-Last modified: Wed May 4 11:48:54 EST 2005
+Last modified: Thu May 12 19:03:42 EST 2005
diff --git a/ghc/docs/comm/the-beast/typecheck.html b/ghc/docs/comm/the-beast/typecheck.html
index e988062..8d22784 100644
--- a/ghc/docs/comm/the-beast/typecheck.html
+++ b/ghc/docs/comm/the-beast/typecheck.html
@@ -28,7 +28,15 @@
href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/typecheck/TcRnTypes.lhs">TcRnTypes.TcId
used to represent identifiers in some signatures during type checking
is, in fact, nothing but a synonym for a plain
- Id.
+ Id.
+
+ It is also noteworthy, that the representations of types changes during
+ type checking from HsType to TypeRep.Type.
+ The latter is a hybrid type representation that
+ is used to type Core, but still contains sufficient information to
+ recover source types. In particular, the type checker maintains and
+ compares types in their Type form.
-Last modified: Mon May 9 11:02:20 EST 2005 +Last modified: Thu May 12 22:52:46 EST 2005
+ 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 the Core type system, which is the type system + used by the by far most important 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.
+
+ Slightly more involved are data types declared via a
+ newtype declaration. These newtypes constitute new type
+ constructors---i.e., they are not just type macros, but introduce new
+ type names. However, provide that a newtype is not recursive, we still
+ want to implement it by its representation type. In other words,
+ tcEqType cannot see through a newtype, but
+ coreEqType can.
+
+ 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.
+
+ 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: Thu May 12 22:47:50 EST 2005 + +
+