+ GHC essentially supports two type systems: (1) the source type + system (which is a heavily extended version of the type system of + Haskell 98) and (2) the Core type system, which is the type system + used by the intermediate language (see also Sugar Free: From Haskell To Core). +
+
+ During parsing and renaming, type information is represented in a form
+ that is very close to Haskell's concrete syntax; it is defined by
+ HsTypes.HsType. In addition, type, class, and instance
+ declarations are maintained in their source form as defined in the
+ module HsDecl. The situation changes during type checking,
+ where types are translated into a second representation, which is
+ defined in the module types/TypeRep.lhs, as type
+ Type. This second representation is peculiar in that it is
+ a hybrid between the source representation of types and the Core
+ representation of types. Using functions, such as
+ Type.coreView and Type.deepCoreView, a value
+ of type Type exhibits its Core representation. On the
+ other hand, pretty printing a Type with
+ TypeRep.pprType yields the type's source representation.
+
+ In fact, the type checker maintains type
+ environments based on Type, but needs to perform type
+ checking on source-level types. As a result, we have functions
+ Type.tcEqType and Type.tcCmpType, which
+ compare types based on their source representation, as well as the
+ function coreEqType, which compares them based on their
+ core representation. The latter is needed during type checking of Core
+ (as performed by the functions in the module
+ coreSyn/CoreLint.lhs).
+
+ Type synonyms in Haskell are essentially a form of macro definitions on
+ the type level. For example, when the type checker compares two type
+ terms, synonyms are always compared in their expanded form. However, to
+ produce good error messages, we like to avoid expanding type synonyms
+ during pretty printing. Hence, Type has a variant
+ NoteTy TyNote Type, where
+
+++data TyNote + = FTVNote TyVarSet -- The free type variables of the noted expression + + | SynNote Type -- Used for type synonyms + -- The Type is always a TyConApp, and is the un-expanded form. + -- The type to which the note is attached is the expanded form.+
+ In other words, a NoteTy represents the expanded form of a
+ type synonym together with a note stating its source form.
+
+ During translation from HsType to Type the
+ function Type.mkSynTy is used to construct representations
+ of applications of type synonyms. It creates a NoteTy node
+ if the synonym is applied to a sufficient number of arguments;
+ otherwise, it builds a simple TyConApp and leaves it to
+ TcMType.checkValidType to pick up invalid unsaturated
+ synonym applications. While creating a NoteTy,
+ mkSynTy also expands the synonym by substituting the type
+ arguments for the parameters of the synonym definition, using
+ Type.substTyWith.
+
+ The function mkSynTy is used indirectly via
+ mkGenTyConApp, mkAppTy, and
+ mkAppTy, which construct type representations involving
+ type applications. The function mkSynTy is also used
+ directly during type checking interface files; this is for tedious
+ reasons to do with forall hoisting - see the comment at
+ TcIface.mkIfTcApp.
+
+ Data types declared by a newtype declarations constitute new
+ type constructors---i.e., they are not just type macros, but introduce
+ new type names. However, provided that a newtype is not recursive, we
+ still want to implement it by its representation type. GHC realises this
+ by providing two flavours of type equality: (1) tcEqType is
+ source-level type equality, which compares newtypes and
+ PredTypes by name, and (2) coreEqType compares
+ them structurally (by using deepCoreView to expand the
+ representation before comparing). The function
+ deepCoreView (via coreView) invokes
+ expandNewTcApp for every type constructor application
+ (TyConApp) to determine whether we are looking at a newtype
+ application that needs to be expanded to its representation type.
+
+ The dictionary translation of type classes, translates each predicate in
+ a type context of a type signature into an additional argument, which
+ carries a dictionary with the functions overloaded by the corresponding
+ class. The Type data type has a special variant
+ PredTy PredType for predicates, where
+
+++data PredType + = ClassP Class [Type] -- Class predicate + | IParam (IPName Name) Type -- Implicit parameter+
+ These types need to be handled as source type during type checking, but
+ turn into their representations when inspected through
+ coreView. The representation is determined by
+ Type.predTypeRep.
+
+ Type constructor applications are represented in Type by
+ the variant TyConApp :: TyCon -> [Type] -> Type. The first
+ argument to TyConApp, namely TyCon.TyCon,
+ distinguishes between function type constructors (variant
+ FunTyCon) and algebraic type constructors (variant
+ AlgTyCon), which arise from data and newtype declarations.
+ The variant AlgTyCon contains all the information available
+ from the data/newtype declaration as well as derived information, such
+ as the Unique and argument variance information. This
+ includes a field algTcRhs :: AlgTyConRhs, where
+ AlgTyConRhs distinguishes three kinds of algebraic data
+ type declarations: (1) declarations that have been exported abstractly,
+ (2) data declarations, and (3) newtype
+ declarations. The last two both include their original right hand side;
+ in addition, the third variant also caches the "ultimate" representation
+ type, which is the right hand side after expanding all type synonyms and
+ non-recursive newtypes.
+
+ Both data and newtype declarations refer to their data constructors
+ represented as DataCon.DataCon, which include all details
+ of their signature (as derived from the original declaration) as well
+ information for code generation, such as their tag value.
+
+ Class declarations turn into values of type Class.Class.
+ They represent methods as the Ids of the dictionary
+ selector functions. Similar selector functions are available for
+ superclass dictionaries.
+
+ Instance declarations turn into values of type
+ InstEnv.Instance, which in interface files are represented
+ as IfaceSyn.IfaceInst. Moreover, the type
+ InstEnv.InstEnv, which is a synonym for UniqFM
+ ClsInstEnv, provides a mapping of classes to their
+ instances---ClsInstEnv is essentially a list of instance
+ declarations.
+
+ +Last modified: Sun Jun 19 13:07:22 EST 2005 + +
+ +