[project @ 2000-10-03 08:43:00 by simonpj]

[ghc-hetmet.git] / ghc / compiler / basicTypes / BasicTypes.lhs

diff --git a/ghc/compiler/basicTypes/BasicTypes.lhs b/ghc/compiler/basicTypes/BasicTypes.lhs
index b0100e6..6a8c583 100644 (file)
--- a/ghc/compiler/basicTypes/BasicTypes.lhs
+++ b/ghc/compiler/basicTypes/BasicTypes.lhs
@@ -34,8 +34,9 @@ module BasicTypes(
        OccInfo(..), seqOccInfo, isFragileOcc, isDeadOcc, isLoopBreaker,
 
        InsideLam, insideLam, notInsideLam,
-       OneBranch, oneBranch, notOneBranch
+       OneBranch, oneBranch, notOneBranch,
 
+        EP(..)
    ) where
 
 #include "HsVersions.h"
@@ -197,6 +198,42 @@ isNonRec Recursive    = False
 isNonRec NonRecursive = True
 \end{code}
 
+%************************************************************************
+%*                                                                     *
+\subsection[Generic]{Generic flag}
+%*                                                                     *
+%************************************************************************
+
+This is the "Embedding-Projection pair" datatype, it contains 
+two pieces of code (normally either RenamedHsExpr's or Id's)
+If we have a such a pair (EP from to), the idea is that 'from' and 'to'
+represents functions of type 
+
+       from :: T -> Tring
+       to   :: Tring -> T
+
+And we should have 
+
+       to (from x) = x
+
+T and Tring are arbitrary, but typically T is the 'main' type while
+Tring is the 'representation' type.  (This just helps us remember 
+whether to use 'from' or 'to'.
+
+\begin{code}
+data EP a = EP { fromEP :: a,  -- :: T -> Tring
+                toEP   :: a }  -- :: Tring -> T
+\end{code}
+
+Embedding-projection pairs are used in several places:
+
+First of all, each type constructor has an EP associated with it, the
+code in EP converts (datatype T) from T to Tring and back again.
+
+Secondly, when we are filling in Generic methods (in the typechecker, 
+tcMethodBinds), we are constructing bimaps by induction on the structure
+of the type of the method signature.
+
 
 %************************************************************************
 %*                                                                     *