Vars, Ids, and
+ TyVars is provided elsewhere.
+
+
+ Sets, Finite Maps, and Environments
+
+ Sets of variables, or more generally names, which are needed throughout
+ the compiler, are provided by the modules VarSet.lhs
+ and NameSet.lhs,
+ respectively. Moreover, frequently maps from variables (or names) to
+ other data is needed. For example, a substitution is represented by a
+ finite map from variable names to expressions. Jobs like this are
+ solved by means of variable and name environments implemented by the
+ modules VarEnv.lhs
+ and NameEnv.lhs.
+
+
VarSet
+ The Module VarSet provides the types VarSet,
+ IdSet, and TyVarSet, which are synonyms in the
+ current implementation, as Var, Id, and
+ TyVar are synonyms. The module provides all the operations
+ that one would expect including the creating of sets from individual
+ variables and lists of variables, union and intersection operations,
+ element checks, deletion, filter, fold, and map functions.
+
+ The implementation is based on UniqSets,
+ which in turn are simply UniqFMs
+ (i.e., finite maps with uniques as keys) that map each unique to the
+ variable that it represents.
+
+
NameSet
+ The Module NameSet provides the same functionality as
+ VarSet only for Names.
+ As for the difference between Names and Vars,
+ a Var is built from a Name plus additional
+ information (mostly importantly type information).
+
+
VarEnv
+ The module VarEnv provides the types VarEnv,
+ IdEnv, and TyVarEnv, which are again
+ synonyms. The provided base functionality is similar to
+ VarSet with the main difference that a type VarEnv
+ T associates a value of type T with each variable in
+ the environment, thus effectively implementing a finite map from
+ variables to values of type T.
+
+ The implementation of VarEnv is also by UniqFM,
+ which entails the slightly surprising implication that it is
+ not possible to retrieve the domain of a variable environment.
+ In other words, there is no function corresponding to
+ VarSet.varSetElems :: VarSet -> [Var] in
+ VarEnv. This is because the UniqFM used to
+ implement VarEnv stores only the unique corresponding to a
+ variable in the environment, but not the entire variable (and there is
+ no mapping from uniques to variables).
+
+ In addition to plain variable environments, the module also contains
+ special substitution environments - the type SubstEnv -
+ that associates variables with a special purpose type
+ SubstResult.
+
+
NameEnv
+ The type NameEnv.NameEnv is like VarEnv only
+ for Names.
+
+