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The monad for renaming, typechecking, desugaring

The renamer, typechecker, interface-file typechecker, and desugarer all share a certain amount in common: they must report errors, handle environments, do I/O, etc. Furthermore, because of Template Haskell we have to interleave renaming and typechecking. So all four share a common monad, called TcRnIf. This infrastructure is defined by the following modules:

• compiler/GHC/Data/IOEnv.hs: extends the IO monad with an environment (just a simple reader monad).
• compiler/GHC/Tc/Types.hs: builds the TcRnIf monad on top of IOEnv:
• compiler/GHC/Tc/Utils/Monad.hs: defines lots of access functions for the renamer, typechecker, and interface typechecker.
• compiler/GHC/HsToCore/Monad.hs: specialises the TcRnIf monad for the desugarer.

The typechecker and renamer use exactly the same monad, TcRn; the desugarer and interface-file checker use different instantiations of TcRnIf. To give you the idea, here is how the TcRn monad looks:

type TcRn a       = TcRnIf TcGblEnv TcLclEnv a
type TcRnIf a b c = IOEnv (Env a b) c

data Env gbl lcl	-- Changes as we move into an expression
  = Env {
	env_top	 :: HscEnv,	-- Top-level stuff that never changes
				-- Includes all info about imported things

	env_us   :: TcRef UniqSupply,	-- Unique supply for local varibles

	env_gbl  :: gbl,	-- Info about things defined at the top level
				-- of the module being compiled

	env_lcl  :: lcl		-- Nested stuff; changes as we go into 
				-- an expression
    }

type RnM  a = TcRn a		-- Historical
type TcM  a = TcRn a		-- Historical

The details of the global environment type TcGblEnv and local environment type TcLclEnv are also defined in compiler/GHC/Tc/Types.hs. Side effecting operations, such as updating the unique supply, are done with TcRefs, which are simply a synonym for IORefs.