Fix Trac #1814 (staging interaction in Template Haskell and GHCi), and add comments

[ghc-hetmet.git] / compiler / typecheck / TcRnTypes.lhs

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index 80e5b0f..eb1cd04 100644 (file)
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -1,4 +1,4 @@
-%
+
 % (c) The University of Glasgow 2006
 % (c) The GRASP Project, Glasgow University, 1992-2002
 %
@@ -7,7 +7,7 @@
 -- The above warning supression flag is a temporary kludge.
 -- While working on this module you are encouraged to remove it and fix
 -- any warnings in the module. See
---     http://hackage.haskell.org/trac/ghc/wiki/CodingStyle#Warnings
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
 -- for details
 
 module TcRnTypes(
@@ -152,7 +152,7 @@ data TcGblEnv
                -- (Ids defined in this module start in the local envt, 
                --  though they move to the global envt during zonking)
 
-       tcg_type_env_var :: TcRef TypeEnv,      
+       tcg_type_env_var :: TcRef TypeEnv,
                -- Used only to initialise the interface-file
                -- typechecker in initIfaceTcRn, so that it can see stuff
                -- bound in this module when dealing with hi-boot recursions
@@ -369,7 +369,7 @@ type ThLevel = Int
        -- Incremented when going inside a bracket,
        -- decremented when going inside a splice
        -- NB: ThLevel is one greater than the 'n' in Fig 2 of the
-       --     original "Template meta-programmign for Haskell" paper
+       --     original "Template meta-programming for Haskell" paper
 
 impLevel, topLevel :: ThLevel
 topLevel = 1   -- Things defined at top level of this module
@@ -470,7 +470,7 @@ data RefinementVisibility
                                        -- GADT refinement
 
 instance Outputable TcTyThing where    -- Debugging only
-   ppr (AGlobal g)      = ppr g
+   ppr (AGlobal g)      = pprTyThing g
    ppr elt@(ATcId {})   = text "Identifier" <> 
                          ifPprDebug (brackets (ppr (tct_id elt) <> dcolon <> ppr (tct_type elt) <> comma
                                 <+> ppr (tct_level elt) <+> ppr (tct_co elt)))
@@ -635,6 +635,17 @@ In addition to the basic Haskell variants of 'Inst's, they can now also
 represent implication constraints 'forall tvs. (reft, given) => wanted'
 and equality constraints 'co :: ty1 ~ ty2'.
 
+NB: Equalities occur in two flavours:
+
+  (1) Dict {tci_pred = EqPred ty1 ty2}
+  (2) EqInst {tci_left = ty1, tci_right = ty2, tci_co = coe}
+
+The former arises from equalities in contexts, whereas the latter is used
+whenever the type checker introduces an equality (e.g., during deferring
+unification).
+
+I am not convinced that this duplication is necessary or useful! -=chak
+
 \begin{code}
 data Inst
   = Dict {
@@ -650,7 +661,7 @@ data Inst
                                    -- Includes coercion variables
                                    --   mentioned in tci_reft
        tci_reft   :: Refinement,
-       tci_given  :: [Inst],       -- Only Dicts
+       tci_given  :: [Inst],       -- Only Dicts and EqInsts
                                    --   (no Methods, LitInsts, ImplicInsts)
        tci_wanted :: [Inst],       -- Only Dicts and ImplicInsts
                                    --   (no Methods or LitInsts)
@@ -704,23 +715,22 @@ data Inst
 
   | EqInst {                     -- delayed unification of the form 
                                  --    co :: ty1 ~ ty2
-       tci_left  :: TcType,      -- ty1
-       tci_right :: TcType,      -- ty2
+       tci_left  :: TcType,      -- ty1    -- both types are...
+       tci_right :: TcType,      -- ty2    -- ...free of boxes
        tci_co    :: Either       -- co
-                       TcTyVar   --    a wanted equation, with a hole, to be 
-                                 --      filled with a witness for the equality
-                                  --        for equation generated by the 
-                                  --        unifier, 'ty1' is the actual and
-                                  --        'ty2' the expected type
-                       Coercion, --    a given equation, with a coercion
-                                 --      witnessing the equality
-                                 --         a coercion that originates from a
-                                 --         signature or a GADT is a CoVar, but
-                                  --         after normalisation of coercions,
-                                 --         they can be arbitrary Coercions 
-                                  --         involving constructors and 
-                                  --         pseudo-constructors like sym and 
-                                  --         trans.
+                       TcTyVar   --  - a wanted equation, with a hole, to be 
+                                 --    filled with a witness for the equality;
+                                  --    for equation arising from deferring
+                                  --    unification, 'ty1' is the actual and
+                                  --    'ty2' the expected type
+                       Coercion, --  - a given equation, with a coercion
+                                 --    witnessing the equality;
+                                 --    a coercion that originates from a
+                                 --    signature or a GADT is a CoVar, but
+                                  --    after normalisation of coercions, they
+                                 --    can be arbitrary Coercions involving
+                                  --    constructors and pseudo-constructors 
+                                  --    like sym and trans.
        tci_loc   :: InstLoc,
 
        tci_name  :: Name       -- Debugging help only: this makes it easier to