Comments and debug output only

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 292c231..8c557d3 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -1872,7 +1872,7 @@ reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
            go wanteds state }
   where
     go []     state = return state
            go wanteds state }
   where
     go []     state = return state

-    go (w:ws) state = do { traceTc (text "reduceList " <+> ppr (w:ws) <+> ppr state)
+    go (w:ws) state = do { traceTc (text "reduceList " <+> (ppr (w:ws) $$ ppr state))

                         ; state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
                         ; go ws state' }
 
                         ; state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
                         ; go ws state' }
 

@@ -2174,7 +2174,8 @@ reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
 Note [Reducing implication constraints]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we are trying to simplify
 Note [Reducing implication constraints]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we are trying to simplify

-       (Ord a, forall b. C a b => (W [a] b, D c b))
+       ( do: Ord a, 
+         ic: (forall b. C a b => (W [a] b, D c b)) )

 where
        instance (C a b, Ord a) => W [a] b
 When solving the implication constraint, we'll start with
 where
        instance (C a b, Ord a) => W [a] b
 When solving the implication constraint, we'll start with

@@ -2182,16 +2183,26 @@ When solving the implication constraint, we'll start with
 in the Avails.  Then we add (C a b -> Given) and solve. Extracting
 the results gives us a binding for the (W [a] b), with an Irred of 
 (Ord a, D c b).  Now, the (Ord a) comes from "outside" the implication,
 in the Avails.  Then we add (C a b -> Given) and solve. Extracting
 the results gives us a binding for the (W [a] b), with an Irred of 
 (Ord a, D c b).  Now, the (Ord a) comes from "outside" the implication,

-but the (D d b) is from "inside".  So we want to generate a Rhs binding
+but the (D d b) is from "inside".  So we want to generate a GenInst

 like this
 
 like this
 

-       ic = /\b \dc:C a b). (df a b dc do, ic' b dc)
-          depending on
-               do :: Ord a
-               ic' :: forall b. C a b => D c b
+   ic = GenInst 
+          [ do  :: Ord a,
+            ic' :: forall b. C a b => D c b]
+          (/\b \(dc:C a b). (df a b dc do, ic' b dc))

 
 

-The 'depending on' part of the Rhs is important, because it drives
-the extractResults code.
+The first arg of GenInst gives the free dictionary variables of the
+second argument -- the "needed givens".  And that list in turn is
+vital because it's used to determine what other dicts must be solved.
+This very list ends up in the second field of the Rhs, and drives
+extractResults.
+
+The need for this field is why we have to return "needed givens"
+from extractResults, reduceContext, checkLoop, and so on.
+
+NB: the "needed givens" in a GenInst or Rhs, may contain two dicts
+with the same type but different Ids, e.g. [d12 :: Eq a, d81 :: Eq a]
+That says we must generate a binding for both d12 and d81.

 
 The "inside" and "outside" distinction is what's going on with 'inner' and
 'outer' in reduceImplication
 
 The "inside" and "outside" distinction is what's going on with 'inner' and
 'outer' in reduceImplication

@@ -2325,6 +2336,8 @@ extractResults :: Avails
               -> TcM ( TcDictBinds,    -- Bindings
                        [Inst],         -- Irreducible ones
                        [Inst])         -- Needed givens, i.e. ones used in the bindings
               -> TcM ( TcDictBinds,    -- Bindings
                        [Inst],         -- Irreducible ones
                        [Inst])         -- Needed givens, i.e. ones used in the bindings

+                               -- Postcondition: needed-givens = free vars( binds ) \ irreds
+                               -- Note [Reducing implication constraints]

 
 extractResults (Avails _ avails) wanteds
   = go avails emptyBag [] [] wanteds
 
 extractResults (Avails _ avails) wanteds
   = go avails emptyBag [] [] wanteds