{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

module TcRnTypes(
	TcRnIf, TcRn, TcM, RnM,	IfM, IfL, IfG, -- The monad is opaque outside this module
	TcRef,

	-- The environment types
	Env(..), 
	TcGblEnv(..), TcLclEnv(..), 
	IfGblEnv(..), IfLclEnv(..), 

	-- Ranamer types
	ErrCtxt, RecFieldEnv(..),
	ImportAvails(..), emptyImportAvails, plusImportAvails, 
	WhereFrom(..), mkModDeps,

	-- Typechecker types
	TcTypeEnv, TcTyThing(..), pprTcTyThingCategory, 

	-- Template Haskell
	ThStage(..), topStage, topAnnStage, topSpliceStage,
	ThLevel, impLevel, outerLevel, thLevel,

	-- Arrows
	ArrowCtxt(NoArrowCtxt), newArrowScope, escapeArrowScope,

	-- Constraints
        Untouchables(..), inTouchableRange, isNoUntouchables,

       -- Canonical constraints
        Xi, Ct(..), Cts, emptyCts, andCts, andManyCts, 
        singleCt, extendCts, isEmptyCts, isCTyEqCan, 
        isCDictCan_Maybe, isCIPCan_Maybe, isCFunEqCan_Maybe,
        isCIrredEvCan, isCNonCanonical,
        SubGoalDepth, ctPred,

        WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
        andWC, addFlats, addImplics, mkFlatWC,

        EvVarX(..), mkEvVarX, evVarOf, evVarX, evVarOfPred,
        WantedEvVar,

        Implication(..),
        CtLoc(..), ctLocSpan, ctLocOrigin, setCtLocOrigin,
	CtOrigin(..), EqOrigin(..), 
        WantedLoc, GivenLoc, GivenKind(..), pushErrCtxt,

	SkolemInfo(..),

        CtFlavor(..), pprFlavorArising, isWanted, 
        isGivenOrSolved, isGiven_maybe, isSolved,
        isDerived,

	-- Pretty printing
        pprEvVarTheta, pprWantedEvVar, pprWantedsWithLocs,
	pprEvVars, pprEvVarWithType, pprWantedEvVarWithLoc,
        pprArising, pprArisingAt,

	-- Misc other types
	TcId, TcIdSet, TcTyVarBind(..), TcTyVarBinds
	
  ) where

#include "HsVersions.h"

import HsSyn
import HscTypes
import TcEvidence( EvBind, EvBindsVar, EvTerm )
import Type
import Class    ( Class )
import TyCon    ( TyCon )
import DataCon  ( DataCon, dataConUserType )
import TcType
import Annotations
import InstEnv
import FamInstEnv
import IOEnv
import RdrName
import Name
import NameEnv
import NameSet
import Avail
import Var
import VarEnv
import Module
import SrcLoc
import VarSet
import ErrUtils
import UniqFM
import UniqSupply
import Unique
import BasicTypes
import Bag
import Outputable
import ListSetOps
import FastString

import Data.Set (Set)

type TcRef a 	 = IORef a
type TcId    	 = Id 			
type TcIdSet 	 = IdSet


type TcRnIf a b c = IOEnv (Env a b) c
type IfM lcl a  = TcRnIf IfGblEnv lcl a		-- Iface stuff

type IfG a  = IfM () a				-- Top level
type IfL a  = IfM IfLclEnv a			-- Nested
type TcRn a = TcRnIf TcGblEnv TcLclEnv a
type RnM  a = TcRn a		-- Historical
type TcM  a = TcRn a		-- Historical

data TcTyVarBind = TcTyVarBind TcTyVar TcType

type TcTyVarBinds = Bag TcTyVarBind

instance Outputable TcTyVarBind where
  ppr (TcTyVarBind tv ty) = ppr tv <+> text ":=" <+> ppr ty

-- We 'stack' these envs through the Reader like monad infastructure
-- as we move into an expression (although the change is focused in
-- the lcl type).
data Env gbl lcl
  = Env {
        env_top  :: HscEnv,  -- Top-level stuff that never changes
                             -- Includes all info about imported things

        env_us   :: {-# UNPACK #-} !(IORef UniqSupply),
                             -- Unique supply for local varibles

        env_gbl  :: gbl,     -- Info about things defined at the top level
                             -- of the module being compiled

        env_lcl  :: lcl      -- Nested stuff; changes as we go into 
    }

-- TcGblEnv describes the top-level of the module at the 
-- point at which the typechecker is finished work.
-- It is this structure that is handed on to the desugarer
-- For state that needs to be updated during the typechecking
-- phase and returned at end, use a TcRef (= IORef).

data TcGblEnv
  = TcGblEnv {
	tcg_mod     :: Module,         -- ^ Module being compiled
	tcg_src     :: HscSource,
          -- ^ What kind of module (regular Haskell, hs-boot, ext-core)

	tcg_rdr_env :: GlobalRdrEnv,   -- ^ Top level envt; used during renaming
	tcg_default :: Maybe [Type],
          -- ^ Types used for defaulting. @Nothing@ => no @default@ decl

	tcg_fix_env   :: FixityEnv,	-- ^ Just for things in this module
	tcg_field_env :: RecFieldEnv,	-- ^ Just for things in this module

	tcg_type_env :: TypeEnv,
          -- ^ Global type env for the module we are compiling now.  All
	  -- TyCons and Classes (for this module) end up in here right away,
	  -- along with their derived constructors, selectors.
	  --
	  -- (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,
		-- 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
		-- Updated at intervals (e.g. after dealing with types and classes)
	
	tcg_inst_env     :: InstEnv,
          -- ^ Instance envt for /home-package/ modules; Includes the dfuns in
	  -- tcg_insts
	tcg_fam_inst_env :: FamInstEnv,	-- ^ Ditto for family instances

		-- Now a bunch of things about this module that are simply 
		-- accumulated, but never consulted until the end.  
		-- Nevertheless, it's convenient to accumulate them along 
		-- with the rest of the info from this module.
	tcg_exports :: [AvailInfo],	-- ^ What is exported
	tcg_imports :: ImportAvails,
          -- ^ Information about what was imported from where, including
	  -- things bound in this module. Also store Safe Haskell info
          -- here about transative trusted packaage requirements.

	tcg_dus :: DefUses,
          -- ^ What is defined in this module and what is used.
          -- The latter is used to generate
          --
          --  (a) version tracking; no need to recompile if these things have
          --      not changed version stamp
          --
          --  (b) unused-import info

	tcg_keep :: TcRef NameSet,
          -- ^ Locally-defined top-level names to keep alive.
          --
          -- "Keep alive" means give them an Exported flag, so that the
          -- simplifier does not discard them as dead code, and so that they
          -- are exposed in the interface file (but not to export to the
          -- user).
          --
          -- Some things, like dict-fun Ids and default-method Ids are "born"
          -- with the Exported flag on, for exactly the above reason, but some
          -- we only discover as we go.  Specifically:
          --
          --   * The to/from functions for generic data types
          --
          --   * Top-level variables appearing free in the RHS of an orphan
          --     rule
          --
          --   * Top-level variables appearing free in a TH bracket

        tcg_th_used :: TcRef Bool,
          -- ^ @True@ <=> Template Haskell syntax used.
          --
          -- We need this so that we can generate a dependency on the
          -- Template Haskell package, becuase the desugarer is going
          -- to emit loads of references to TH symbols.  The reference
          -- is implicit rather than explicit, so we have to zap a
          -- mutable variable.

        tcg_th_splice_used :: TcRef Bool,
          -- ^ @True@ <=> A Template Haskell splice was used.
          --
          -- Splices disable recompilation avoidance (see #481)

	tcg_dfun_n  :: TcRef OccSet,
          -- ^ Allows us to choose unique DFun names.

	-- The next fields accumulate the payload of the module
	-- The binds, rules and foreign-decl fiels are collected
	-- initially in un-zonked form and are finally zonked in tcRnSrcDecls

        tcg_rn_exports :: Maybe [Located (IE Name)],
        tcg_rn_imports :: [LImportDecl Name],
		-- Keep the renamed imports regardless.  They are not 
		-- voluminous and are needed if you want to report unused imports

        tcg_used_rdrnames :: TcRef (Set RdrName),
		-- The set of used *imported* (not locally-defined) RdrNames
		-- Used only to report unused import declarations

	tcg_rn_decls :: Maybe (HsGroup Name),
          -- ^ Renamed decls, maybe.  @Nothing@ <=> Don't retain renamed
          -- decls.

        tcg_dependent_files :: TcRef [FilePath], -- ^ dependencies from addDependentFile

        tcg_ev_binds  :: Bag EvBind,	    -- Top-level evidence bindings
	tcg_binds     :: LHsBinds Id,	    -- Value bindings in this module
        tcg_sigs      :: NameSet, 	    -- ...Top-level names that *lack* a signature
        tcg_imp_specs :: [LTcSpecPrag],     -- ...SPECIALISE prags for imported Ids
	tcg_warns     :: Warnings,	    -- ...Warnings and deprecations
	tcg_anns      :: [Annotation],      -- ...Annotations
        tcg_tcs       :: [TyCon],           -- ...TyCons and Classes
	tcg_insts     :: [Instance],	    -- ...Instances
        tcg_fam_insts :: [FamInst],         -- ...Family instances
        tcg_rules     :: [LRuleDecl Id],    -- ...Rules
        tcg_fords     :: [LForeignDecl Id], -- ...Foreign import & exports
        tcg_vects     :: [LVectDecl Id],    -- ...Vectorisation declarations

	tcg_doc_hdr   :: Maybe LHsDocString, -- ^ Maybe Haddock header docs
        tcg_hpc       :: AnyHpcUsage,        -- ^ @True@ if any part of the
                                             --  prog uses hpc instrumentation.

        tcg_main      :: Maybe Name,         -- ^ The Name of the main
                                             -- function, if this module is
                                             -- the main module.
        tcg_safeInfer :: TcRef Bool          -- Has the typechecker infered this
                                             -- module as -XSafe (Safe Haskell)
    }

data RecFieldEnv 
  = RecFields (NameEnv [Name])	-- Maps a constructor name *in this module*
				-- to the fields for that constructor
	      NameSet		-- Set of all fields declared *in this module*;
				-- used to suppress name-shadowing complaints
				-- when using record wild cards
				-- E.g.  let fld = e in C {..}
	-- This is used when dealing with ".." notation in record 
	-- construction and pattern matching.
	-- The FieldEnv deals *only* with constructors defined in *this*
	-- module.  For imported modules, we get the same info from the
	-- TypeEnv

data IfGblEnv 
  = IfGblEnv {
	-- The type environment for the module being compiled,
	-- in case the interface refers back to it via a reference that
	-- was originally a hi-boot file.
	-- We need the module name so we can test when it's appropriate
	-- to look in this env.
	if_rec_types :: Maybe (Module, IfG TypeEnv)
		-- Allows a read effect, so it can be in a mutable
		-- variable; c.f. handling the external package type env
		-- Nothing => interactive stuff, no loops possible
    }

data IfLclEnv
  = IfLclEnv {
	-- The module for the current IfaceDecl
	-- So if we see   f = \x -> x
	-- it means M.f = \x -> x, where M is the if_mod
	if_mod :: Module,

	-- The field is used only for error reporting
	-- if (say) there's a Lint error in it
	if_loc :: SDoc,
		-- Where the interface came from:
		--	.hi file, or GHCi state, or ext core
		-- plus which bit is currently being examined

	if_tv_env  :: UniqFM TyVar,	-- Nested tyvar bindings
		      	     		-- (and coercions)
	if_id_env  :: UniqFM Id		-- Nested id binding
    }

data TcLclEnv		-- Changes as we move inside an expression
			-- Discarded after typecheck/rename; not passed on to desugarer
  = TcLclEnv {
	tcl_loc  :: SrcSpan,		-- Source span
	tcl_ctxt :: [ErrCtxt],		-- Error context, innermost on top
	tcl_errs :: TcRef Messages,	-- Place to accumulate errors

	tcl_th_ctxt    :: ThStage,	      -- Template Haskell context
	tcl_arrow_ctxt :: ArrowCtxt,	      -- Arrow-notation context

	tcl_rdr :: LocalRdrEnv,		-- Local name envt
		-- Maintained during renaming, of course, but also during
		-- type checking, solely so that when renaming a Template-Haskell
		-- splice we have the right environment for the renamer.
		-- 
		--   Does *not* include global name envt; may shadow it
		--   Includes both ordinary variables and type variables;
		--   they are kept distinct because tyvar have a different
		--   occurrence contructor (Name.TvOcc)
		-- We still need the unsullied global name env so that
    		--   we can look up record field names

	tcl_env  :: TcTypeEnv,    -- The local type environment: Ids and
			          -- TyVars defined in this module
					
	tcl_tyvars :: TcRef TcTyVarSet,	-- The "global tyvars"
			-- Namely, the in-scope TyVars bound in tcl_env, 
			-- plus the tyvars mentioned in the types of Ids bound
			-- in tcl_lenv. 
                        -- Why mutable? see notes with tcGetGlobalTyVars

	tcl_lie   :: TcRef WantedConstraints,    -- Place to accumulate type constraints

	-- TcMetaTyVars have 
	tcl_meta  :: TcRef Unique,  -- The next free unique for TcMetaTyVars
		     		    -- Guaranteed to be allocated linearly
	tcl_untch :: Unique	    -- Any TcMetaTyVar with 
		     		    --     unique >= tcl_untch is touchable
		     		    --     unique <  tcl_untch is untouchable
    }

type TcTypeEnv = NameEnv TcTyThing


{- Note [Given Insts]
   ~~~~~~~~~~~~~~~~~~
Because of GADTs, we have to pass inwards the Insts provided by type signatures 
and existential contexts. Consider
	data T a where { T1 :: b -> b -> T [b] }
	f :: Eq a => T a -> Bool
	f (T1 x y) = [x]==[y]

The constructor T1 binds an existential variable 'b', and we need Eq [b].
Well, we have it, because Eq a refines to Eq [b], but we can only spot that if we 
pass it inwards.

-}

---------------------------
-- Template Haskell stages and levels 
---------------------------

data ThStage	-- See Note [Template Haskell state diagram] in TcSplice
  = Splice	-- Top-level splicing
		-- This code will be run *at compile time*;
		--   the result replaces the splice
		-- Binding level = 0
 
  | Comp   	-- Ordinary Haskell code
		-- Binding level = 1

  | Brack  			-- Inside brackets 
      ThStage 			--   Binding level = level(stage) + 1
      (TcRef [PendingSplice])	--   Accumulate pending splices here
      (TcRef WantedConstraints)	--     and type constraints here

topStage, topAnnStage, topSpliceStage :: ThStage
topStage       = Comp
topAnnStage    = Splice
topSpliceStage = Splice

instance Outputable ThStage where
   ppr Splice        = text "Splice"
   ppr Comp	     = text "Comp"
   ppr (Brack s _ _) = text "Brack" <> parens (ppr s)

type ThLevel = Int	
        -- See Note [Template Haskell levels] in TcSplice
	-- 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-programming for Haskell" paper

impLevel, outerLevel :: ThLevel
impLevel = 0	-- Imported things; they can be used inside a top level splice
outerLevel = 1	-- Things defined outside brackets
-- NB: Things at level 0 are not *necessarily* imported.
--	eg  $( \b -> ... )   here b is bound at level 0
--
-- For example: 
--	f = ...
--	g1 = $(map ...)		is OK
--	g2 = $(f ...)		is not OK; because we havn't compiled f yet

thLevel :: ThStage -> ThLevel
thLevel Splice        = 0
thLevel Comp          = 1
thLevel (Brack s _ _) = thLevel s + 1

---------------------------
-- Arrow-notation context
---------------------------

{-
In arrow notation, a variable bound by a proc (or enclosed let/kappa)
is not in scope to the left of an arrow tail (-<) or the head of (|..|).
For example

	proc x -> (e1 -< e2)

Here, x is not in scope in e1, but it is in scope in e2.  This can get
a bit complicated:

	let x = 3 in
	proc y -> (proc z -> e1) -< e2

Here, x and z are in scope in e1, but y is not.  We implement this by
recording the environment when passing a proc (using newArrowScope),
and returning to that (using escapeArrowScope) on the left of -< and the
head of (|..|).
-}

data ArrowCtxt
  = NoArrowCtxt
  | ArrowCtxt (Env TcGblEnv TcLclEnv)

-- Record the current environment (outside a proc)
newArrowScope :: TcM a -> TcM a
newArrowScope
  = updEnv $ \env ->
	env { env_lcl = (env_lcl env) { tcl_arrow_ctxt = ArrowCtxt env } }

-- Return to the stored environment (from the enclosing proc)
escapeArrowScope :: TcM a -> TcM a
escapeArrowScope
  = updEnv $ \ env -> case tcl_arrow_ctxt (env_lcl env) of
	NoArrowCtxt -> env
	ArrowCtxt env' -> env'

---------------------------
-- TcTyThing
---------------------------

data TcTyThing
  = AGlobal TyThing		-- Used only in the return type of a lookup

  | ATcId   {		-- Ids defined in this module; may not be fully zonked
	tct_id     :: TcId,		
	tct_closed :: TopLevelFlag,   -- See Note [Bindings with closed types]
	tct_level  :: ThLevel }

  | ATyVar  Name TcType		-- The type to which the lexically scoped type vaiable
				-- is currently refined. We only need the Name
				-- for error-message purposes; it is the corresponding
				-- Name in the domain of the envt

  | AThing  TcKind   -- Used temporarily, during kind checking, for the
		     --	tycons and clases in this recursive group
                     -- Can be a mono-kind or a poly-kind; in TcTyClsDcls see
                     -- Note [Type checking recursive type and class declarations]

  | ANothing                    -- see Note [ANothing]

{-
Note [ANothing]
~~~~~~~~~~~~~~~

We don't want to allow promotion in a strongly connected component
when kind checking.

Consider:
  data T f = K (f (K Any))

When kind checking the `data T' declaration the local env contains the
mappings:
  T -> AThing <some initial kind>
  K -> ANothing

ANothing is only used for DataCons, and only used during type checking
in tcTyClGroup.
-}


instance Outputable TcTyThing where	-- Debugging only
   ppr (AGlobal g)      = pprTyThing g
   ppr elt@(ATcId {})   = text "Identifier" <> 
			  brackets (ppr (tct_id elt) <> dcolon 
                                 <> ppr (varType (tct_id elt)) <> comma
				 <+> ppr (tct_closed elt) <> comma
				 <+> ppr (tct_level elt))
   ppr (ATyVar tv _)    = text "Type variable" <+> quotes (ppr tv)
   ppr (AThing k)       = text "AThing" <+> ppr k
   ppr ANothing         = text "ANothing"

pprTcTyThingCategory :: TcTyThing -> SDoc
pprTcTyThingCategory (AGlobal thing) = pprTyThingCategory thing
pprTcTyThingCategory (ATyVar {})     = ptext (sLit "Type variable")
pprTcTyThingCategory (ATcId {})      = ptext (sLit "Local identifier")
pprTcTyThingCategory (AThing {})     = ptext (sLit "Kinded thing")
pprTcTyThingCategory ANothing        = ptext (sLit "Opaque thing")

type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, Message))
	-- Monadic so that we have a chance
	-- to deal with bound type variables just before error
	-- message construction

	-- Bool:  True <=> this is a landmark context; do not
	--		   discard it when trimming for display

-- | 'ImportAvails' summarises what was imported from where, irrespective of
-- whether the imported things are actually used or not.  It is used:
--
--  * when processing the export list,
--
--  * when constructing usage info for the interface file,
--
--  * to identify the list of directly imported modules for initialisation
--    purposes and for optimised overlap checking of family instances,
--
--  * when figuring out what things are really unused
--
data ImportAvails 
   = ImportAvails {
	imp_mods :: ImportedMods,
	  --      = ModuleEnv [(ModuleName, Bool, SrcSpan, Bool)],
          -- ^ Domain is all directly-imported modules
          -- The 'ModuleName' is what the module was imported as, e.g. in
          -- @
          --     import Foo as Bar
          -- @
          -- it is @Bar@.
          --
          -- The 'Bool' means:
          --
          --  - @True@ => import was @import Foo ()@
          --
          --  - @False@ => import was some other form
          --
          -- Used
          --
          --   (a) to help construct the usage information in the interface
          --       file; if we import somethign we need to recompile if the
          --       export version changes
          --
          --   (b) to specify what child modules to initialise
          --
          -- We need a full ModuleEnv rather than a ModuleNameEnv here,
          -- because we might be importing modules of the same name from
          -- different packages. (currently not the case, but might be in the
          -- future).

        imp_dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface),
          -- ^ Home-package modules needed by the module being compiled
          --
          -- It doesn't matter whether any of these dependencies
          -- are actually /used/ when compiling the module; they
          -- are listed if they are below it at all.  For
          -- example, suppose M imports A which imports X.  Then
          -- compiling M might not need to consult X.hi, but X
          -- is still listed in M's dependencies.

        imp_dep_pkgs :: [PackageId],
          -- ^ Packages needed by the module being compiled, whether directly,
          -- or via other modules in this package, or via modules imported
          -- from other packages.
        
        imp_trust_pkgs :: [PackageId],
          -- ^ This is strictly a subset of imp_dep_pkgs and records the
          -- packages the current module needs to trust for Safe Haskell
          -- compilation to succeed. A package is required to be trusted if
          -- we are dependent on a trustworthy module in that package.
          -- While perhaps making imp_dep_pkgs a tuple of (PackageId, Bool)
          -- where True for the bool indicates the package is required to be
          -- trusted is the more logical  design, doing so complicates a lot
          -- of code not concerned with Safe Haskell.
          -- See Note [RnNames . Tracking Trust Transitively]

        imp_trust_own_pkg :: Bool,
          -- ^ Do we require that our own package is trusted?
          -- This is to handle efficiently the case where a Safe module imports
          -- a Trustworthy module that resides in the same package as it.
          -- See Note [RnNames . Trust Own Package]

        imp_orphs :: [Module],
          -- ^ Orphan modules below us in the import tree (and maybe including
          -- us for imported modules)

        imp_finsts :: [Module]
          -- ^ Family instance modules below us in the import tree (and maybe
          -- including us for imported modules)
      }

mkModDeps :: [(ModuleName, IsBootInterface)]
	  -> ModuleNameEnv (ModuleName, IsBootInterface)
mkModDeps deps = foldl add emptyUFM deps
	       where
		 add env elt@(m,_) = addToUFM env m elt

emptyImportAvails :: ImportAvails
emptyImportAvails = ImportAvails { imp_mods          = emptyModuleEnv,
                                   imp_dep_mods      = emptyUFM,
                                   imp_dep_pkgs      = [],
                                   imp_trust_pkgs    = [],
                                   imp_trust_own_pkg = False,
                                   imp_orphs         = [],
                                   imp_finsts        = [] }

-- | Union two ImportAvails
--
-- This function is a key part of Import handling, basically
-- for each import we create a seperate ImportAvails structure
-- and then union them all together with this function.
plusImportAvails ::  ImportAvails ->  ImportAvails ->  ImportAvails
plusImportAvails
  (ImportAvails { imp_mods = mods1,
                  imp_dep_mods = dmods1, imp_dep_pkgs = dpkgs1,
                  imp_trust_pkgs = tpkgs1, imp_trust_own_pkg = tself1,
                  imp_orphs = orphs1, imp_finsts = finsts1 })
  (ImportAvails { imp_mods = mods2,
                  imp_dep_mods = dmods2, imp_dep_pkgs = dpkgs2,
                  imp_trust_pkgs = tpkgs2, imp_trust_own_pkg = tself2,
                  imp_orphs = orphs2, imp_finsts = finsts2 })
  = ImportAvails { imp_mods          = plusModuleEnv_C (++) mods1 mods2,
                   imp_dep_mods      = plusUFM_C plus_mod_dep dmods1 dmods2,
                   imp_dep_pkgs      = dpkgs1 `unionLists` dpkgs2,
                   imp_trust_pkgs    = tpkgs1 `unionLists` tpkgs2,
                   imp_trust_own_pkg = tself1 || tself2,
                   imp_orphs         = orphs1 `unionLists` orphs2,
                   imp_finsts        = finsts1 `unionLists` finsts2 }
  where
    plus_mod_dep (m1, boot1) (m2, boot2) 
        = WARN( not (m1 == m2), (ppr m1 <+> ppr m2) $$ (ppr boot1 <+> ppr boot2) )
                -- Check mod-names match
          (m1, boot1 && boot2) -- If either side can "see" a non-hi-boot interface, use that

data WhereFrom 
  = ImportByUser IsBootInterface	-- Ordinary user import (perhaps {-# SOURCE #-})
  | ImportBySystem			-- Non user import.

instance Outputable WhereFrom where
  ppr (ImportByUser is_boot) | is_boot     = ptext (sLit "{- SOURCE -}")
			     | otherwise   = empty
  ppr ImportBySystem     		   = ptext (sLit "{- SYSTEM -}")

-- Types without any type functions inside.  However, note that xi
-- types CAN contain unexpanded type synonyms; however, the
-- (transitive) expansions of those type synonyms will not contain any
-- type functions.
type Xi = Type       -- In many comments, "xi" ranges over Xi

type Cts = Bag Ct

type SubGoalDepth = Int -- An ever increasing number used to restrict 
                        -- simplifier iterations. Bounded by -fcontext-stack.

data Ct
  -- Atomic canonical constraints 
  = CDictCan {  -- e.g.  Num xi
      cc_id     :: EvVar,
      cc_flavor :: CtFlavor, 
      cc_class  :: Class, 
      cc_tyargs :: [Xi],

      cc_depth  :: SubGoalDepth -- Simplification depth of this constraint
                       -- See Note [WorkList]
    }

  | CIPCan {	-- ?x::tau
      -- See note [Canonical implicit parameter constraints].
      cc_id     :: EvVar,
      cc_flavor :: CtFlavor,
      cc_ip_nm  :: IPName Name,
      cc_ip_ty  :: TcTauType, -- Not a Xi! See same not as above
      cc_depth  :: SubGoalDepth        -- See Note [WorkList]
    }

  | CIrredEvCan {  -- These stand for yet-unknown predicates
      cc_id     :: EvVar,
      cc_flavor :: CtFlavor,
      cc_ty     :: Xi, -- cc_ty is flat hence it may only be of the form (tv xi1 xi2 ... xin)
                       -- Since, if it were a type constructor application, that'd make the
                       -- whole constraint a CDictCan, CIPCan, or CTyEqCan. And it can't be
                       -- a type family application either because it's a Xi type.
      cc_depth :: SubGoalDepth -- See Note [WorkList]
    }

  | CTyEqCan {  -- tv ~ xi	(recall xi means function free)
       -- Invariant: 
       --   * tv not in tvs(xi)   (occurs check)
       --   * typeKind xi `compatKind` typeKind tv
       --       See Note [Spontaneous solving and kind compatibility]
       --   * We prefer unification variables on the left *JUST* for efficiency
      cc_id     :: EvVar, 
      cc_flavor :: CtFlavor, 
      cc_tyvar  :: TcTyVar, 
      cc_rhs    :: Xi,

      cc_depth :: SubGoalDepth -- See Note [WorkList] 
    }

  | CFunEqCan {  -- F xis ~ xi  
                 -- Invariant: * isSynFamilyTyCon cc_fun 
                 --            * typeKind (F xis) `compatKind` typeKind xi
      cc_id     :: EvVar,
      cc_flavor :: CtFlavor, 
      cc_fun    :: TyCon,	-- A type function
      cc_tyargs :: [Xi],	-- Either under-saturated or exactly saturated
      cc_rhs    :: Xi,      	--    *never* over-saturated (because if so
      		      		--    we should have decomposed)

      cc_depth  :: SubGoalDepth -- See Note [WorkList]
                   
    }

  | CNonCanonical { -- See Note [NonCanonical Semantics] 
      cc_id     :: EvVar,
      cc_flavor :: CtFlavor, 
      cc_depth  :: SubGoalDepth
    }

ctPred :: Ct -> PredType 
ctPred (CNonCanonical { cc_id = v }) = evVarPred v
ctPred (CDictCan { cc_class = cls, cc_tyargs = xis }) 
  = mkClassPred cls xis
ctPred (CTyEqCan { cc_tyvar = tv, cc_rhs = xi }) 
  = mkEqPred (mkTyVarTy tv, xi)
ctPred (CFunEqCan { cc_fun = fn, cc_tyargs = xis1, cc_rhs = xi2 }) 
  = mkEqPred(mkTyConApp fn xis1, xi2)
ctPred (CIPCan { cc_ip_nm = nm, cc_ip_ty = xi }) 
  = mkIPPred nm xi
ctPred (CIrredEvCan { cc_ty = xi }) = xi

instance Outputable Ct where
  ppr ct = ppr (cc_flavor ct) <> braces (ppr (cc_depth ct))
                  <+> ppr ev_var <+> dcolon <+> ppr (ctPred ct)
                  <+> parens (text ct_sort)
         where ev_var  = cc_id ct
               ct_sort = case ct of 
                           CTyEqCan {}      -> "CTyEqCan"
                           CFunEqCan {}     -> "CFunEqCan"
                           CNonCanonical {} -> "CNonCanonical"
                           CDictCan {}      -> "CDictCan"
                           CIPCan {}        -> "CIPCan"
                           CIrredEvCan {}   -> "CIrredEvCan"

singleCt :: Ct -> Cts 
singleCt = unitBag 

andCts :: Cts -> Cts -> Cts 
andCts = unionBags

extendCts :: Cts -> Ct -> Cts 
extendCts = snocBag 

andManyCts :: [Cts] -> Cts 
andManyCts = unionManyBags

emptyCts :: Cts 
emptyCts = emptyBag

isEmptyCts :: Cts -> Bool
isEmptyCts = isEmptyBag

isCTyEqCan :: Ct -> Bool 
isCTyEqCan (CTyEqCan {})  = True 
isCTyEqCan (CFunEqCan {}) = False
isCTyEqCan _              = False 

isCDictCan_Maybe :: Ct -> Maybe Class
isCDictCan_Maybe (CDictCan {cc_class = cls })  = Just cls
isCDictCan_Maybe _              = Nothing

isCIPCan_Maybe :: Ct -> Maybe (IPName Name)
isCIPCan_Maybe  (CIPCan {cc_ip_nm = nm }) = Just nm
isCIPCan_Maybe _                = Nothing

isCIrredEvCan :: Ct -> Bool
isCIrredEvCan (CIrredEvCan {}) = True
isCIrredEvCan _                = False

isCFunEqCan_Maybe :: Ct -> Maybe TyCon
isCFunEqCan_Maybe (CFunEqCan { cc_fun = tc }) = Just tc
isCFunEqCan_Maybe _ = Nothing

isCNonCanonical :: Ct -> Bool
isCNonCanonical (CNonCanonical {}) = True 
isCNonCanonical _ = False 

data WantedConstraints
  = WC { wc_flat  :: Cts                -- Unsolved constraints, all wanted
       , wc_impl  :: Bag Implication
       , wc_insol :: Cts               -- Insoluble constraints, can be
                                       -- wanted, given, or derived
                                       -- See Note [Insoluble constraints]
    }

emptyWC :: WantedConstraints
emptyWC = WC { wc_flat = emptyBag, wc_impl = emptyBag, wc_insol = emptyBag }

mkFlatWC :: [Ct] -> WantedConstraints
mkFlatWC cts 
  = WC { wc_flat = listToBag cts, wc_impl = emptyBag, wc_insol = emptyBag }

isEmptyWC :: WantedConstraints -> Bool
isEmptyWC (WC { wc_flat = f, wc_impl = i, wc_insol = n })
  = isEmptyBag f && isEmptyBag i && isEmptyBag n

insolubleWC :: WantedConstraints -> Bool
-- True if there are any insoluble constraints in the wanted bag
insolubleWC wc = not (isEmptyBag (wc_insol wc))
               || anyBag ic_insol (wc_impl wc)

andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints
andWC (WC { wc_flat = f1, wc_impl = i1, wc_insol = n1 })
      (WC { wc_flat = f2, wc_impl = i2, wc_insol = n2 })
  = WC { wc_flat  = f1 `unionBags` f2
       , wc_impl  = i1 `unionBags` i2
       , wc_insol = n1 `unionBags` n2 }

addFlats :: WantedConstraints -> Bag WantedEvVar -> WantedConstraints
addFlats wc wevs 
  = wc { wc_flat = wc_flat wc `unionBags` cts }
  where cts = mapBag mk_noncan wevs 
        mk_noncan (EvVarX v wl) 
          = CNonCanonical { cc_id = v, cc_flavor = Wanted wl, cc_depth = 0}

addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints
addImplics wc implic = wc { wc_impl = wc_impl wc `unionBags` implic }

instance Outputable WantedConstraints where
  ppr (WC {wc_flat = f, wc_impl = i, wc_insol = n})
   = ptext (sLit "WC") <+> braces (vcat
        [ if isEmptyBag f then empty else
          ptext (sLit "wc_flat =")  <+> pprBag ppr f
        , if isEmptyBag i then empty else
          ptext (sLit "wc_impl =")  <+> pprBag ppr i
        , if isEmptyBag n then empty else
          ptext (sLit "wc_insol =") <+> pprBag ppr n ])

pprBag :: (a -> SDoc) -> Bag a -> SDoc
pprBag pp b = foldrBag (($$) . pp) empty b

data Untouchables = NoUntouchables
                  | TouchableRange
                          Unique  -- Low end
                          Unique  -- High end
 -- A TcMetaTyvar is *touchable* iff its unique u satisfies
 --   u >= low
 --   u < high

instance Outputable Untouchables where
  ppr NoUntouchables = ptext (sLit "No untouchables")
  ppr (TouchableRange low high) = ptext (sLit "Touchable range:") <+> 
                                  ppr low <+> char '-' <+> ppr high

isNoUntouchables :: Untouchables -> Bool
isNoUntouchables NoUntouchables      = True
isNoUntouchables (TouchableRange {}) = False

inTouchableRange :: Untouchables -> TcTyVar -> Bool
inTouchableRange NoUntouchables _ = True
inTouchableRange (TouchableRange low high) tv 
  = uniq >= low && uniq < high
  where
    uniq = varUnique tv

-- EvVar defined in module Var.lhs:
-- Evidence variables include all *quantifiable* constraints
--   dictionaries
--   implicit parameters
--   coercion variables

data Implication
  = Implic {  
      ic_untch :: Untouchables, -- Untouchables: unification variables
                                -- free in the environment
      ic_env   :: TcTypeEnv,    -- The type environment
                                -- Used only when generating error messages
	  -- Generally, ic_untch is a superset of tvsof(ic_env)
	  -- However, we don't zonk ic_env when zonking the Implication
	  -- Instead we do that when generating a skolem-escape error message

      ic_skols  :: TcTyVarSet,   -- Introduced skolems 
      		   	         -- See Note [Skolems in an implication]

      ic_given  :: [EvVar],      -- Given evidence variables
      		   		 --   (order does not matter)
      ic_loc   :: GivenLoc,      -- Binding location of the implication,
                                 --   which is also the location of all the
                                 --   given evidence variables

      ic_wanted :: WantedConstraints,  -- The wanted
      ic_insol  :: Bool,               -- True iff insolubleWC ic_wanted is true

      ic_binds  :: EvBindsVar   -- Points to the place to fill in the
                                -- abstraction and bindings
    }

instance Outputable Implication where
  ppr (Implic { ic_untch = untch, ic_skols = skols, ic_given = given
              , ic_wanted = wanted
              , ic_binds = binds, ic_loc = loc })
   = ptext (sLit "Implic") <+> braces 
     (sep [ ptext (sLit "Untouchables = ") <+> ppr untch
          , ptext (sLit "Skolems = ") <+> ppr skols
          , ptext (sLit "Given = ") <+> pprEvVars given
          , ptext (sLit "Wanted = ") <+> ppr wanted
          , ptext (sLit "Binds = ") <+> ppr binds
          , pprSkolInfo (ctLocOrigin loc)
          , ppr (ctLocSpan loc) ])

data EvVarX a = EvVarX EvVar a
     -- An evidence variable with accompanying info

type WantedEvVar   = EvVarX WantedLoc     -- The location where it arose


instance Outputable (EvVarX a) where
  ppr (EvVarX ev _) = pprEvVarWithType ev
  -- If you want to see the associated info,
  -- use a more specific printing function

mkEvVarX :: EvVar -> a -> EvVarX a
mkEvVarX = EvVarX

evVarOf :: EvVarX a -> EvVar
evVarOf (EvVarX ev _) = ev

evVarX :: EvVarX a -> a
evVarX (EvVarX _ a) = a

evVarOfPred :: EvVarX a -> PredType
evVarOfPred wev = evVarPred (evVarOf wev)

pprEvVars :: [EvVar] -> SDoc	-- Print with their types
pprEvVars ev_vars = vcat (map pprEvVarWithType ev_vars)

pprEvVarTheta :: [EvVar] -> SDoc
pprEvVarTheta ev_vars = pprTheta (map evVarPred ev_vars)
                              
pprEvVarWithType :: EvVar -> SDoc
pprEvVarWithType v = ppr v <+> dcolon <+> pprType (evVarPred v)

pprWantedsWithLocs :: WantedConstraints -> SDoc
pprWantedsWithLocs wcs
  =  vcat [ pprBag ppr (wc_flat wcs)
          , pprBag ppr (wc_impl wcs)
          , pprBag ppr (wc_insol wcs) ]

pprWantedEvVarWithLoc, pprWantedEvVar :: WantedEvVar -> SDoc
pprWantedEvVarWithLoc (EvVarX v loc) = hang (pprEvVarWithType v)
                                          2 (pprArisingAt loc)
pprWantedEvVar        (EvVarX v _)   = pprEvVarWithType v

data CtFlavor
  = Given GivenLoc GivenKind -- We have evidence for this constraint in TcEvBinds
  | Derived WantedLoc        -- Derived's are just hints for unifications 
  | Wanted WantedLoc         -- We have no evidence bindings for this constraint. 

data GivenKind
  = GivenOrig   -- Originates in some given, such as signature or pattern match
  | GivenSolved (Maybe EvTerm) 
                -- Is given as result of being solved, maybe provisionally on
                -- some other wanted constraints. We cache the evidence term 
                -- sometimes here as well /as well as/ in the EvBinds, 
                -- see Note [Optimizing Spontaneously Solved Coercions]

instance Outputable CtFlavor where
  ppr (Given _ GivenOrig)        = ptext (sLit "[G]")
  ppr (Given _ (GivenSolved {})) = ptext (sLit "[S]") -- Print [S] for Given/Solved's
  ppr (Wanted {})                = ptext (sLit "[W]")
  ppr (Derived {})               = ptext (sLit "[D]")

pprFlavorArising :: CtFlavor -> SDoc
pprFlavorArising (Derived wl)   = pprArisingAt wl
pprFlavorArising (Wanted  wl)   = pprArisingAt wl
pprFlavorArising (Given gl _)   = pprArisingAt gl

isWanted :: CtFlavor -> Bool
isWanted (Wanted {}) = True
isWanted _           = False

isGivenOrSolved :: CtFlavor -> Bool
isGivenOrSolved (Given {}) = True
isGivenOrSolved _ = False

isSolved :: CtFlavor -> Bool
isSolved (Given _ (GivenSolved {})) = True
isSolved _ = False

isGiven_maybe :: CtFlavor -> Maybe GivenKind 
isGiven_maybe (Given _ gk) = Just gk
isGiven_maybe _            = Nothing

isDerived :: CtFlavor -> Bool 
isDerived (Derived {}) = True
isDerived _            = False

data CtLoc orig = CtLoc orig SrcSpan [ErrCtxt]

type WantedLoc = CtLoc CtOrigin      -- Instantiation for wanted constraints
type GivenLoc  = CtLoc SkolemInfo    -- Instantiation for given constraints

ctLocSpan :: CtLoc o -> SrcSpan
ctLocSpan (CtLoc _ s _) = s

ctLocOrigin :: CtLoc o -> o
ctLocOrigin (CtLoc o _ _) = o

setCtLocOrigin :: CtLoc o -> o' -> CtLoc o'
setCtLocOrigin (CtLoc _ s c) o = CtLoc o s c

pushErrCtxt :: orig -> ErrCtxt -> CtLoc orig -> CtLoc orig
pushErrCtxt o err (CtLoc _ s errs) = CtLoc o s (err:errs)

pprArising :: CtOrigin -> SDoc
-- Used for the main, top-level error message
-- We've done special processing for TypeEq and FunDep origins
pprArising (TypeEqOrigin {}) = empty
pprArising FunDepOrigin      = empty
pprArising orig              = text "arising from" <+> ppr orig

pprArisingAt :: Outputable o => CtLoc o -> SDoc
pprArisingAt (CtLoc o s _) = sep [ text "arising from" <+> ppr o
                                 , text "at" <+> ppr s]

-- SkolemInfo gives the origin of *given* constraints
--   a) type variables are skolemised
--   b) an implication constraint is generated
data SkolemInfo
  = SigSkol UserTypeCtxt	-- A skolem that is created by instantiating
            Type                -- a programmer-supplied type signature
				-- Location of the binding site is on the TyVar

	-- The rest are for non-scoped skolems
  | ClsSkol Class	-- Bound at a class decl
  | InstSkol 		-- Bound at an instance decl
  | DataSkol            -- Bound at a data type declaration
  | FamInstSkol         -- Bound at a family instance decl
  | PatSkol 	        -- An existential type variable bound by a pattern for
      DataCon           -- a data constructor with an existential type.
      (HsMatchContext Name)	
	     --	e.g.   data T = forall a. Eq a => MkT a
	     --        f (MkT x) = ...
	     -- The pattern MkT x will allocate an existential type
	     -- variable for 'a'.  

  | ArrowSkol 	  	-- An arrow form (see TcArrows)

  | IPSkol [IPName Name]  -- Binding site of an implicit parameter

  | RuleSkol RuleName	-- The LHS of a RULE

  | InferSkol [(Name,TcType)]
                        -- We have inferred a type for these (mutually-recursivive)
                        -- polymorphic Ids, and are now checking that their RHS
                        -- constraints are satisfied.

  | BracketSkol         -- Template Haskell bracket

  | UnifyForAllSkol     -- We are unifying two for-all types
       TcType

  | UnkSkol             -- Unhelpful info (until I improve it)

instance Outputable SkolemInfo where
  ppr = pprSkolInfo

pprSkolInfo :: SkolemInfo -> SDoc
-- Complete the sentence "is a rigid type variable bound by..."
pprSkolInfo (SigSkol (FunSigCtxt f) ty)
                            = hang (ptext (sLit "the type signature for"))
                                 2 (ppr f <+> dcolon <+> ppr ty)
pprSkolInfo (SigSkol cx ty) = hang (pprUserTypeCtxt cx <> colon)
                                 2 (ppr ty)
pprSkolInfo (IPSkol ips)    = ptext (sLit "the implicit-parameter bindings for")
                              <+> pprWithCommas ppr ips
pprSkolInfo (ClsSkol cls)   = ptext (sLit "the class declaration for") <+> quotes (ppr cls)
pprSkolInfo InstSkol        = ptext (sLit "the instance declaration")
pprSkolInfo DataSkol        = ptext (sLit "the data type declaration")
pprSkolInfo FamInstSkol     = ptext (sLit "the family instance declaration")
pprSkolInfo BracketSkol     = ptext (sLit "a Template Haskell bracket")
pprSkolInfo (RuleSkol name) = ptext (sLit "the RULE") <+> doubleQuotes (ftext name)
pprSkolInfo ArrowSkol       = ptext (sLit "the arrow form")
pprSkolInfo (PatSkol dc mc)  = sep [ ptext (sLit "a pattern with constructor")
                                   , nest 2 $ ppr dc <+> dcolon
                                              <+> ppr (dataConUserType dc) <> comma
                                  , ptext (sLit "in") <+> pprMatchContext mc ]
pprSkolInfo (InferSkol ids) = sep [ ptext (sLit "the inferred type of")
                                  , vcat [ ppr name <+> dcolon <+> ppr ty
                                         | (name,ty) <- ids ]]
pprSkolInfo (UnifyForAllSkol ty) = ptext (sLit "the type") <+> ppr ty

-- UnkSkol
-- For type variables the others are dealt with by pprSkolTvBinding.  
-- For Insts, these cases should not happen
pprSkolInfo UnkSkol = WARN( True, text "pprSkolInfo: UnkSkol" ) ptext (sLit "UnkSkol")

-- CtOrigin gives the origin of *wanted* constraints
data CtOrigin
  = OccurrenceOf Name		-- Occurrence of an overloaded identifier
  | AppOrigin	 		-- An application of some kind

  | SpecPragOrigin Name		-- Specialisation pragma for identifier

  | TypeEqOrigin EqOrigin

  | IPOccOrigin  (IPName Name)	-- Occurrence of an implicit parameter

  | LiteralOrigin (HsOverLit Name)	-- Occurrence of a literal
  | NegateOrigin			-- Occurrence of syntactic negation

  | ArithSeqOrigin (ArithSeqInfo Name) -- [x..], [x..y] etc
  | PArrSeqOrigin  (ArithSeqInfo Name) -- [:x..y:] and [:x,y..z:]
  | SectionOrigin
  | TupleOrigin			       -- (..,..)
  | AmbigOrigin Name	-- f :: ty
  | ExprSigOrigin	-- e :: ty
  | PatSigOrigin	-- p :: ty
  | PatOrigin	        -- Instantiating a polytyped pattern at a constructor
  | RecordUpdOrigin
  | ViewPatOrigin

  | ScOrigin	        -- Typechecking superclasses of an instance declaration
  | DerivOrigin		-- Typechecking deriving
  | StandAloneDerivOrigin -- Typechecking stand-alone deriving
  | DefaultOrigin	-- Typechecking a default decl
  | DoOrigin		-- Arising from a do expression
  | MCompOrigin         -- Arising from a monad comprehension
  | IfOrigin            -- Arising from an if statement
  | ProcOrigin		-- Arising from a proc expression
  | AnnOrigin           -- An annotation
  | FunDepOrigin

data EqOrigin 
  = UnifyOrigin 
       { uo_actual   :: TcType
       , uo_expected :: TcType }

instance Outputable CtOrigin where
  ppr orig = pprO orig

pprO :: CtOrigin -> SDoc
pprO (OccurrenceOf name)   = hsep [ptext (sLit "a use of"), quotes (ppr name)]
pprO AppOrigin             = ptext (sLit "an application")
pprO (SpecPragOrigin name) = hsep [ptext (sLit "a specialisation pragma for"), quotes (ppr name)]
pprO (IPOccOrigin name)    = hsep [ptext (sLit "a use of implicit parameter"), quotes (ppr name)]
pprO RecordUpdOrigin       = ptext (sLit "a record update")
pprO (AmbigOrigin name)    = ptext (sLit "the ambiguity check for") <+> quotes (ppr name)
pprO ExprSigOrigin         = ptext (sLit "an expression type signature")
pprO PatSigOrigin          = ptext (sLit "a pattern type signature")
pprO PatOrigin             = ptext (sLit "a pattern")
pprO ViewPatOrigin         = ptext (sLit "a view pattern")
pprO IfOrigin              = ptext (sLit "an if statement")
pprO (LiteralOrigin lit)   = hsep [ptext (sLit "the literal"), quotes (ppr lit)]
pprO (ArithSeqOrigin seq)  = hsep [ptext (sLit "the arithmetic sequence"), quotes (ppr seq)]
pprO (PArrSeqOrigin seq)   = hsep [ptext (sLit "the parallel array sequence"), quotes (ppr seq)]
pprO SectionOrigin	   = ptext (sLit "an operator section")
pprO TupleOrigin	   = ptext (sLit "a tuple")
pprO NegateOrigin	   = ptext (sLit "a use of syntactic negation")
pprO ScOrigin	           = ptext (sLit "the superclasses of an instance declaration")
pprO DerivOrigin	   = ptext (sLit "the 'deriving' clause of a data type declaration")
pprO StandAloneDerivOrigin = ptext (sLit "a 'deriving' declaration")
pprO DefaultOrigin	   = ptext (sLit "a 'default' declaration")
pprO DoOrigin	           = ptext (sLit "a do statement")
pprO MCompOrigin           = ptext (sLit "a statement in a monad comprehension")
pprO ProcOrigin	           = ptext (sLit "a proc expression")
pprO (TypeEqOrigin eq)     = ptext (sLit "an equality") <+> ppr eq
pprO AnnOrigin             = ptext (sLit "an annotation")
pprO FunDepOrigin          = ptext (sLit "a functional dependency")

instance Outputable EqOrigin where
  ppr (UnifyOrigin t1 t2) = ppr t1 <+> char '~' <+> ppr t2