%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 19921998
%
TcInstDecls: Typechecking instance declarations
\begin{code}
module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
#include "HsVersions.h"
import HsSyn
import TcBinds
import TcTyClsDecls
import TcClassDcl
import TcPat ( addInlinePrags )
import TcRnMonad
import TcMType
import TcType
import BuildTyCl
import Inst
import InstEnv
import FamInst
import FamInstEnv
import TcDeriv
import TcEnv
import TcHsType
import TcUnify
import MkCore ( nO_METHOD_BINDING_ERROR_ID )
import Type
import TcEvidence
import TyCon
import DataCon
import Class
import Var
import VarEnv
import VarSet ( mkVarSet, varSetElems )
import Pair
import CoreUnfold ( mkDFunUnfolding )
import CoreSyn ( Expr(Var), CoreExpr, varToCoreExpr )
import PrelNames ( typeableClassNames )
import Bag
import BasicTypes
import DynFlags
import FastString
import Id
import MkId
import Name
import NameSet
import Outputable
import SrcLoc
import Util
import Control.Monad
import Data.Maybe
import Maybes ( orElse )
\end{code}
Typechecking instance declarations is done in two passes. The first
pass, made by @tcInstDecls1@, collects information to be used in the
second pass.
This preprocessed info includes the asyetunprocessed bindings
inside the instance declaration. These are typechecked in the second
pass, when the classinstance envs and GVE contain all the info from
all the instance and value decls. Indeed that's the reason we need
two passes over the instance decls.
Note [How instance declarations are translated]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here is how we translation instance declarations into Core
Running example:
class C a where
op1, op2 :: Ix b => a -> b -> b
op2 = <dmrhs>
instance C a => C [a]
op1 = <rhs>
===>
op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
op1 = ...
op2 = ...
$dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
$dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dmrhs>
op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
op1_i = /\a. \(d:C a).
let this :: C [a]
this = df_i a d
local_op1 :: forall b. Ix b => [a] -> b -> b
local_op1 = <rhs>
in local_op1 a d
op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
df_i :: forall a. C a -> C [a]
df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
Note [Instances and loop breakers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Note that df_i may be mutually recursive with both op1_i and op2_i.
It's crucial that df_i is not chosen as the loop breaker, even
though op1_i has a (userspecified) INLINE pragma.
* Instead the idea is to inline df_i into op1_i, which may then select
methods from the MkC record, and thereby break the recursion with
df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
the same type, it won't mention df_i, so there won't be recursion in
the first place.)
* If op1_i is marked INLINE by the user there's a danger that we won't
inline df_i in it, and that in turn means that (since it'll be a
loopbreaker because df_i isn't), op1_i will ironically never be
inlined. But this is OK: the recursion breaking happens by way of
a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
Note [ClassOp/DFun selection]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
One thing we see a lot is stuff like
op2 (df d1 d2)
where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
'op2' and 'df' to get
case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
MkD _ op2 _ _ _ -> op2
And that will reduce to ($cop2 d1 d2) which is what we wanted.
But it's tricky to make this work in practice, because it requires us to
inline both 'op2' and 'df'. But neither is keen to inline without having
seen the other's result; and it's very easy to get code bloat (from the
big intermediate) if you inline a bit too much.
Instead we use a cunning trick.
* We arrange that 'df' and 'op2' NEVER inline.
* We arrange that 'df' is ALWAYS defined in the sylised form
df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
* We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
that lists its methods.
* We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
a suitable constructor application
were.
* We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
iff its argument satisfies exprIsConApp_maybe. This is done in
MkId mkDictSelId
* We make 'df' CONLIKE, so that shared uses stil match; eg
let d = df d1 d2
in ...(op2 d)...(op1 d)...
Note [Singlemethod classes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the class has just one method (or, more accurately, just one element
of {superclasses + methods}), then we use a different strategy.
class C a where op :: a -> a
instance C a => C [a] where op = <blah>
We translate the class decl into a newtype, which just gives a
toplevel axiom. The "constructor" MkC expands to a cast, as does the
classop selector.
axiom Co:C a :: C a ~ (a->a)
op :: forall a. C a -> (a -> a)
op a d = d |> (Co:C a)
MkC :: forall a. (a->a) -> C a
MkC = /\a.\op. op |> (sym Co:C a)
The clever RULE stuff doesn't work now, because ($df a d) isn't
a constructor application, so exprIsConApp_maybe won't return
Just <blah>.
Instead, we simply rely on the fact that casts are cheap:
$df :: forall a. C a => C [a]
$df = /\a. \d. MkC [a] ($cop_list a d)
= $cop_list |> forall a. C a -> (sym (Co:C [a]))
$cop_list :: forall a. C a => [a] -> [a]
$cop_list = <blah>
So if we see
(op ($df a d))
we'll inline 'op' and '$df', since both are simply casts, and
good things happen.
Why do we use this different strategy? Because otherwise we
end up with noninlined dictionaries that look like
$df = $cop |> blah
which adds an extra indirection to every use, which seems stupid. See
Trac #4138 for an example (although the regression reported there
wasn't due to the indirction).
There is an awkward wrinkle though: we want to be very
careful when we have
instance C a => C [a] where
op = ...
then we'll get an INLINE pragma on $cop_list but it's important that
$cop_list only inlines when it's applied to *two* arguments (the
dictionary and the list argument). So we nust not etaexpand $df
above. We ensure that this doesn't happen by putting an INLINE
pragma on the dfun itself; after all, it ends up being just a cast.
There is one more dark corner to the INLINE story, even more deeply
buried. Consider this (Trac #3772):
class DeepSeq a => C a where
gen :: Int -> a
instance C a => C [a] where
gen n = ...
class DeepSeq a where
deepSeq :: a -> b -> b
instance DeepSeq a => DeepSeq [a] where
deepSeq xs b = foldr deepSeq b xs
That gives rise to these defns:
$cdeepSeq :: DeepSeq a -> [a] -> b -> b
$cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ...
$fDeepSeq[] :: DeepSeq a -> DeepSeq [a]
$fDeepSeq[] a d = $cdeepSeq a d |> blah
$cp1 a d :: C a => DeepSep [a]
$cp1 a d = $fDeepSep[] a (scsel a d)
$fC[] :: C a => C [a]
$fC[] a d = MkC ($cp1 a d) ($cgen a d)
Here $cp1 is the code that generates the superclass for C [a]. The
issue is this: we must not etaexpand $cp1 either, or else $fDeepSeq[]
and then $cdeepSeq will inline there, which is definitely wrong. Like
on the dfun, we solve this by adding an INLINE pragma to $cp1.
Note [Subtle interaction of recursion and overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this
class C a where { op1,op2 :: a -> a }
instance C a => C [a] where
op1 x = op2 x ++ op2 x
op2 x = ...
instance C [Int] where
...
When typechecking the C [a] instance, we need a C [a] dictionary (for
the call of op2). If we look up in the instance environment, we find
an overlap. And in *general* the right thing is to complain (see Note
[Overlapping instances] in InstEnv). But in *this* case it's wrong to
complain, because we just want to delegate to the op2 of this same
instance.
Why is this justified? Because we generate a (C [a]) constraint in
a context in which 'a' cannot be instantiated to anything that matches
other overlapping instances, or else we would not be excecuting this
version of op1 in the first place.
It might even be a bit disguised:
nullFail :: C [a] => [a] -> [a]
nullFail x = op2 x ++ op2 x
instance C a => C [a] where
op1 x = nullFail x
Precisely this is used in package 'regexbase', module Context.hs.
See the overlapping instances for RegexContext, and the fact that they
call 'nullFail' just like the example above. The DoCon package also
does the same thing; it shows up in module Fraction.hs
Conclusion: when typechecking the methods in a C [a] instance, we want to
treat the 'a' as an *existential* type variable, in the sense described
by Note [Binding when looking up instances]. That is why isOverlappableTyVar
responds True to an InstSkol, which is the kind of skolem we use in
tcInstDecl2.
Note [Tricky type variable scoping]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In our example
class C a where
op1, op2 :: Ix b => a -> b -> b
op2 = <dmrhs>
instance C a => C [a]
op1 = <rhs>
note that 'a' and 'b' are *both* in scope in <dmrhs>, but only 'a' is
in scope in <rhs>. In particular, we must make sure that 'b' is in
scope when typechecking <dmrhs>. This is achieved by subFunTys,
which brings appropriate tyvars into scope. This happens for both
<dmrhs> and for <rhs>, but that doesn't matter: the *renamer* will have
complained if 'b' is mentioned in <rhs>.
%************************************************************************
%* *
\subsection{Extracting instance decls}
%* *
%************************************************************************
Gather up the instance declarations from their various sources
\begin{code}
tcInstDecls1
:: [LTyClDecl Name]
-> [LInstDecl Name]
-> [LDerivDecl Name]
-> TcM (TcGblEnv,
[InstInfo Name],
HsValBinds Name)
tcInstDecls1 tycl_decls inst_decls deriv_decls
= checkNoErrs $
do {
; idx_tycons <- mapAndRecoverM tcTopFamInstDecl $
filter (isFamInstDecl . unLoc) tycl_decls
; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
; let { (local_info,
at_tycons_s) = unzip local_info_tycons
; at_idx_tycons = concat at_tycons_s ++ idx_tycons
; at_things = map ATyCon at_idx_tycons }
; tcExtendGlobalEnvImplicit at_things $ do
{ tcg_env <- tcAddImplicits at_things
; setGblEnv tcg_env $
addInsts local_info $
addFamInsts at_idx_tycons $ do {
failIfErrsM
; (gbl_env, deriv_inst_info, deriv_binds)
<- tcDeriving tycl_decls inst_decls deriv_decls
; dflags <- getDOpts
; when (safeLanguageOn dflags) $
mapM_ (\x -> when (typInstCheck x)
(addErrAt (getSrcSpan $ iSpec x) typInstErr))
local_info
; when (safeInferOn dflags) $
mapM_ (\x -> when (typInstCheck x) recordUnsafeInfer) local_info
; return ( gbl_env
, (bagToList deriv_inst_info) ++ local_info
, deriv_binds)
}}}
where
typInstCheck ty = is_cls (iSpec ty) `elem` typeableClassNames
typInstErr = ptext $ sLit $ "Can't create hand written instances of Typeable in Safe"
++ " Haskell! Can only derive them"
addInsts :: [InstInfo Name] -> TcM a -> TcM a
addInsts infos thing_inside
= tcExtendLocalInstEnv (map iSpec infos) thing_inside
addFamInsts :: [TyCon] -> TcM a -> TcM a
addFamInsts tycons thing_inside
= tcExtendLocalFamInstEnv (map mkLocalFamInst tycons) thing_inside
\end{code}
\begin{code}
tcLocalInstDecl1 :: LInstDecl Name
-> TcM (InstInfo Name, [TyCon])
tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
= setSrcSpan loc $
addErrCtxt (instDeclCtxt1 poly_ty) $
do { is_boot <- tcIsHsBoot
; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
badBootDeclErr
; (tyvars, theta, clas, inst_tys) <- tcHsInstHead InstDeclCtxt poly_ty
; let mini_env = mkVarEnv (classTyVars clas `zip` inst_tys)
; traceTc "tcLocalInstDecl" (ppr poly_ty)
; idx_tycons0 <- tcExtendTyVarEnv tyvars $
mapAndRecoverM (tcAssocDecl clas mini_env) ats
; let defined_ats = mkNameSet $ map (tcdName . unLoc) ats
check_at_instance (fam_tc, defs)
| tyConName fam_tc `elemNameSet` defined_ats = return (Nothing, [])
| null defs = return (Just (tyConName fam_tc), [])
| otherwise = do
defs' <- forM defs $ \(ATD tvs pat_tys rhs _loc) -> do
let mini_env_subst = mkTvSubst (mkInScopeSet (mkVarSet tvs)) mini_env
tvs' = varSetElems (tyVarsOfType rhs')
pat_tys' = substTys mini_env_subst pat_tys
rhs' = substTy mini_env_subst rhs
rep_tc_name <- newFamInstTyConName (noLoc (tyConName fam_tc)) pat_tys'
buildSynTyCon rep_tc_name tvs'
(SynonymTyCon rhs')
(mkArrowKinds (map tyVarKind tvs') (typeKind rhs'))
NoParentTyCon (Just (fam_tc, pat_tys'))
return (Nothing, defs')
; missing_at_stuff <- mapM check_at_instance (classATItems clas)
; let (omitted, idx_tycons1) = unzip missing_at_stuff
; warn <- woptM Opt_WarnMissingMethods
; mapM_ (warnTc warn . omittedATWarn) (catMaybes omitted)
; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
; overlap_flag <- getOverlapFlag
; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
ispec = mkLocalInstance dfun overlap_flag
inst_info = InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags False }
; return (inst_info, idx_tycons0 ++ concat idx_tycons1) }
\end{code}
%************************************************************************
%* *
Type checking family instances
%* *
%************************************************************************
Family instances are somewhat of a hybrid. They are processed together with
class instance heads, but can contain data constructors and hence they share a
lot of kinding and type checking code with ordinary algebraic data types (and
GADTs).
\begin{code}
tcTopFamInstDecl :: LTyClDecl Name -> TcM TyCon
tcTopFamInstDecl (L loc decl)
= setSrcSpan loc $
tcAddDeclCtxt decl $
tcFamInstDecl TopLevel decl
tcFamInstDecl :: TopLevelFlag -> TyClDecl Name -> TcM TyCon
tcFamInstDecl top_lvl decl
= do {
; traceTc "tcFamInstDecl" (ppr decl)
; let fam_tc_lname = tcdLName decl
; type_families <- xoptM Opt_TypeFamilies
; is_boot <- tcIsHsBoot
; checkTc type_families $ badFamInstDecl fam_tc_lname
; checkTc (not is_boot) $ badBootFamInstDeclErr
; fam_tc <- tcLookupLocatedTyCon fam_tc_lname
; checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
; when (isTopLevel top_lvl && isTyConAssoc fam_tc)
(addErr $ assocInClassErr fam_tc_lname)
; tc <- tcFamInstDecl1 fam_tc decl
; checkValidTyCon tc
; return tc }
tcFamInstDecl1 :: TyCon -> TyClDecl Name -> TcM TyCon
tcFamInstDecl1 fam_tc (decl@TySynonym {})
= do {
; (t_tvs, t_typats, t_rhs) <- tcSynFamInstDecl fam_tc decl
; checkValidFamInst t_typats t_rhs
; rep_tc_name <- newFamInstTyConName (tcdLName decl) t_typats
; buildSynTyCon rep_tc_name t_tvs
(SynonymTyCon t_rhs)
(typeKind t_rhs)
NoParentTyCon (Just (fam_tc, t_typats))
}
tcFamInstDecl1 fam_tc (decl@TyData { tcdND = new_or_data, tcdCtxt = ctxt
, tcdTyVars = tvs, tcdTyPats = Just pats
, tcdCons = cons})
= do {
checkTc (isFamilyTyCon fam_tc) (notFamily fam_tc)
; checkTc (isAlgTyCon fam_tc) (wrongKindOfFamily fam_tc)
; tcFamTyPats fam_tc tvs pats (\_always_star -> kcDataDecl decl) $
\tvs' pats' resultKind -> do
{ mapM_ checkTyFamFreeness pats'
; checkTc (isLiftedTypeKind resultKind) $ tooFewParmsErr (tyConArity fam_tc)
; stupid_theta <- tcHsKindedContext =<< kcHsContext ctxt
; dataDeclChecks (tcdName decl) new_or_data stupid_theta cons
; rep_tc_name <- newFamInstTyConName (tcdLName decl) pats'
; let ex_ok = True
; fixM (\ rep_tycon -> do
{ let orig_res_ty = mkTyConApp fam_tc pats'
; data_cons <- tcConDecls new_or_data ex_ok rep_tycon
(tvs', orig_res_ty) cons
; tc_rhs <-
case new_or_data of
DataType -> return (mkDataTyConRhs data_cons)
NewType -> ASSERT( not (null data_cons) )
mkNewTyConRhs rep_tc_name rep_tycon (head data_cons)
; buildAlgTyCon rep_tc_name tvs' stupid_theta tc_rhs Recursive
h98_syntax NoParentTyCon (Just (fam_tc, pats'))
})
} }
where
h98_syntax = case cons of
L _ (ConDecl { con_res = ResTyGADT _ }) : _ -> False
_ -> True
tcFamInstDecl1 _ d = pprPanic "tcFamInstDecl1" (ppr d)
tcAssocDecl :: Class
-> VarEnv Type
-> LTyClDecl Name
-> TcM TyCon
tcAssocDecl clas mini_env (L loc decl)
= setSrcSpan loc $
tcAddDeclCtxt decl $
do { at_tc <- tcFamInstDecl NotTopLevel decl
; let Just (fam_tc, at_tys) = tyConFamInst_maybe at_tc
; checkTc (Just clas == tyConAssoc_maybe fam_tc)
(badATErr (className clas) (tyConName at_tc))
; zipWithM_ check_arg (tyConTyVars fam_tc) at_tys
; return at_tc }
where
check_arg fam_tc_tv at_ty
| Just inst_ty <- lookupVarEnv mini_env fam_tc_tv
= checkTc (inst_ty `eqType` at_ty)
(wrongATArgErr at_ty inst_ty)
| otherwise
= return ()
\end{code}
%************************************************************************
%* *
Typechecking instance declarations, pass 2
%* *
%************************************************************************
\begin{code}
tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
-> TcM (LHsBinds Id)
tcInstDecls2 tycl_decls inst_decls
= do {
let class_decls = filter (isClassDecl . unLoc) tycl_decls
; dm_binds_s <- mapM tcClassDecl2 class_decls
; let dm_binds = unionManyBags dm_binds_s
; let dm_ids = collectHsBindsBinders dm_binds
; inst_binds_s <- tcExtendLetEnv TopLevel dm_ids $
mapM tcInstDecl2 inst_decls
; return (dm_binds `unionBags` unionManyBags inst_binds_s) }
\end{code}
See Note [Default methods and instances]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The default method Ids are already in the type environment (see Note
[Default method Ids and Template Haskell] in TcTyClsDcls), BUT they
don't have their InlinePragmas yet. Usually that would not matter,
because the simplifier propagates information from binding site to
use. But, unusually, when compiling instance decls we *copy* the
INLINE pragma from the default method to the method for that
particular operation (see Note [INLINE and default methods] below).
So right here in tcInstDecls2 we must reextend the type envt with
the default method Ids replete with their INLINE pragmas. Urk.
\begin{code}
tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
= recoverM (return emptyLHsBinds) $
setSrcSpan loc $
addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
do {
; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType (idType dfun_id)
; let (clas, inst_tys) = tcSplitDFunHead inst_head
(class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
; dfun_ev_vars <- newEvVars dfun_theta
; (sc_args, sc_binds)
<- mapAndUnzipM (tcSuperClass inst_tyvars dfun_ev_vars)
(sc_sels `zip` sc_theta')
; spec_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds
; (meth_ids, meth_binds)
<- tcExtendTyVarEnv inst_tyvars $
tcInstanceMethods dfun_id clas inst_tyvars dfun_ev_vars
inst_tys spec_info
op_items ibinds
; self_dict <- newDict clas inst_tys
; let class_tc = classTyCon clas
[dict_constr] = tyConDataCons class_tc
dict_bind = mkVarBind self_dict (L loc con_app_args)
con_app_tys = wrapId (mkWpTyApps inst_tys)
(dataConWrapId dict_constr)
con_app_scs = mkHsWrap (mkWpEvApps (map mk_sc_ev_term sc_args)) con_app_tys
con_app_args = foldl mk_app con_app_scs $
map (wrapId arg_wrapper) meth_ids
mk_app :: HsExpr Id -> HsExpr Id -> HsExpr Id
mk_app fun arg = HsApp (L loc fun) (L loc arg)
mk_sc_ev_term :: EvVar -> EvTerm
mk_sc_ev_term sc
| null inst_tv_tys
, null dfun_ev_vars = EvId sc
| otherwise = EvDFunApp sc inst_tv_tys dfun_ev_vars
inst_tv_tys = mkTyVarTys inst_tyvars
arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps inst_tv_tys
dfun_id_w_fun
| isNewTyCon class_tc
= dfun_id `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
| otherwise
= dfun_id `setIdUnfolding` mkDFunUnfolding dfun_ty dfun_args
`setInlinePragma` dfunInlinePragma
dfun_args :: [CoreExpr]
dfun_args = map varToCoreExpr sc_args ++
map Var meth_ids
export = ABE { abe_wrap = idHsWrapper, abe_poly = dfun_id_w_fun
, abe_mono = self_dict, abe_prags = SpecPrags spec_inst_prags }
main_bind = AbsBinds { abs_tvs = inst_tyvars
, abs_ev_vars = dfun_ev_vars
, abs_exports = [export]
, abs_ev_binds = emptyTcEvBinds
, abs_binds = unitBag dict_bind }
; return (unitBag (L loc main_bind) `unionBags`
listToBag meth_binds `unionBags`
unionManyBags sc_binds)
}
where
dfun_ty = idType dfun_id
dfun_id = instanceDFunId ispec
loc = getSrcSpan dfun_id
tcSuperClass :: [TcTyVar] -> [EvVar]
-> (Id, PredType)
-> TcM (TcId, LHsBinds TcId)
tcSuperClass tyvars ev_vars (sc_sel, sc_pred)
= do { (ev_binds, sc_dict)
<- newImplication InstSkol tyvars ev_vars $
emitWanted ScOrigin sc_pred
; uniq <- newUnique
; let sc_op_ty = mkForAllTys tyvars $ mkPiTypes ev_vars (varType sc_dict)
sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
(getName sc_sel)
sc_op_id = mkLocalId sc_op_name sc_op_ty
sc_op_bind = mkVarBind sc_op_id (L noSrcSpan $ wrapId sc_wrapper sc_dict)
sc_wrapper = mkWpTyLams tyvars
<.> mkWpLams ev_vars
<.> mkWpLet ev_binds
; return (sc_op_id, unitBag sc_op_bind) }
tcSpecInstPrags :: DFunId -> InstBindings Name
-> TcM ([Located TcSpecPrag], PragFun)
tcSpecInstPrags _ (NewTypeDerived {})
= return ([], \_ -> [])
tcSpecInstPrags dfun_id (VanillaInst binds uprags _)
= do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
filter isSpecInstLSig uprags
; return (spec_inst_prags, mkPragFun uprags binds) }
\end{code}
Note [Superclass loop avoidance]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following (extreme) situation:
class C a => D a where ...
instance D [a] => D [a] where ...
Although this looks wrong (assume D [a] to prove D [a]), it is only a
more extreme case of what happens with recursive dictionaries, and it
can, just about, make sense because the methods do some work before
recursing.
To implement the dfun we must generate code for the superclass C [a],
which we had better not get by superclass selection from the supplied
argument:
dfun :: forall a. D [a] -> D [a]
dfun = \d::D [a] -> MkD (scsel d) ..
Rather, we want to get it by finding an instance for (C [a]). We
achieve this by
not making the superclasses of a "wanted"
available for solving wanted constraints.
Test case SCLoop tests this fix.
Note [SPECIALISE instance pragmas]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
instance (Ix a, Ix b) => Ix (a,b) where
range (x,y) = ...
We do *not* want to make a specialised version of the dictionary
function. Rather, we want specialised versions of each method.
Thus we should generate something like this:
$dfIx :: (Ix a, Ix x) => Ix (a,b)
$dfIx da db = Ix ($crange da db) (...other methods...)
$dfIxPair :: (Ix a, Ix x) => Ix (a,b)
$dfIxPair = Ix ($crangePair da db) (...other methods...)
$crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
$crange da db = <blah>
Note that
* The RULE is unaffected by the specialisation. We don't want to
specialise $dfIx, because then it would need a specialised RULE
which is a pain. The single RULE works fine at all specialisations.
See Note [How instance declarations are translated] above
* Instead, we want to specialise the *method*, $crange
In practice, rather than faking up a SPECIALISE pragama for each
method (which is painful, since we'd have to figure out its
specialised type), we call tcSpecPrag *as if* were going to specialise
$dfIx
SpecPrag which, as it turns out, can be used unchanged for each method.
The "it turns out" bit is delicate, but it works fine!
\begin{code}
tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
= addErrCtxt (spec_ctxt prag) $
do { let name = idName dfun_id
; (tyvars, theta, clas, tys) <- tcHsInstHead SpecInstCtxt hs_ty
; let spec_dfun_ty = mkDictFunTy tyvars theta clas tys
; co_fn <- tcSubType (SpecPragOrigin name) SpecInstCtxt
(idType dfun_id) spec_dfun_ty
; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
where
spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
tcSpecInst _ _ = panic "tcSpecInst"
\end{code}
%************************************************************************
%* *
Typechecking an instance method
%* *
%************************************************************************
tcInstanceMethod
Make the method bindings, as a [(NonRec, HsBinds)], one per method
Remembering to use fresh Name (the instance method Name) as the binder
Bring the instance method Ids into scope, for the benefit of tcInstSig
Use sig_fn mapping instance method Name -> instance tyvars
Ditto prag_fn
Use tcValBinds to do the checking
\begin{code}
tcInstanceMethods :: DFunId -> Class -> [TcTyVar]
-> [EvVar]
-> [TcType]
-> ([Located TcSpecPrag], PragFun)
-> [(Id, DefMeth)]
-> InstBindings Name
-> TcM ([Id], [LHsBind Id])
tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
(spec_inst_prags, prag_fn)
op_items (VanillaInst binds _ standalone_deriv)
= mapAndUnzipM tc_item op_items
where
tc_item :: (Id, DefMeth) -> TcM (Id, LHsBind Id)
tc_item (sel_id, dm_info)
= case findMethodBind (idName sel_id) binds of
Just user_bind -> tc_body sel_id standalone_deriv user_bind
Nothing -> traceTc "tc_def" (ppr sel_id) >>
tc_default sel_id dm_info
tc_body :: Id -> Bool -> LHsBind Name -> TcM (TcId, LHsBind Id)
tc_body sel_id generated_code rn_bind
= add_meth_ctxt sel_id generated_code rn_bind $
do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
inst_tys sel_id
; let prags = prag_fn (idName sel_id)
; meth_id1 <- addInlinePrags meth_id prags
; spec_prags <- tcSpecPrags meth_id1 prags
; bind <- tcInstanceMethodBody InstSkol
tyvars dfun_ev_vars
meth_id1 local_meth_id meth_sig_fn
(mk_meth_spec_prags meth_id1 spec_prags)
rn_bind
; return (meth_id1, bind) }
tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id)
tc_default sel_id (GenDefMeth dm_name)
= do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id dm_name
; tc_body sel_id False meth_bind }
tc_default sel_id NoDefMeth
= do { traceTc "tc_def: warn" (ppr sel_id)
; warnMissingMethod sel_id
; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
inst_tys sel_id
; return (meth_id, mkVarBind meth_id $
mkLHsWrap lam_wrapper error_rhs) }
where
error_rhs = L loc $ HsApp error_fun error_msg
error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars
tc_default sel_id (DefMeth dm_name)
= do {
; self_dict <- newDict clas inst_tys
; let self_ev_bind = EvBind self_dict
(EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars)
; (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
inst_tys sel_id
; dm_id <- tcLookupId dm_name
; let dm_inline_prag = idInlinePragma dm_id
rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $
HsVar dm_id
meth_bind = mkVarBind local_meth_id (L loc rhs)
meth_id1 = meth_id `setInlinePragma` dm_inline_prag
export = ABE { abe_wrap = idHsWrapper, abe_poly = meth_id1
, abe_mono = local_meth_id
, abe_prags = mk_meth_spec_prags meth_id1 [] }
bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
, abs_exports = [export]
, abs_ev_binds = EvBinds (unitBag self_ev_bind)
, abs_binds = unitBag meth_bind }
; return (meth_id1, L loc bind) }
mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> TcSpecPrags
mk_meth_spec_prags meth_id spec_prags_for_me
= SpecPrags (spec_prags_for_me ++
[ L loc (SpecPrag meth_id wrap inl)
| L loc (SpecPrag _ wrap inl) <- spec_inst_prags])
loc = getSrcSpan dfun_id
meth_sig_fn _ = Just ([],loc)
add_meth_ctxt sel_id generated_code rn_bind thing
| generated_code = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys rn_bind) thing
| otherwise = thing
tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
_ op_items (NewTypeDerived coi _)
= do { rep_d_stuff <- checkConstraints InstSkol tyvars dfun_ev_vars $
emitWanted ScOrigin rep_pred
; mapAndUnzipM (tc_item rep_d_stuff) op_items }
where
loc = getSrcSpan dfun_id
Just (init_inst_tys, _) = snocView inst_tys
rep_ty = pFst (tcCoercionKind co)
rep_pred = mkClassPred clas (init_inst_tys ++ [rep_ty])
co = mkTcSymCo (mkTcInstCos coi (mkTyVarTys tyvars))
tc_item :: (TcEvBinds, EvVar) -> (Id, DefMeth) -> TcM (TcId, LHsBind TcId)
tc_item (rep_ev_binds, rep_d) (sel_id, _)
= do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
inst_tys sel_id
; let meth_rhs = wrapId (mk_op_wrapper sel_id rep_d) sel_id
meth_bind = mkVarBind local_meth_id (L loc meth_rhs)
export = ABE { abe_wrap = idHsWrapper, abe_poly = meth_id
, abe_mono = local_meth_id, abe_prags = noSpecPrags }
bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
, abs_exports = [export]
, abs_ev_binds = rep_ev_binds
, abs_binds = unitBag $ meth_bind }
; return (meth_id, L loc bind) }
mk_op_wrapper :: Id -> EvVar -> HsWrapper
mk_op_wrapper sel_id rep_d
= WpCast (liftTcCoSubstWith sel_tvs (map mkTcReflCo init_inst_tys ++ [co])
local_meth_ty)
<.> WpEvApp (EvId rep_d)
<.> mkWpTyApps (init_inst_tys ++ [rep_ty])
where
(sel_tvs, sel_rho) = tcSplitForAllTys (idType sel_id)
(_, local_meth_ty) = tcSplitPredFunTy_maybe sel_rho
`orElse` pprPanic "tcInstanceMethods" (ppr sel_id)
mkMethIds :: Class -> [TcTyVar] -> [EvVar] -> [TcType] -> Id -> TcM (TcId, TcId)
mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id
= do { uniq <- newUnique
; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
; local_meth_name <- newLocalName sel_name
; let meth_id = mkLocalId meth_name meth_ty
local_meth_id = mkLocalId local_meth_name local_meth_ty
; return (meth_id, local_meth_id) }
where
local_meth_ty = instantiateMethod clas sel_id inst_tys
meth_ty = mkForAllTys tyvars $ mkPiTypes dfun_ev_vars local_meth_ty
sel_name = idName sel_id
wrapId :: HsWrapper -> id -> HsExpr id
wrapId wrapper id = mkHsWrap wrapper (HsVar id)
derivBindCtxt :: Id -> Class -> [Type ] -> LHsBind Name -> SDoc
derivBindCtxt sel_id clas tys _bind
= vcat [ ptext (sLit "When typechecking the code for ") <+> quotes (ppr sel_id)
, nest 2 (ptext (sLit "in a standalone derived instance for")
<+> quotes (pprClassPred clas tys) <> colon)
, nest 2 $ ptext (sLit "To see the code I am typechecking, use -ddump-deriv") ]
warnMissingMethod :: Id -> TcM ()
warnMissingMethod sel_id
= do { warn <- woptM Opt_WarnMissingMethods
; traceTc "warn" (ppr sel_id <+> ppr warn <+> ppr (not (startsWithUnderscore (getOccName sel_id))))
; warnTc (warn
&& not (startsWithUnderscore (getOccName sel_id)))
(ptext (sLit "No explicit method nor default method for")
<+> quotes (ppr sel_id)) }
\end{code}
Note [Export helper functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We arrange to export the "helper functions" of an instance declaration,
so that they are not subject to preInlineUnconditionally, even if their
RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
the dict fun as Ids, not as CoreExprs, so we can't substitute a
nonvariable for them.
We could change this by making DFunUnfoldings have CoreExprs, but it
seems a bit simpler this way.
Note [Default methods in instances]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this
class Baz v x where
foo :: x -> x
foo y = <blah>
instance Baz Int Int
From the class decl we get
$dmfoo :: forall v x. Baz v x => x -> x
$dmfoo y = <blah>
Notice that the type is ambiguous. That's fine, though. The instance
decl generates
$dBazIntInt = MkBaz fooIntInt
fooIntInt = $dmfoo Int Int $dBazIntInt
BUT this does mean we must generate the dictionary translation of
fooIntInt directly, rather than generating sourcecode and
typechecking it. That was the bug in Trac #1061. In any case it's
less work to generate the translated version!
Note [INLINE and default methods]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Default methods need special case. They are supposed to behave rather like
macros. For exmample
class Foo a where
op1, op2 :: Bool -> a -> a
op1 b x = op2 (not b) x
instance Foo Int where
op2 b x = <blah>
The instance declaration should behave
just as if 'op1' had been defined with the
code, and INLINE pragma, from its original
definition.
That is, just as if you'd written
instance Foo Int where
op2 b x = <blah>
op1 b x = op2 (not b) x
So for the above example we generate:
$dmop1 d b x = op2 d (not b) x
$fFooInt = MkD $cop1 $cop2
$cop1 = $dmop1 $fFooInt
$cop2 = <blah>
Note carefullly:
* We *copy* any INLINE pragma from the default method $dmop1 to the
instance $cop1. Otherwise we'll just inline the former in the
latter and stop, which isn't what the user expected
* Regardless of its pragma, we give the default method an
unfolding with an InlineCompulsory source. That means
that it'll be inlined at every use site, notably in
each instance declaration, such as $cop1. This inlining
must happen even though
a) $dmop1 is not saturated in $cop1
b) $cop1 itself has an INLINE pragma
It's vital that $dmop1 *is* inlined in this way, to allow the mutual
recursion between $fooInt and $cop1 to be broken
* To communicate the need for an InlineCompulsory to the desugarer
(which makes the Unfoldings), we use the IsDefaultMethod constructor
in TcSpecPrags.
%************************************************************************
%* *
\subsection{Error messages}
%* *
%************************************************************************
\begin{code}
instDeclCtxt1 :: LHsType Name -> SDoc
instDeclCtxt1 hs_inst_ty
= inst_decl_ctxt (case unLoc hs_inst_ty of
HsForAllTy _ _ _ (L _ ty') -> ppr ty'
_ -> ppr hs_inst_ty)
instDeclCtxt2 :: Type -> SDoc
instDeclCtxt2 dfun_ty
= inst_decl_ctxt (ppr (mkClassPred cls tys))
where
(_,_,cls,tys) = tcSplitDFunTy dfun_ty
inst_decl_ctxt :: SDoc -> SDoc
inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
omittedATWarn :: Name -> SDoc
omittedATWarn at
= ptext (sLit "No explicit AT declaration for") <+> quotes (ppr at)
badBootFamInstDeclErr :: SDoc
badBootFamInstDeclErr
= ptext (sLit "Illegal family instance in hs-boot file")
notFamily :: TyCon -> SDoc
notFamily tycon
= vcat [ ptext (sLit "Illegal family instance for") <+> quotes (ppr tycon)
, nest 2 $ parens (ppr tycon <+> ptext (sLit "is not an indexed type family"))]
tooFewParmsErr :: Arity -> SDoc
tooFewParmsErr arity
= ptext (sLit "Family instance has too few parameters; expected") <+>
ppr arity
assocInClassErr :: Located Name -> SDoc
assocInClassErr name
= ptext (sLit "Associated type") <+> quotes (ppr name) <+>
ptext (sLit "must be inside a class instance")
badFamInstDecl :: Located Name -> SDoc
badFamInstDecl tc_name
= vcat [ ptext (sLit "Illegal family instance for") <+>
quotes (ppr tc_name)
, nest 2 (parens $ ptext (sLit "Use -XTypeFamilies to allow indexed type families")) ]
\end{code}