\begin{code}
module TcSimplify(
simplifyInfer, quantifyPred,
simplifyAmbiguityCheck,
simplifyDefault,
simplifyRule, simplifyTop, simplifyInteractive,
solveWantedsTcM
) where
#include "HsVersions.h"
import TcRnTypes
import TcRnMonad
import TcErrors
import TcMType as TcM
import TcType
import TcSMonad as TcS
import TcInteract
import Kind ( defaultKind_maybe )
import Inst
import FunDeps ( growThetaTyVars )
import Type ( classifyPredType, PredTree(..), getClassPredTys_maybe )
import Class ( Class )
import Var
import Unique
import VarSet
import VarEnv
import TcEvidence
import Name
import Bag
import ListSetOps
import Util
import PrelInfo
import PrelNames
import Class ( classKey )
import BasicTypes ( RuleName )
import Outputable
import FastString
import TrieMap ()
\end{code}
*********************************************************************************
* *
* External interface *
* *
*********************************************************************************
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
simplifyTop wanteds
= do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds
; ev_binds_var <- newTcEvBinds
; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top
; binds1 <- TcRnMonad.getTcEvBinds ev_binds_var
; traceTc "End simplifyTop }" empty
; traceTc "reportUnsolved {" empty
; binds2 <- reportUnsolved zonked_final_wc
; traceTc "reportUnsolved }" empty
; return (binds1 `unionBags` binds2) }
where
simpl_top :: WantedConstraints -> TcS WantedConstraints
simpl_top wanteds
= do { wc_first_go <- nestTcS (solve_wanteds_and_drop wanteds)
; try_tyvar_defaulting wc_first_go }
try_tyvar_defaulting :: WantedConstraints -> TcS WantedConstraints
try_tyvar_defaulting wc
| isEmptyWC wc
= return wc
| otherwise
= do { free_tvs <- TcS.zonkTyVarsAndFV (tyVarsOfWC wc)
; let meta_tvs = varSetElems (filterVarSet isMetaTyVar free_tvs)
; meta_tvs' <- mapM defaultTyVar meta_tvs
; if meta_tvs' == meta_tvs
then try_class_defaulting wc
else do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
; try_class_defaulting wc_residual } }
try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
try_class_defaulting wc
| isEmptyWC wc || insolubleWC wc
= return wc
| otherwise
= do { something_happened <- applyDefaultingRules (approximateWC wc)
; if something_happened
then do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
; try_class_defaulting wc_residual }
else return wc }
\end{code}
Note [Must simplify after defaulting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may have a deeply buried constraint
(t:*) ~ (a:Open)
which we couldn't solve because of the kind incompatibility, and 'a' is free.
Then when we default 'a' we can solve the constraint. And we want to do
that before starting in on type classes. We MUST do it before reporting
errors, because it isn't an error! Trac #7967 was due to this.
Note [Top-level Defaulting Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We have considered two design choices for where/when to apply defaulting.
(i) Do it in SimplCheck mode only /whenever/ you try to solve some
flat constraints, maybe deep inside the context of implications.
This used to be the case in GHC 7.4.1.
(ii) Do it in a tight loop at simplifyTop, once all other constraint has
finished. This is the current story.
Option (i) had many disadvantages:
a) First it was deep inside the actual solver,
b) Second it was dependent on the context (Infer a type signature,
or Check a type signature, or Interactive) since we did not want
to always start defaulting when inferring (though there is an exception to
this see Note [Default while Inferring])
c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs:
f :: Int -> Bool
f x = const True (\y -> let w :: a -> a
w a = const a (y+1)
in w y)
We will get an implication constraint (for beta the type of y):
[untch=beta] forall a. 0 => Num beta
which we really cannot default /while solving/ the implication, since beta is
untouchable.
Instead our new defaulting story is to pull defaulting out of the solver loop and
go with option (i), implemented at SimplifyTop. Namely:
- First have a go at solving the residual constraint of the whole program
- Try to approximate it with a flat constraint
- Figure out derived defaulting equations for that flat constraint
- Go round the loop again if you did manage to get some equations
Now, that has to do with class defaulting. However there exists type variable /kind/
defaulting. Again this is done at the top-level and the plan is:
- At the top-level, once you had a go at solving the constraint, do
figure out /all/ the touchable unification variables of the wanted constraints.
- Apply defaulting to their kinds
More details in Note [DefaultTyVar].
\begin{code}
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
= traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >>
simplifyTop wanteds
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds
= traceTc "simplifyInteractive" empty >>
simplifyTop wanteds
simplifyDefault :: ThetaType
-> TcM ()
simplifyDefault theta
= do { traceTc "simplifyInteractive" empty
; wanted <- newFlatWanteds DefaultOrigin theta
; (unsolved, _binds) <- solveWantedsTcM (mkFlatWC wanted)
; traceTc "reportUnsolved {" empty
; reportAllUnsolved unsolved
; traceTc "reportUnsolved }" empty
; return () }
\end{code}
*********************************************************************************
* *
* Inference
* *
***********************************************************************************
\begin{code}
simplifyInfer :: Bool
-> Bool
-> [(Name, TcTauType)]
-> WantedConstraints
-> TcM ([TcTyVar],
[EvVar],
Bool,
TcEvBinds)
simplifyInfer _top_lvl apply_mr name_taus wanteds
| isEmptyWC wanteds
= do { gbl_tvs <- tcGetGlobalTyVars
; qtkvs <- quantifyTyVars gbl_tvs (tyVarsOfTypes (map snd name_taus))
; traceTc "simplifyInfer: emtpy WC" (ppr name_taus $$ ppr qtkvs)
; return (qtkvs, [], False, emptyTcEvBinds) }
| otherwise
= do { traceTc "simplifyInfer {" $ vcat
[ ptext (sLit "binds =") <+> ppr name_taus
, ptext (sLit "closed =") <+> ppr _top_lvl
, ptext (sLit "apply_mr =") <+> ppr apply_mr
, ptext (sLit "(unzonked) wanted =") <+> ppr wanteds
]
; ev_binds_var <- newTcEvBinds
; wanted_transformed <- solveWantedsTcMWithEvBinds ev_binds_var wanteds $
solve_wanteds_and_drop
; tc_lcl_env <- TcRnMonad.getLclEnv
; let untch = tcl_untch tc_lcl_env
; quant_pred_candidates
<- if insolubleWC wanted_transformed
then return []
else do { gbl_tvs <- tcGetGlobalTyVars
; let quant_cand = approximateWC wanted_transformed
meta_tvs = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand))
; ((flats, _insols), _extra_binds) <- runTcS $
do { mapM_ (promoteAndDefaultTyVar untch gbl_tvs) meta_tvs
; _implics <- solveInteract quant_cand
; getInertUnsolved }
; return (map ctPred $ filter isWantedCt (bagToList flats)) }
; gbl_tvs <- tcGetGlobalTyVars
; zonked_tau_tvs <- TcM.zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
; let poly_qtvs = growThetaTyVars quant_pred_candidates zonked_tau_tvs
`minusVarSet` gbl_tvs
pbound = filter (quantifyPred poly_qtvs) quant_pred_candidates
constrained_tvs = tyVarsOfTypes pbound `unionVarSet` gbl_tvs
mr_bites = apply_mr && not (null pbound)
; (qtvs, bound) <- if mr_bites
then do { qtvs <- quantifyTyVars constrained_tvs zonked_tau_tvs
; return (qtvs, []) }
else do { qtvs <- quantifyTyVars gbl_tvs poly_qtvs
; return (qtvs, pbound) }
; traceTc "simplifyWithApprox" $
vcat [ ptext (sLit "quant_pred_candidates =") <+> ppr quant_pred_candidates
, ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs
, ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs
, ptext (sLit "pbound =") <+> ppr pbound
, ptext (sLit "bbound =") <+> ppr bound
, ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs
, ptext (sLit "constrained_tvs =") <+> ppr constrained_tvs
, ptext (sLit "mr_bites =") <+> ppr mr_bites
, ptext (sLit "qtvs =") <+> ppr qtvs ]
; if null qtvs && null bound
then do { traceTc "} simplifyInfer/no implication needed" empty
; emitConstraints wanted_transformed
; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
else do
{
let minimal_flat_preds = mkMinimalBySCs bound
skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
| (name, ty) <- name_taus ]
; minimal_bound_ev_vars <- mapM TcM.newEvVar minimal_flat_preds
; let implic = Implic { ic_untch = pushUntouchables untch
, ic_skols = qtvs
, ic_fsks = []
, ic_given = minimal_bound_ev_vars
, ic_wanted = wanted_transformed
, ic_insol = False
, ic_binds = ev_binds_var
, ic_info = skol_info
, ic_env = tc_lcl_env }
; emitImplication implic
; traceTc "} simplifyInfer/produced residual implication for quantification" $
vcat [ ptext (sLit "implic =") <+> ppr implic
, ptext (sLit "qtvs =") <+> ppr qtvs
, ptext (sLit "spb =") <+> ppr quant_pred_candidates
, ptext (sLit "bound =") <+> ppr bound ]
; return ( qtvs, minimal_bound_ev_vars
, mr_bites, TcEvBinds ev_binds_var) } }
quantifyPred :: TyVarSet
-> PredType -> Bool
quantifyPred qtvs pred
| isIPPred pred = True
| otherwise = tyVarsOfType pred `intersectsVarSet` qtvs
\end{code}
Note [Inheriting implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
f x = (x::Int) + ?y
where f is *not* a top-level binding.
From the RHS of f we'll get the constraint (?y::Int).
There are two types we might infer for f:
f :: Int -> Int
(so we get ?y from the context of f's definition), or
f :: (?y::Int) => Int -> Int
At first you might think the first was better, because then
?y behaves like a free variable of the definition, rather than
having to be passed at each call site. But of course, the WHOLE
IDEA is that ?y should be passed at each call site (that's what
dynamic binding means) so we'd better infer the second.
BOTTOM LINE: when *inferring types* you must quantify over implicit
parameters, *even if* they don't mention the bound type variables.
Reason: because implicit parameters, uniquely, have local instance
declarations. See the predicate quantifyPred.
Note [Quantification with errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we find that the RHS of the definition has some absolutely-insoluble
constraints, we abandon all attempts to find a context to quantify
over, and instead make the function fully-polymorphic in whatever
type we have found. For two reasons
a) Minimise downstream errors
b) Avoid spurious errors from this function
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Our current plan is that defaulting only happens at simplifyTop and
not simplifyInfer. This may lead to some insoluble deferred constraints
Example:
instance D g => C g Int b
constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha
type inferred = gamma -> gamma
Now, if we try to default (alpha := Int) we will be able to refine the implication to
(forall b. 0 => C gamma Int b)
which can then be simplified further to
(forall b. 0 => D gamma)
Finally we /can/ approximate this implication with (D gamma) and infer the quantified
type: forall g. D g => g -> g
Instead what will currently happen is that we will get a quantified type
(forall g. g -> g) and an implication:
forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha
which, even if the simplifyTop defaults (alpha := Int) we will still be left with an
unsolvable implication:
forall g. 0 => (forall b. 0 => D g)
The concrete example would be:
h :: C g a s => g -> a -> ST s a
f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1)
But it is quite tedious to do defaulting and resolve the implication constraints and
we have not observed code breaking because of the lack of defaulting in inference so
we don't do it for now.
Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we quantify over a constraint, in simplifyInfer we need to
quantify over a constraint that is minimal in some sense: For
instance, if the final wanted constraint is (Eq alpha, Ord alpha),
we'd like to quantify over Ord alpha, because we can just get Eq alpha
from superclass selection from Ord alpha. This minimization is what
mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
to check the original wanted.
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-------- NB NB NB (Jun 12) -------------
This note not longer applies; see the notes with Trac #4361.
But I'm leaving it in here so we remember the issue.)
----------------------------------------
When inferring the type of a let-binding, with simplifyInfer,
try to avoid unnecessarily simplifying class constraints.
Doing so aids sharing, but it also helps with delicate
situations like
instance C t => C [t] where ..
f :: C [t] => ....
f x = let g y = ...(constraint C [t])...
in ...
When inferring a type for 'g', we don't want to apply the
instance decl, because then we can't satisfy (C t). So we
just notice that g isn't quantified over 't' and partition
the constraints before simplifying.
This only half-works, but then let-generalisation only half-works.
*********************************************************************************
* *
* RULES *
* *
***********************************************************************************
See note [Simplifying RULE consraints] in TcRule
Note [RULE quanfification over equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Decideing which equalities to quantify over is tricky:
* We do not want to quantify over insoluble equalities (Int ~ Bool)
(a) because we prefer to report a LHS type error
(b) because if such things end up in 'givens' we get a bogus
"inaccessible code" error
* But we do want to quantify over things like (a ~ F b), where
F is a type function.
The difficulty is that it's hard to tell what is insoluble!
So we see whether the simplificaiotn step yielded any type errors,
and if so refrain from quantifying over *any* equalites.
\begin{code}
simplifyRule :: RuleName
-> WantedConstraints
-> WantedConstraints
-> TcM ([EvVar], WantedConstraints)
simplifyRule name lhs_wanted rhs_wanted
= do {
(resid_wanted, _) <- solveWantedsTcM (lhs_wanted `andWC` rhs_wanted)
; zonked_lhs_flats <- zonkCts (wc_flat lhs_wanted)
; let (q_cts, non_q_cts) = partitionBag quantify_me zonked_lhs_flats
quantify_me
| insolubleWC resid_wanted = quantify_insol
| otherwise = quantify_normal
quantify_insol ct = not (isEqPred (ctPred ct))
quantify_normal ct
| EqPred t1 t2 <- classifyPredType (ctPred ct)
= not (t1 `eqType` t2)
| otherwise
= True
; traceTc "simplifyRule" $
vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
, text "zonked_lhs_flats" <+> ppr zonked_lhs_flats
, text "q_cts" <+> ppr q_cts
, text "non_q_cts" <+> ppr non_q_cts ]
; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
, lhs_wanted { wc_flat = non_q_cts }) }
\end{code}
*********************************************************************************
* *
* Main Simplifier *
* *
***********************************************************************************
Note [Deferring coercion errors to runtime]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
While developing, sometimes it is desirable to allow compilation to succeed even
if there are type errors in the code. Consider the following case:
module Main where
a :: Int
a = 'a'
main = print "b"
Even though `a` is ill-typed, it is not used in the end, so if all that we're
interested in is `main` it is handy to be able to ignore the problems in `a`.
Since we treat type equalities as evidence, this is relatively simple. Whenever
we run into a type mismatch in TcUnify, we normally just emit an error. But it
is always safe to defer the mismatch to the main constraint solver. If we do
that, `a` will get transformed into
co :: Int ~ Char
co = ...
a :: Int
a = 'a' `cast` co
The constraint solver would realize that `co` is an insoluble constraint, and
emit an error with `reportUnsolved`. But we can also replace the right-hand side
of `co` with `error "Deferred type error: Int ~ Char"`. This allows the program
to compile, and it will run fine unless we evaluate `a`. This is what
`deferErrorsToRuntime` does.
It does this by keeping track of which errors correspond to which coercion
in TcErrors (with ErrEnv). TcErrors.reportTidyWanteds does not print the errors
and does not fail if -fdefer-type-errors is on, so that we can continue
compilation. The errors are turned into warnings in `reportUnsolved`.
Note [Zonk after solving]
~~~~~~~~~~~~~~~~~~~~~~~~~
We zonk the result immediately after constraint solving, for two reasons:
a) because zonkWC generates evidence, and this is the moment when we
have a suitable evidence variable to hand.
Note that *after* solving the constraints are typically small, so the
overhead is not great.
\begin{code}
solveWantedsTcMWithEvBinds :: EvBindsVar
-> WantedConstraints
-> (WantedConstraints -> TcS WantedConstraints)
-> TcM WantedConstraints
solveWantedsTcMWithEvBinds ev_binds_var wc tcs_action
= do { traceTc "solveWantedsTcMWithEvBinds" $ text "wanted=" <+> ppr wc
; wc2 <- runTcSWithEvBinds ev_binds_var (tcs_action wc)
; zonkWC ev_binds_var wc2 }
solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
solveWantedsTcM wanted
= do { ev_binds_var <- newTcEvBinds
; wanteds' <- solveWantedsTcMWithEvBinds ev_binds_var wanted solve_wanteds_and_drop
; binds <- TcRnMonad.getTcEvBinds ev_binds_var
; return (wanteds', binds) }
solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints)
solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted
; return (dropDerivedWC wc) }
solve_wanteds :: WantedConstraints -> TcS WantedConstraints
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })
= do { traceTcS "solveWanteds {" (ppr wanted)
; traceTcS "solveFlats {" empty
; let all_flats = flats `unionBags` insols
; impls_from_flats <- solveInteract all_flats
; traceTcS "solveFlats end }" (ppr impls_from_flats)
; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
; (unsolved_flats, insoluble_flats) <- getInertUnsolved
; let wc = WC { wc_flat = unsolved_flats
, wc_impl = unsolved_implics
, wc_insol = insoluble_flats }
; bb <- getTcEvBindsMap
; tb <- getTcSTyBindsMap
; traceTcS "solveWanteds }" $
vcat [ text "unsolved_flats =" <+> ppr unsolved_flats
, text "unsolved_implics =" <+> ppr unsolved_implics
, text "current evbinds =" <+> ppr (evBindMapBinds bb)
, text "current tybinds =" <+> vcat (map ppr (varEnvElts tb))
, text "final wc =" <+> ppr wc ]
; return wc }
simpl_loop :: Int
-> Bag Implication
-> TcS (Bag Implication)
simpl_loop n implics
| n > 10
= traceTcS "solveWanteds: loop!" empty >> return implics
| otherwise
= do { (floated_eqs, unsolved_implics) <- solveNestedImplications implics
; if isEmptyBag floated_eqs
then return unsolved_implics
else
do {
impls_from_eqs <- solveInteract floated_eqs
; simpl_loop (n+1) (unsolved_implics `unionBags` impls_from_eqs)} }
solveNestedImplications :: Bag Implication
-> TcS (Cts, Bag Implication)
solveNestedImplications implics
| isEmptyBag implics
= return (emptyBag, emptyBag)
| otherwise
= do { inerts <- getTcSInerts
; let thinner_inerts = prepareInertsForImplications inerts
; traceTcS "solveNestedImplications starting {" $
vcat [ text "original inerts = " <+> ppr inerts
, text "thinner_inerts = " <+> ppr thinner_inerts ]
; (floated_eqs, unsolved_implics)
<- flatMapBagPairM (solveImplication thinner_inerts) implics
; traceTcS "solveNestedImplications end }" $
vcat [ text "all floated_eqs =" <+> ppr floated_eqs
, text "unsolved_implics =" <+> ppr unsolved_implics ]
; return (floated_eqs, unsolved_implics) }
solveImplication :: InertSet
-> Implication
-> TcS (Cts,
Bag Implication)
solveImplication inerts
imp@(Implic { ic_untch = untch
, ic_binds = ev_binds
, ic_skols = skols
, ic_fsks = old_fsks
, ic_given = givens
, ic_wanted = wanteds
, ic_info = info
, ic_env = env })
= do { traceTcS "solveImplication {" (ppr imp)
; (new_fsks, residual_wanted)
<- nestImplicTcS ev_binds untch inerts $
do { solveInteractGiven (mkGivenLoc info env) old_fsks givens
; residual_wanted <- solve_wanteds wanteds
; more_fsks <- getFlattenSkols
; return (more_fsks ++ old_fsks, residual_wanted) }
; (floated_eqs, final_wanted)
<- floatEqualities (skols ++ new_fsks) givens residual_wanted
; let res_implic | isEmptyWC final_wanted
= emptyBag
| otherwise
= unitBag (imp { ic_fsks = new_fsks
, ic_wanted = dropDerivedWC final_wanted
, ic_insol = insolubleWC final_wanted })
; evbinds <- getTcEvBindsMap
; traceTcS "solveImplication end }" $ vcat
[ text "floated_eqs =" <+> ppr floated_eqs
, text "new_fsks =" <+> ppr new_fsks
, text "res_implic =" <+> ppr res_implic
, text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]
; return (floated_eqs, res_implic) }
\end{code}
\begin{code}
floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints
-> TcS (Cts, WantedConstraints)
floatEqualities skols can_given wanteds@(WC { wc_flat = flats })
| hasEqualities can_given
= return (emptyBag, wanteds)
| otherwise
= do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats
; untch <- TcS.getUntouchables
; mapM_ (promoteTyVar untch) (varSetElems (tyVarsOfCts float_eqs))
; ty_binds <- getTcSTyBindsMap
; traceTcS "floatEqualities" (vcat [ text "Flats =" <+> ppr flats
, text "Floated eqs =" <+> ppr float_eqs
, text "Ty binds =" <+> ppr ty_binds])
; return (float_eqs, wanteds { wc_flat = remaining_flats }) }
where
skol_set = fixVarSet mk_next (mkVarSet skols)
mk_next tvs = foldrBag grow_one tvs flats
grow_one (CFunEqCan { cc_tyargs = xis, cc_rhs = rhs }) tvs
| intersectsVarSet tvs (tyVarsOfTypes xis)
= tvs `unionVarSet` tyVarsOfType rhs
grow_one _ tvs = tvs
is_floatable :: Ct -> Bool
is_floatable ct = isEqPred pred && skol_set `disjointVarSet` tyVarsOfType pred
where
pred = ctPred ct
promoteTyVar :: Untouchables -> TcTyVar -> TcS ()
promoteTyVar untch tv
| isFloatedTouchableMetaTyVar untch tv
= do { cloned_tv <- TcS.cloneMetaTyVar tv
; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch
; setWantedTyBind tv (mkTyVarTy rhs_tv) }
| otherwise
= return ()
promoteAndDefaultTyVar :: Untouchables -> TcTyVarSet -> TyVar -> TcS ()
promoteAndDefaultTyVar untch gbl_tvs tv
= do { tv1 <- if tv `elemVarSet` gbl_tvs
then return tv
else defaultTyVar tv
; promoteTyVar untch tv1 }
defaultTyVar :: TcTyVar -> TcS TcTyVar
defaultTyVar the_tv
| Just default_k <- defaultKind_maybe (tyVarKind the_tv)
= do { tv' <- TcS.cloneMetaTyVar the_tv
; let new_tv = setTyVarKind tv' default_k
; traceTcS "defaultTyVar" (ppr the_tv <+> ppr new_tv)
; setWantedTyBind the_tv (mkTyVarTy new_tv)
; return new_tv }
| otherwise = return the_tv
approximateWC :: WantedConstraints -> Cts
approximateWC wc
= float_wc emptyVarSet wc
where
float_wc :: TcTyVarSet -> WantedConstraints -> Cts
float_wc trapping_tvs (WC { wc_flat = flats, wc_impl = implics })
= filterBag is_floatable flats `unionBags`
do_bag (float_implic new_trapping_tvs) implics
where
new_trapping_tvs = fixVarSet grow trapping_tvs
is_floatable ct = tyVarsOfCt ct `disjointVarSet` new_trapping_tvs
grow tvs = foldrBag grow_one tvs flats
grow_one ct tvs | ct_tvs `intersectsVarSet` tvs = tvs `unionVarSet` ct_tvs
| otherwise = tvs
where
ct_tvs = tyVarsOfCt ct
float_implic :: TcTyVarSet -> Implication -> Cts
float_implic trapping_tvs imp
| hasEqualities (ic_given imp)
= emptyCts
| otherwise
= float_wc new_trapping_tvs (ic_wanted imp)
where
new_trapping_tvs = trapping_tvs `extendVarSetList` ic_skols imp
`extendVarSetList` ic_fsks imp
do_bag :: (a -> Bag c) -> Bag a -> Bag c
do_bag f = foldrBag (unionBags.f) emptyBag
\end{code}
Note [ApproximateWC]
~~~~~~~~~~~~~~~~~~~~
approximateWC takes a constraint, typically arising from the RHS of a
let-binding whose type we are *inferring*, and extracts from it some
*flat* constraints that we might plausibly abstract over. Of course
the top-level flat constraints are plausible, but we also float constraints
out from inside, if they are not captured by skolems.
The same function is used when doing type-class defaulting (see the call
to applyDefaultingRules) to extract constraints that that might be defaulted.
There are two caveats:
1. We do *not* float anything out if the implication binds equality
constraints, because that defeats the OutsideIn story. Consider
data T a where
TInt :: T Int
MkT :: T a
f TInt = 3::Int
We get the implication (a ~ Int => res ~ Int), where so far we've decided
f :: T a -> res
We don't want to float (res~Int) out because then we'll infer
f :: T a -> Int
which is only on of the possible types. (GHC 7.6 accidentally *did*
float out of such implications, which meant it would happily infer
non-principal types.)
2. We do not float out an inner constraint that shares a type variable
(transitively) with one that is trapped by a skolem. Eg
forall a. F a ~ beta, Integral beta
We don't want to float out (Integral beta). Doing so would be bad
when defaulting, because then we'll default beta:=Integer, and that
makes the error message much worse; we'd get
Can't solve F a ~ Integer
rather than
Can't solve Integral (F a)
Moreover, floating out these "contaminated" constraints doesn't help
when generalising either. If we generalise over (Integral b), we still
can't solve the retained implication (forall a. F a ~ b). Indeed,
arguably that too would be a harder error to understand.
Note [DefaultTyVar]
~~~~~~~~~~~~~~~~~~~
defaultTyVar is used on any un-instantiated meta type variables to
default the kind of OpenKind and ArgKind etc to *. This is important
to ensure that instance declarations match. For example consider
instance Show (a->b)
foo x = show (\_ -> True)
Then we'll get a constraint (Show (p ->q)) where p has kind ArgKind,
and that won't match the typeKind (*) in the instance decl. See tests
tc217 and tc175.
We look only at touchable type variables. No further constraints
are going to affect these type variables, so it's time to do it by
hand. However we aren't ready to default them fully to () or
whatever, because the type-class defaulting rules have yet to run.
An important point is that if the type variable tv has kind k and the
default is default_k we do not simply generate [D] (k ~ default_k) because:
(1) k may be ArgKind and default_k may be * so we will fail
(2) We need to rewrite all occurrences of the tv to be a type
variable with the right kind and we choose to do this by rewriting
the type variable /itself/ by a new variable which does have the
right kind.
Note [Promote _and_ default when inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we are inferring a type, we simplify the constraint, and then use
approximateWC to produce a list of candidate constraints. Then we MUST
a) Promote any meta-tyvars that have been floated out by
approximateWC, to restore invariant (MetaTvInv) described in
Note [Untouchable type variables] in TcType.
b) Default the kind of any meta-tyyvars that are not mentioned in
in the environment.
To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we
have an instance (C ((x:*) -> Int)). The instance doesn't match -- but it
should! If we don't solve the constraint, we'll stupidly quantify over
(C (a->Int)) and, worse, in doing so zonkQuantifiedTyVar will quantify over
(b:*) instead of (a:OpenKind), which can lead to disaster; see Trac #7332.
Trac #7641 is a simpler example.
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For ordinary pattern matches (including existentials) we float
equalities out of implications, for instance:
data T where
MkT :: Eq a => a -> T
f x y = case x of MkT _ -> (y::Int)
We get the implication constraint (x::T) (y::alpha):
forall a. [untouchable=alpha] Eq a => alpha ~ Int
We want to float out the equality into a scope where alpha is no
longer untouchable, to solve the implication!
But we cannot float equalities out of implications whose givens may
yield or contain equalities:
data T a where
T1 :: T Int
T2 :: T Bool
T3 :: T a
h :: T a -> a -> Int
f x y = case x of
T1 -> y::Int
T2 -> y::Bool
T3 -> h x y
We generate constraint, for (x::T alpha) and (y :: beta):
[untouchables = beta] (alpha ~ Int => beta ~ Int) -- From 1st branch
[untouchables = beta] (alpha ~ Bool => beta ~ Bool) -- From 2nd branch
(alpha ~ beta) -- From 3rd branch
If we float the equality (beta ~ Int) outside of the first implication and
the equality (beta ~ Bool) out of the second we get an insoluble constraint.
But if we just leave them inside the implications we unify alpha := beta and
solve everything.
Principle:
We do not want to float equalities out which may need the given *evidence*
to become soluble.
Consequence: classes with functional dependencies don't matter (since there is
no evidence for a fundep equality), but equality superclasses do matter (since
they carry evidence).
Note [Float equalities from under a skolem binding]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You might worry about skolem escape with all this floating.
For example, consider
[2] forall a. (a ~ F beta[2] delta,
Maybe beta[2] ~ gamma[1])
The (Maybe beta ~ gamma) doesn't mention 'a', so we float it, and
solve with gamma := beta. But what if later delta:=Int, and
F b Int = b.
Then we'd get a ~ beta[2], and solve to get beta:=a, and now the
skolem has escaped!
But it's ok: when we float (Maybe beta[2] ~ gamma[1]), we promote beta[2]
to beta[1], and that means the (a ~ beta[1]) will be stuck, as it should be.
Previously we tried to "grow" the skol_set with the constraints, to get
all the tyvars that could *conceivably* unify with the skolems, but that
was far too conservative (Trac #7804). Example: this should be fine:
f :: (forall a. a -> Proxy x -> Proxy (F x)) -> Int
f = error "Urk" :: (forall a. a -> Proxy x -> Proxy (F x)) -> Int
BUT (sigh) we have to be careful. Here are some edge cases:
a) [2]forall a. (F a delta[1] ~ beta[2], delta[1] ~ Maybe beta[2])
b) [2]forall a. (F b ty ~ beta[2], G beta[2] ~ gamma[2])
c) [2]forall a. (F a ty ~ beta[2], delta[1] ~ Maybe beta[2])
In (a) we *must* float out the second equality,
else we can't solve at all (Trac #7804).
In (b) we *must not* float out the second equality.
It will ultimately be solved (by flattening) in situ, but if we
float it we'll promote beta,gamma, and render the first equality insoluble.
In (c) it would be OK to float the second equality but better not to.
If we flatten we see (delta[1] ~ Maybe (F a ty)), which is a
skolem-escape problem. If we float the secodn equality we'll
end up with (F a ty ~ beta'[1]), which is a less explicable error.
Hence we start with the skolems, grow them by the CFunEqCans, and
float ones that don't mention the grown variables. Seems very ad hoc.
Note [Promoting unification variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we float an equality out of an implication we must "promote" free
unification variables of the equality, in order to maintain Invariant
(MetaTvInv) from Note [Untouchable type variables] in TcType. for the
leftover implication.
This is absolutely necessary. Consider the following example. We start
with two implications and a class with a functional dependency.
class C x y | x -> y
instance C [a] [a]
(I1) [untch=beta]forall b. 0 => F Int ~ [beta]
(I2) [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c]
We float (F Int ~ [beta]) out of I1, and we float (F Int ~ [[alpha]]) out of I2.
They may react to yield that (beta := [alpha]) which can then be pushed inwards
the leftover of I2 to get (C [alpha] [a]) which, using the FunDep, will mean that
(alpha := a). In the end we will have the skolem 'b' escaping in the untouchable
beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs:
class C x y | x -> y where
op :: x -> y -> ()
instance C [a] [a]
type family F a :: *
h :: F Int -> ()
h = undefined
data TEx where
TEx :: a -> TEx
f (x::beta) =
let g1 :: forall b. b -> ()
g1 _ = h [x]
g2 z = case z of TEx y -> (h [[undefined]], op x [y])
in (g1 '3', g2 undefined)
Note [Solving Family Equations]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
After we are done with simplification we may be left with constraints of the form:
[Wanted] F xis ~ beta
If 'beta' is a touchable unification variable not already bound in the TyBinds
then we'd like to create a binding for it, effectively "defaulting" it to be 'F xis'.
When is it ok to do so?
1) 'beta' must not already be defaulted to something. Example:
[Wanted] F Int ~ beta <~ Will default [beta := F Int]
[Wanted] F Char ~ beta <~ Already defaulted, can't default again. We
have to report this as unsolved.
2) However, we must still do an occurs check when defaulting (F xis ~ beta), to
set [beta := F xis] only if beta is not among the free variables of xis.
3) Notice that 'beta' can't be bound in ty binds already because we rewrite RHS
of type family equations. See Inert Set invariants in TcInteract.
This solving is now happening during zonking, see Note [Unflattening while zonking]
in TcMType.
*********************************************************************************
* *
* Defaulting and disamgiguation *
* *
*********************************************************************************
\begin{code}
applyDefaultingRules :: Cts -> TcS Bool
applyDefaultingRules wanteds
| isEmptyBag wanteds
= return False
| otherwise
= do { traceTcS "applyDefaultingRules { " $
text "wanteds =" <+> ppr wanteds
; info@(default_tys, _) <- getDefaultInfo
; let groups = findDefaultableGroups info wanteds
; traceTcS "findDefaultableGroups" $ vcat [ text "groups=" <+> ppr groups
, text "info=" <+> ppr info ]
; something_happeneds <- mapM (disambigGroup default_tys) groups
; traceTcS "applyDefaultingRules }" (ppr something_happeneds)
; return (or something_happeneds) }
\end{code}
\begin{code}
findDefaultableGroups
:: ( [Type]
, (Bool,Bool) )
-> Cts
-> [[(Ct,Class,TcTyVar)]]
findDefaultableGroups (default_tys, (ovl_strings, extended_defaults)) wanteds
| null default_tys = []
| otherwise = filter is_defaultable_group (equivClasses cmp_tv unaries)
where
unaries :: [(Ct, Class, TcTyVar)]
non_unaries :: [Ct]
(unaries, non_unaries) = partitionWith find_unary (bagToList wanteds)
find_unary cc
| Just (cls,[ty]) <- getClassPredTys_maybe (ctPred cc)
, Just tv <- tcGetTyVar_maybe ty
, isMetaTyVar tv
= Left (cc, cls, tv)
find_unary cc = Right cc
bad_tvs :: TcTyVarSet
bad_tvs = foldr (unionVarSet . tyVarsOfCt) emptyVarSet non_unaries
cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
is_defaultable_group ds@((_,_,tv):_)
= let b1 = isTyConableTyVar tv
b2 = not (tv `elemVarSet` bad_tvs)
b4 = defaultable_classes [cls | (_,cls,_) <- ds]
in (b1 && b2 && b4)
is_defaultable_group [] = panic "defaultable_group"
defaultable_classes clss
| extended_defaults = any isInteractiveClass clss
| otherwise = all is_std_class clss && (any is_num_class clss)
isInteractiveClass cls
= is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
disambigGroup :: [Type]
-> [(Ct, Class, TcTyVar)]
-> TcS Bool
disambigGroup [] _grp
= return False
disambigGroup (default_ty:default_tys) group
= do { traceTcS "disambigGroup {" (ppr group $$ ppr default_ty)
; success <- tryTcS $
do { setWantedTyBind the_tv default_ty
; implics_from_defaulting <- solveInteract wanteds
; MASSERT(isEmptyBag implics_from_defaulting)
; checkAllSolved }
; if success then
do { setWantedTyBind the_tv default_ty
; wrapWarnTcS $ warnDefaulting wanteds default_ty
; traceTcS "disambigGroup succeeded }" (ppr default_ty)
; return True }
else
do { traceTcS "disambigGroup failed, will try other default types }"
(ppr default_ty)
; disambigGroup default_tys group } }
where
((_,_,the_tv):_) = group
wanteds = listToBag (map fstOf3 group)
\end{code}
Note [Avoiding spurious errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When doing the unification for defaulting, we check for skolem
type variables, and simply don't default them. For example:
f = (*) -- Monomorphic
g :: Num a => a -> a
g x = f x x
Here, we get a complaint when checking the type signature for g,
that g isn't polymorphic enough; but then we get another one when
dealing with the (Num a) context arising from f's definition;
we try to unify a with Int (to default it), but find that it's
already been unified with the rigid variable from g's type sig