path: root/compiler/typecheck/TcClassDcl.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%

Typechecking class declarations

\begin{code}
{-# 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://ghc.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

module TcClassDcl ( tcClassSigs, tcClassDecl2, 
		    findMethodBind, instantiateMethod, tcInstanceMethodBody,
		    tcClassMinimalDef,
                    HsSigFun, mkHsSigFun, lookupHsSig, emptyHsSigs,
		    tcMkDeclCtxt, tcAddDeclCtxt, badMethodErr
		  ) where

#include "HsVersions.h"

import HsSyn
import TcEnv
import TcPat( addInlinePrags )
import TcEvidence( idHsWrapper )
import TcBinds
import TcUnify
import TcHsType
import TcMType
import Type     ( getClassPredTys_maybe )
import TcType
import TcRnMonad
import BuildTyCl( TcMethInfo, defaultClassMinimalDef )
import Class
import Id
import Name
import NameEnv
import NameSet
import Var
import Outputable
import SrcLoc
import Maybes
import BasicTypes
import Bag
import FastString
import BooleanFormula (impliesAtom, isUnsatisfied, pprBooleanFormulaNice)
import Util

import Control.Monad
\end{code}


Dictionary handling
~~~~~~~~~~~~~~~~~~~
Every class implicitly declares a new data type, corresponding to dictionaries
of that class. So, for example:

	class (D a) => C a where
	  op1 :: a -> a
	  op2 :: forall b. Ord b => a -> b -> b

would implicitly declare

	data CDict a = CDict (D a)	
			     (a -> a)
			     (forall b. Ord b => a -> b -> b)

(We could use a record decl, but that means changing more of the existing apparatus.
One step at at time!)

For classes with just one superclass+method, we use a newtype decl instead:

	class C a where
	  op :: forallb. a -> b -> b

generates

	newtype CDict a = CDict (forall b. a -> b -> b)

Now DictTy in Type is just a form of type synomym: 
	DictTy c t = TyConTy CDict `AppTy` t

Death to "ExpandingDicts".


%************************************************************************
%*									*
		Type-checking the class op signatures
%*									*
%************************************************************************

\begin{code}
tcClassSigs :: Name	             -- Name of the class
	    -> [LSig Name]
	    -> LHsBinds Name
	    -> TcM ([TcMethInfo],    -- Exactly one for each method
                    NameEnv Type)    -- Types of the generic-default methods
tcClassSigs clas sigs def_methods
  = do { traceTc "tcClassSigs 1" (ppr clas)

       ; gen_dm_prs <- concat <$> mapM (addLocM tc_gen_sig) gen_sigs
       ; let gen_dm_env = mkNameEnv gen_dm_prs

       ; op_info <- concat <$> mapM (addLocM (tc_sig gen_dm_env)) vanilla_sigs

       ; let op_names = mkNameSet [ n | (n,_,_) <- op_info ]
       ; sequence_ [ failWithTc (badMethodErr clas n)
                   | n <- dm_bind_names, not (n `elemNameSet` op_names) ]
		   -- Value binding for non class-method (ie no TypeSig)

       ; sequence_ [ failWithTc (badGenericMethod clas n)
                   | (n,_) <- gen_dm_prs, not (n `elem` dm_bind_names) ]
		   -- Generic signature without value binding

       ; traceTc "tcClassSigs 2" (ppr clas)
       ; return (op_info, gen_dm_env) }
  where
    vanilla_sigs = [L loc (nm,ty) | L loc (TypeSig    nm ty) <- sigs]
    gen_sigs     = [L loc (nm,ty) | L loc (GenericSig nm ty) <- sigs]
    dm_bind_names :: [Name]	-- These ones have a value binding in the class decl
    dm_bind_names = [op | L _ (FunBind {fun_id = L _ op}) <- bagToList def_methods]

    tc_sig genop_env (op_names, op_hs_ty)
      = do { traceTc "ClsSig 1" (ppr op_names)
           ; op_ty <- tcClassSigType op_hs_ty	-- Class tyvars already in scope
           ; traceTc "ClsSig 2" (ppr op_names)
           ; return [ (op_name, f op_name, op_ty) | L _ op_name <- op_names ] }
           where
             f nm | nm `elemNameEnv` genop_env = GenericDM
                  | nm `elem` dm_bind_names    = VanillaDM
                  | otherwise                  = NoDM

    tc_gen_sig (op_names, gen_hs_ty)
      = do { gen_op_ty <- tcClassSigType gen_hs_ty
           ; return [ (op_name, gen_op_ty) | L _ op_name <- op_names ] }
\end{code}


%************************************************************************
%*									*
		Class Declarations
%*									*
%************************************************************************

\begin{code}
tcClassDecl2 :: LTyClDecl Name		-- The class declaration
	     -> TcM (LHsBinds Id)

tcClassDecl2 (L loc (ClassDecl {tcdLName = class_name, tcdSigs = sigs, 
				tcdMeths = default_binds}))
  = recoverM (return emptyLHsBinds)	$
    setSrcSpan loc		   	$
    do  { clas <- tcLookupLocatedClass class_name

	-- We make a separate binding for each default method.
	-- At one time I used a single AbsBinds for all of them, thus
	-- AbsBind [d] [dm1, dm2, dm3] { dm1 = ...; dm2 = ...; dm3 = ... }
	-- But that desugars into
	--	ds = \d -> (..., ..., ...)
	--	dm1 = \d -> case ds d of (a,b,c) -> a
	-- And since ds is big, it doesn't get inlined, so we don't get good
	-- default methods.  Better to make separate AbsBinds for each
	; let
	      (tyvars, _, _, op_items) = classBigSig clas
              prag_fn     = mkPragFun sigs default_binds
	      sig_fn	  = mkHsSigFun sigs
              clas_tyvars = snd (tcSuperSkolTyVars tyvars)
	      pred  	  = mkClassPred clas (mkTyVarTys clas_tyvars)
	; this_dict <- newEvVar pred

	; traceTc "TIM2" (ppr sigs)
	; let tc_dm = tcDefMeth clas clas_tyvars
				this_dict default_binds 
	      			sig_fn prag_fn

	; dm_binds <- tcExtendTyVarEnv clas_tyvars $
                      mapM tc_dm op_items

	; return (unionManyBags dm_binds) }

tcClassDecl2 d = pprPanic "tcClassDecl2" (ppr d)
    
tcDefMeth :: Class -> [TyVar] -> EvVar -> LHsBinds Name
          -> HsSigFun -> PragFun -> ClassOpItem
          -> TcM (LHsBinds TcId)
-- Generate code for polymorphic default methods only (hence DefMeth)
-- (Generic default methods have turned into instance decls by now.)
-- This is incompatible with Hugs, which expects a polymorphic 
-- default method for every class op, regardless of whether or not 
-- the programmer supplied an explicit default decl for the class.  
-- (If necessary we can fix that, but we don't have a convenient Id to hand.)
tcDefMeth clas tyvars this_dict binds_in hs_sig_fn prag_fn (sel_id, dm_info)
  = case dm_info of
      NoDefMeth          -> do { mapM_ (addLocM (badDmPrag sel_id)) prags
                               ; return emptyBag }
      DefMeth dm_name    -> tc_dm dm_name 
      GenDefMeth dm_name -> tc_dm dm_name 
  where
    sel_name           = idName sel_id
    prags              = prag_fn sel_name
    (dm_bind,bndr_loc) = findMethodBind sel_name binds_in
	                 `orElse` pprPanic "tcDefMeth" (ppr sel_id)

    -- Eg.   class C a where
    --          op :: forall b. Eq b => a -> [b] -> a
    --		gen_op :: a -> a
    -- 		generic gen_op :: D a => a -> a
    -- The "local_dm_ty" is precisely the type in the above
    -- type signatures, ie with no "forall a. C a =>" prefix

    tc_dm dm_name 
      = do { dm_id <- tcLookupId dm_name
	   ; local_dm_name <- setSrcSpan bndr_loc (newLocalName sel_name)
 	     -- Base the local_dm_name on the selector name, because
 	     -- type errors from tcInstanceMethodBody come from here

           ; dm_id_w_inline <- addInlinePrags dm_id prags
           ; spec_prags     <- tcSpecPrags dm_id prags

           ; let local_dm_ty = instantiateMethod clas dm_id (mkTyVarTys tyvars)
                 hs_ty       = lookupHsSig hs_sig_fn sel_name 
                               `orElse` pprPanic "tc_dm" (ppr sel_name)

           ; local_dm_sig <- instTcTySig hs_ty local_dm_ty local_dm_name
           ; warnTc (not (null spec_prags))
                    (ptext (sLit "Ignoring SPECIALISE pragmas on default method") 
                     <+> quotes (ppr sel_name))

           ; tc_bind <- tcInstanceMethodBody (ClsSkol clas) tyvars [this_dict]
                                             dm_id_w_inline local_dm_sig
                                             IsDefaultMethod dm_bind

           ; return (unitBag tc_bind) }

---------------
tcInstanceMethodBody :: SkolemInfo -> [TcTyVar] -> [EvVar]
                     -> Id -> TcSigInfo
          	     -> TcSpecPrags -> LHsBind Name 
          	     -> TcM (LHsBind Id)
tcInstanceMethodBody skol_info tyvars dfun_ev_vars
                     meth_id local_meth_sig
		     specs (L loc bind)
  = do	{ let local_meth_id = sig_id local_meth_sig
              lm_bind = L loc (bind { fun_id = L loc (idName local_meth_id) })
                             -- Substitute the local_meth_name for the binder
			     -- NB: the binding is always a FunBind
	; (ev_binds, (tc_bind, _, _)) 
               <- checkConstraints skol_info tyvars dfun_ev_vars $
	          tcPolyCheck NonRecursive no_prag_fn local_meth_sig [lm_bind]

        ; let export = ABE { abe_wrap = idHsWrapper, abe_poly = meth_id
                           , abe_mono = local_meth_id, abe_prags = specs }
              full_bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
                                   , abs_exports = [export]
                                   , abs_ev_binds = ev_binds
                                   , abs_binds = tc_bind }

        ; return (L loc full_bind) } 
  where
    no_prag_fn  _ = []		-- No pragmas for local_meth_id; 
    		    		-- they are all for meth_id

---------------
tcClassMinimalDef :: Name -> [LSig Name] -> [TcMethInfo] -> TcM ClassMinimalDef
tcClassMinimalDef _clas sigs op_info
  = case findMinimalDef sigs of
      Nothing -> return defMindef
      Just mindef -> do
        -- warn if the given mindef does not imply the default one
        whenIsJust (isUnsatisfied (mindef `impliesAtom`) defMindef) $
          warnTc True . warningMinimalDefIncomplete
        return mindef
  where
    defMindef = defaultClassMinimalDef op_info
\end{code}

\begin{code}
instantiateMethod :: Class -> Id -> [TcType] -> TcType
-- Take a class operation, say  
--	op :: forall ab. C a => forall c. Ix c => (b,c) -> a
-- Instantiate it at [ty1,ty2]
-- Return the "local method type": 
--	forall c. Ix x => (ty2,c) -> ty1
instantiateMethod clas sel_id inst_tys
  = ASSERT( ok_first_pred ) local_meth_ty
  where
    (sel_tyvars,sel_rho) = tcSplitForAllTys (idType sel_id)
    rho_ty = ASSERT( length sel_tyvars == length inst_tys )
    	     substTyWith sel_tyvars inst_tys sel_rho

    (first_pred, local_meth_ty) = tcSplitPredFunTy_maybe rho_ty
    		`orElse` pprPanic "tcInstanceMethod" (ppr sel_id)

    ok_first_pred = case getClassPredTys_maybe first_pred of
		      Just (clas1, _tys) -> clas == clas1
                      Nothing -> False
	      -- The first predicate should be of form (C a b)
	      -- where C is the class in question


---------------------------
type HsSigFun = NameEnv (LHsType Name)

emptyHsSigs :: HsSigFun
emptyHsSigs = emptyNameEnv

mkHsSigFun :: [LSig Name] -> HsSigFun
mkHsSigFun sigs = mkNameEnv [(n, hs_ty) 
                            | L _ (TypeSig ns hs_ty) <- sigs
                            , L _ n <- ns ]

lookupHsSig :: HsSigFun -> Name -> Maybe (LHsType Name)
lookupHsSig = lookupNameEnv

---------------------------
findMethodBind	:: Name  	        -- Selector name
          	-> LHsBinds Name 	-- A group of bindings
		-> Maybe (LHsBind Name, SrcSpan)
          	-- Returns the binding, and the binding 
                -- site of the method binder
findMethodBind sel_name binds
  = foldlBag mplus Nothing (mapBag f binds)
  where 
    f bind@(L _ (FunBind { fun_id = L bndr_loc op_name }))
             | op_name == sel_name
    	     = Just (bind, bndr_loc)
    f _other = Nothing

---------------------------
findMinimalDef :: [LSig Name] -> Maybe ClassMinimalDef
findMinimalDef = firstJusts . map toMinimalDef
  where
    toMinimalDef (L _ (MinimalSig bf)) = Just (fmap unLoc bf)
    toMinimalDef _ = Nothing
\end{code}

Note [Polymorphic methods]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
    class Foo a where
	op :: forall b. Ord b => a -> b -> b -> b
    instance Foo c => Foo [c] where
        op = e

When typechecking the binding 'op = e', we'll have a meth_id for op
whose type is
      op :: forall c. Foo c => forall b. Ord b => [c] -> b -> b -> b

So tcPolyBinds must be capable of dealing with nested polytypes; 
and so it is. See TcBinds.tcMonoBinds (with type-sig case).

Note [Silly default-method bind]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we pass the default method binding to the type checker, it must
look like    op2 = e
not  	     $dmop2 = e
otherwise the "$dm" stuff comes out error messages.  But we want the
"$dm" to come out in the interface file.  So we typecheck the former,
and wrap it in a let, thus
	  $dmop2 = let op2 = e in op2
This makes the error messages right.


%************************************************************************
%*									*
		Error messages
%*									*
%************************************************************************

\begin{code}
tcMkDeclCtxt :: TyClDecl Name -> SDoc
tcMkDeclCtxt decl = hsep [ptext (sLit "In the"), pprTyClDeclFlavour decl, 
                      ptext (sLit "declaration for"), quotes (ppr (tcdName decl))]

tcAddDeclCtxt :: TyClDecl Name -> TcM a -> TcM a
tcAddDeclCtxt decl thing_inside
  = addErrCtxt (tcMkDeclCtxt decl) thing_inside

badMethodErr :: Outputable a => a -> Name -> SDoc
badMethodErr clas op
  = hsep [ptext (sLit "Class"), quotes (ppr clas), 
	  ptext (sLit "does not have a method"), quotes (ppr op)]

badGenericMethod :: Outputable a => a -> Name -> SDoc
badGenericMethod clas op
  = hsep [ptext (sLit "Class"), quotes (ppr clas), 
	  ptext (sLit "has a generic-default signature without a binding"), quotes (ppr op)]

{-
badGenericInstanceType :: LHsBinds Name -> SDoc
badGenericInstanceType binds
  = vcat [ptext (sLit "Illegal type pattern in the generic bindings"),
	  nest 2 (ppr binds)]

missingGenericInstances :: [Name] -> SDoc
missingGenericInstances missing
  = ptext (sLit "Missing type patterns for") <+> pprQuotedList missing
	  
dupGenericInsts :: [(TyCon, InstInfo a)] -> SDoc
dupGenericInsts tc_inst_infos
  = vcat [ptext (sLit "More than one type pattern for a single generic type constructor:"),
	  nest 2 (vcat (map ppr_inst_ty tc_inst_infos)),
	  ptext (sLit "All the type patterns for a generic type constructor must be identical")
    ]
  where 
    ppr_inst_ty (_,inst) = ppr (simpleInstInfoTy inst)
-}
badDmPrag :: Id -> Sig Name -> TcM ()
badDmPrag sel_id prag
  = addErrTc (ptext (sLit "The") <+> hsSigDoc prag <+> ptext (sLit "for default method") 
              <+> quotes (ppr sel_id) 
              <+> ptext (sLit "lacks an accompanying binding"))

warningMinimalDefIncomplete :: ClassMinimalDef -> SDoc
warningMinimalDefIncomplete mindef
  = vcat [ ptext (sLit "The MINIMAL pragma does not require:")
         , nest 2 (pprBooleanFormulaNice mindef)
         , ptext (sLit "but there is no default implementation.") ]
\end{code}