RdrNames, Modules, and OccNames

When the parser parses an identifier, it generates a RdrName. A RdrName is pretty much just a string, or a pair of strings, for a qualified name, such as M.x. Here's the data type declaration, from compiler/basicTypes/RdrName.lhs:

data RdrName 
  = Unqual OccName
	-- Used for ordinary, unqualified occurrences 

  | Qual ModuleName OccName
	-- A qualified name written by the user in 
	--  *source* code.  The module isn't necessarily 
	-- the module where the thing is defined; 
	-- just the one from which it is imported

  | Orig Module OccName
	-- An original name; the module is the *defining* module.
	-- This is used when GHC generates code that will be fed
	-- into the renamer (e.g. from deriving clauses), but where
	-- we want to say "Use Prelude.map dammit".  
 
  | Exact Name
	-- We know exactly the Name. This is used 
	--  (a) when the parser parses built-in syntax like "[]" 
	--	and "(,)", but wants a RdrName from it
	--  (b) when converting names to the RdrNames in IfaceTypes
	--	Here an Exact RdrName always contains an External Name
	--	(Internal Names are converted to simple Unquals)
	--  (c) by Template Haskell, when TH has generated a unique name

The Module and ModuleName types

In GHC, a module is uniquely defined by a pair of the module name and the package where the module is defined. The details are in compiler/basicTypes/Module.lhs and compiler/main/PackageConfig.hs, but here are the key definitions:

newtype PackageId = PId FastString

newtype ModuleName = ModuleName FastString

data Module = Module {
   modulePackageId :: !PackageId,  -- pkg-1.0
   moduleName      :: !ModuleName  -- A.B.C  }

You'll notice that a Qual RdrName contains a ModuleName; which module is referred to depends on the import declarations in that module. In contrast, a Orig RdrName refers to a unique Module.

The OccName type

An OccName is more-or-less just a string, like "foo" or "Tree", giving the (unqualified) name of an entity. Well, not quite just a string, because in Haskell a name like "C" could mean a type constructor or data constructor, depending on context. So GHC defines a type OccName? (defined in basicTypes/OccName.lhs) that is a pair of a FastString and a NameSpace indicating which name space the name is drawn from. The data type is defined (abstractly) in compiler/basicTypes/OccName.lhs:

data OccName = OccName NameSpace EncodedFS

The EncodedFS is a synonym for FastString indicating that the string is Z-encoded. (Details in compiler/basicTypes/OccName.lhs.) Z-encoding encodes funny characters like '%' and '$' into alphabetic characters, like "zp" and "zd", so that they can be used in object-file symbol tables without confusing linkers and suchlike.

The name spaces are:

data NameSpace = VarName	-- Variables, including "source" data constructors
	       | DataName	-- "Real" data constructors 
	       | TvName		-- Type variables
	       | TcClsName	-- Type constructors and classes; Haskell has them
				-- in the same name space for now.