Simplify the implementation of Implicit Parameters

This patch re-implements implicit parameters via a class
with a functional dependency:

    class IP (n::Symbol) a | n -> a where
      ip :: a

This definition is in the library module GHC.IP. Notice
how it use a type-literal, so we can have constraints like
   IP "x" Int
Now all the functional dependency machinery works right to make
implicit parameters behave as they should.

Much special-case processing for implicit parameters can be removed
entirely. One particularly nice thing is not having a dedicated
"original-name cache" for implicit parameters (the nsNames field of
NameCache).  But many other cases disappear:

  * BasicTypes.IPName
  * IPTyCon constructor in Tycon.TyCon
  * CIPCan constructor  in TcRnTypes.Ct
  * IPPred constructor  in Types.PredTree

Implicit parameters remain special in a few ways:

 * Special syntax.  Eg the constraint (IP "x" Int) is parsed
   and printed as (?x::Int).  And we still have local bindings
   for implicit parameters, and occurrences thereof.

 * A implicit-parameter binding  (let ?x = True in e) amounts
   to a local instance declaration, which we have not had before.
   It just generates an implication contraint (easy), but when
   going under it we must purge any existing bindings for
   ?x in the inert set.  See Note [Shadowing of Implicit Parameters]
   in TcSimplify

 * TcMType.sizePred classifies implicit parameter constraints as size-0,
   as before the change

There are accompanying patches to libraries 'base' and 'haddock'

All the work was done by Iavor Diatchki

1 files changed, 1 insertions, 40 deletions

diff --git a/compiler/prelude/TysWiredIn.lhs b/compiler/prelude/TysWiredIn.lhs
index 60518bfd9f..78e1f74b4d 100644
--- a/compiler/prelude/TysWiredIn.lhs
+++ b/compiler/prelude/TysWiredIn.lhs
@@ -72,8 +72,6 @@ module TysWiredIn (
         -- * Equality predicates
         eqTyCon_RDR, eqTyCon, eqTyConName, eqBoxDataCon,
 
-        -- * Implicit parameter predicates
-        mkIPName
     ) where
 
 #include "HsVersions.h"
@@ -85,7 +83,6 @@ import PrelNames
 import TysPrim
 
 -- others:
-import Coercion
 import Constants	( mAX_TUPLE_SIZE )
 import Module		( Module )
 import Type             ( mkTyConApp )
@@ -95,7 +92,7 @@ import TyCon
 import TypeRep
 import RdrName
 import Name
-import BasicTypes       ( TupleSort(..), tupleSortBoxity, IPName(..), 
+import BasicTypes       ( TupleSort(..), tupleSortBoxity,
                           Arity, RecFlag(..), Boxity(..), HsBang(..) )
 import ForeignCall
 import Unique           ( incrUnique, mkTupleTyConUnique,
@@ -254,9 +251,6 @@ pcTyCon is_enum is_rec name cType tyvars cons
 pcDataCon :: Name -> [TyVar] -> [Type] -> TyCon -> DataCon
 pcDataCon = pcDataConWithFixity False
 
-pcDataCon' :: Name -> Unique -> [TyVar] -> [Type] -> TyCon -> DataCon
-pcDataCon' = pcDataConWithFixity' False
-
 pcDataConWithFixity :: Bool -> Name -> [TyVar] -> [Type] -> TyCon -> DataCon
 pcDataConWithFixity infx n = pcDataConWithFixity' infx n (incrUnique (nameUnique n))
 -- The Name's unique is the first of two free uniques;
@@ -395,39 +389,6 @@ unboxedPairDataCon :: DataCon
 unboxedPairDataCon = tupleCon   UnboxedTuple 2
 \end{code}
 
-%************************************************************************
-%*                                                                      *
-\subsection[TysWiredIn-ImplicitParams]{Special type constructors for implicit parameters}
-%*                                                                      *
-%************************************************************************
-
-\begin{code}
-mkIPName :: FastString
-         -> Unique -> Unique -> Unique -> Unique
-         -> IPName Name
-mkIPName ip tycon_u datacon_u dc_wrk_u co_ax_u = name_ip
-  where
-    name_ip = IPName tycon_name
-
-    tycon_name = mkPrimTyConName ip tycon_u tycon
-    tycon      = mkAlgTyCon tycon_name
-                   (liftedTypeKind `mkArrowKind` constraintKind)
-                   [alphaTyVar]
-                   Nothing
-                   []      -- No stupid theta
-                   (NewTyCon { data_con    = datacon, 
-                               nt_rhs      = mkTyVarTy alphaTyVar,
-                               nt_etad_rhs = ([alphaTyVar], mkTyVarTy alphaTyVar),
-                               nt_co       = mkNewTypeCo co_ax_name tycon [alphaTyVar] (mkTyVarTy alphaTyVar) })
-                   (IPTyCon name_ip)
-                   NonRecursive
-                   False
-
-    datacon_name = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "IPBox") datacon_u datacon
-    datacon      = pcDataCon' datacon_name dc_wrk_u [alphaTyVar] [mkTyVarTy alphaTyVar] tycon
-
-    co_ax_name = mkPrimTyConName ip co_ax_u tycon
-\end{code}
 
 %************************************************************************
 %*									*