[project @ 2003-09-11 14:20:40 by simonpj]

--------------------------
	    Allow recursive dictionaries
	   --------------------------

In response to various bleatings, here's a lovely fix that involved simply
inverting two lines of code, to allow recursive dictionaries.  Here's
the comment.  (typecheck/should_run/tc030 tests it)

Note [RECURSIVE DICTIONARIES]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
    data D r = ZeroD | SuccD (r (D r));

    instance (Eq (r (D r))) => Eq (D r) where
        ZeroD     == ZeroD     = True
        (SuccD a) == (SuccD b) = a == b
        _         == _         = False;

    equalDC :: D [] -> D [] -> Bool;
    equalDC = (==);

We need to prove (Eq (D [])).  Here's how we go:

	d1 : Eq (D [])

by instance decl, holds if
	d2 : Eq [D []]
	where 	d1 = dfEqD d2

by instance decl of Eq, holds if
	d3 : D []
	where	d2 = dfEqList d2
		d1 = dfEqD d2

But now we can "tie the knot" to give

	d3 = d1
	d2 = dfEqList d2
	d1 = dfEqD d2

and it'll even run!  The trick is to put the thing we are trying to prove
(in this case Eq (D []) into the database before trying to prove its
contributing clauses.

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 754af98..1eadf03 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1430,8 +1430,13 @@ reduce stack try_me wanted state
     ; ReduceMe ->              -- It should be reduced
        lookupInst wanted             `thenM` \ lookup_result ->
        case lookup_result of
-           GenInst wanteds' rhs -> reduceList stack try_me wanteds' state      `thenM` \ state' ->
-                                   addWanted state' wanted rhs wanteds'
+           GenInst wanteds' rhs -> addWanted state wanted rhs wanteds'         `thenM` \ state' ->
+                                   reduceList stack try_me wanteds' state'
+               -- Experiment with doing addWanted *before* the reduceList, 
+               -- which has the effect of adding the thing we are trying
+               -- to prove to the database before trying to prove the things it
+               -- needs.  See note [RECURSIVE DICTIONARIES]
+
            SimpleInst rhs       -> addWanted state wanted rhs []
 
            NoInstance ->    -- No such instance!
@@ -1599,6 +1604,42 @@ Now we implement the Right Solution, which is to check for loops directly
 when adding superclasses.  It's a bit like the occurs check in unification.
 
 
+Note [RECURSIVE DICTIONARIES]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider 
+    data D r = ZeroD | SuccD (r (D r));
+    
+    instance (Eq (r (D r))) => Eq (D r) where
+        ZeroD     == ZeroD     = True
+        (SuccD a) == (SuccD b) = a == b
+        _         == _         = False;
+    
+    equalDC :: D [] -> D [] -> Bool;
+    equalDC = (==);
+
+We need to prove (Eq (D [])).  Here's how we go:
+
+       d1 : Eq (D [])
+
+by instance decl, holds if
+       d2 : Eq [D []]
+       where   d1 = dfEqD d2
+
+by instance decl of Eq, holds if
+       d3 : D []
+       where   d2 = dfEqList d2
+               d1 = dfEqD d2
+
+But now we can "tie the knot" to give
+
+       d3 = d1
+       d2 = dfEqList d2
+       d1 = dfEqD d2
+
+and it'll even run!  The trick is to put the thing we are trying to prove
+(in this case Eq (D []) into the database before trying to prove its
+contributing clauses.
+       
 
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