GHC.Driver.Monad

A monad that has all the features needed by GHC API calls.

In short, a GHC monad

• allows embedding of IO actions,

• can log warnings,

• allows handling of (extensible) exceptions, and

• maintains a current session.

If you do not use Ghc or GhcT, make sure to call initGhcMonad before any call to the GHC API functions can occur.

A minimal implementation of a GhcMonad. If you need a custom monad, e.g., to maintain additional state consider wrapping this monad or using GhcT.

A monad transformer to add GHC specific features to another monad.

Note that the wrapped monad must support IO and handling of exceptions.

Reflect a computation in the Ghc monad into the IO monad.

You can use this to call functions returning an action in the Ghc monad inside an IO action. This is needed for some (too restrictive) callback arguments of some library functions:

libFunc :: String -> (Int -> IO a) -> IO a
ghcFunc :: Int -> Ghc a

ghcFuncUsingLibFunc :: String -> Ghc a -> Ghc a
ghcFuncUsingLibFunc str =
  reifyGhc $ \s ->
    libFunc $ \i -> do
      reflectGhc (ghcFunc i) s

Grabs the DynFlags from the Session

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

The Session is a handle to the complete state of a compilation session. A compilation session consists of a set of modules constituting the current program or library, the context for interactive evaluation, and various caches.

Call the argument with the current session.

Set the current session to the result of applying the current session to the argument.

Set the current session to the result of applying the current session to the argument.

Call an action with a temporarily modified Session.

Modify the logger

Push a log hook on the stack

Pop a log hook from the stack

Put a log message

Put a log message

Time an action