dependencies are processed automatically (and silently).

Modules are compiled as per the hscTarget flag (-fasm, -fvia-C, or interpreter) in DynFlags, independent of GHC mode.

Interactive evaluation

Interactive evaluation ala GHCi is done by runStmt. But first, this is always done under a current context, i.e., which modules are in scope. Most probably you want to have at least the Prelude and those you loaded in the previous section. How to manipulate the context:

setContext :: Session
           -> [Module]    -- their top levels will be visible
           -> [Module]    -- their exports will be visible
           -> IO ()
getContext :: Session -> IO ([Module], [Module])
findModule :: Session -> ModuleName -> Maybe PackageId -> IO Module
mkModule :: PackageId -> ModuleName -> Module
mkModuleName :: String -> ModuleName
PackageConfig.stringToPackageId :: String -> PackageId

Every module given to setContext must be either in a package known to the session or has been loaded as per the previous subsection. Example:

-- equivalent to GHCi's :m Prelude Control.Monad *Main
prelude <- findModule session (mkModuleName "Prelude") Nothing
monad <- findModule session (mkModuleName "Control.Monad") Nothing
usermod <- findModule session (mkModuleName "Main") Nothing  -- we have loaded this
setContext session [usermod] [prelude,monad]

You can also be specific about packages. You can also use mkModule instead of findModule, or even some module query functions in the next subsection.

Having set a useful context, we're now ready to evaluate.

runStmt :: Session -> String -> IO RunResult
data RunResult
    = RunOk [Name]    -- names bound by the expression
    | RunFailed
    | RunException GHC.IOBase.Exception  -- that's Control.Exception.Exception


runStmt session "let n = 2 + 2"  -- n is bound
runStmt session "n"              -- 4 is printed (note "it" is bound)

(Interactive evaluation works in BatchCompile mode too! There are still other subtle differences, so this is not recommended.)


-- Get module dependency graph
getModuleGraph :: Session -> IO ModuleGraph    -- ModuleGraph = [ModSummary]
-- Get bindings
getBindings :: Session -> IO [TyThing]


Compiler messages are controlled by verbosity and routed through a callback mechanism. These are fields in DynFlags:

data DynFlags = DynFlags { ...,
    verbosity :: Int,
    log_action :: Severity -> SrcLoc.SrcSpan -> Outputable.PprStyle -> ErrUtils.Message -> IO () }

The verbosity field corresponds to the command line -v flag. A low verbosity means log_action is seldom called.

You can set the callback to your logger, like

f <- getSessionDynFlags session
setSessionDynFlags session f{log_action = my_log_action}

You can set verbosity by setting the field or using parseDynamicFlags (introduced above) to parse "-v" strings.

Interactive mode example

The file Media:Interactive.hs (also requires Media:MyPrelude.hs) serves as an example for using GHC as a library in interactive mode. It also shows how to replace some of the standard prelude functions with modified versions. See the comments in the code for further information.

Using the GHC library from inside GHCi

This works, to some extent. However, beware about loading object code, because there is only a single linker symbol table in the runtime, so GHCi will be sharing the symbol table with the new GHC session.

$ ghci -package ghc
Prelude> :m + GHC PackageConfig
Prelude GHC> session <- newSession Interactive (Just "/usr/local/lib/ghc-6.6")
Prelude GHC> setSessionDynFlags session =<< getSessionDynFlags session
Prelude GHC> setContext session [] [mkModule (stringToPackageId "base") (mkModuleName "Prelude")]
Prelude GHC> runStmt session "let add1 x = x + 1"
Prelude GHC> runStmt session "add1 2"