{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1998 \section[TyCoRep]{Type and Coercion - friends' interface} Note [The Type-related module hierarchy] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Class CoAxiom TyCon imports Class, CoAxiom TyCoRep imports Class, CoAxiom, TyCon TyCoPpr imports TyCoRep TyCoFVs imports TyCoRep TyCoSubst imports TyCoRep, TyCoFVs, TyCoPpr TyCoTidy imports TyCoRep, TyCoFVs TysPrim imports TyCoRep ( including mkTyConTy ) Coercion imports Type -} -- We expose the relevant stuff from this module via the Type module {-# OPTIONS_HADDOCK not-home #-} {-# LANGUAGE CPP, DeriveDataTypeable, MultiWayIf, PatternSynonyms, BangPatterns #-} module TyCoRep ( TyThing(..), tyThingCategory, pprTyThingCategory, pprShortTyThing, -- * Types Type( TyVarTy, AppTy, TyConApp, ForAllTy , LitTy, CastTy, CoercionTy , FunTy, ft_arg, ft_res, ft_af ), -- Export the type synonym FunTy too TyLit(..), KindOrType, Kind, KnotTied, PredType, ThetaType, -- Synonyms ArgFlag(..), AnonArgFlag(..), ForallVisFlag(..), -- * Coercions Coercion(..), UnivCoProvenance(..), CoercionHole(..), coHoleCoVar, setCoHoleCoVar, CoercionN, CoercionR, CoercionP, KindCoercion, MCoercion(..), MCoercionR, MCoercionN, -- * Functions over types mkTyConTy, mkTyVarTy, mkTyVarTys, mkTyCoVarTy, mkTyCoVarTys, mkFunTy, mkVisFunTy, mkInvisFunTy, mkVisFunTys, mkInvisFunTys, mkForAllTy, mkForAllTys, mkPiTy, mkPiTys, -- * Functions over binders TyCoBinder(..), TyCoVarBinder, TyBinder, binderVar, binderVars, binderType, binderArgFlag, delBinderVar, isInvisibleArgFlag, isVisibleArgFlag, isInvisibleBinder, isVisibleBinder, isTyBinder, isNamedBinder, -- * Functions over coercions pickLR, -- * Sizes typeSize, coercionSize, provSize ) where #include "HsVersions.h" import GhcPrelude import {-# SOURCE #-} TyCoPpr ( pprType, pprCo, pprTyLit ) -- Transitively pulls in a LOT of stuff, better to break the loop import {-# SOURCE #-} ConLike ( ConLike(..), conLikeName ) -- friends: import IfaceType import Var import VarSet import Name hiding ( varName ) import TyCon import CoAxiom -- others import BasicTypes ( LeftOrRight(..), pickLR ) import Outputable import FastString import Util -- libraries import qualified Data.Data as Data hiding ( TyCon ) import Data.IORef ( IORef ) -- for CoercionHole {- %************************************************************************ %* * TyThing %* * %************************************************************************ Despite the fact that DataCon has to be imported via a hi-boot route, this module seems the right place for TyThing, because it's needed for funTyCon and all the types in TysPrim. It is also SOURCE-imported into Name.hs Note [ATyCon for classes] ~~~~~~~~~~~~~~~~~~~~~~~~~ Both classes and type constructors are represented in the type environment as ATyCon. You can tell the difference, and get to the class, with isClassTyCon :: TyCon -> Bool tyConClass_maybe :: TyCon -> Maybe Class The Class and its associated TyCon have the same Name. -} -- | A global typecheckable-thing, essentially anything that has a name. -- Not to be confused with a 'TcTyThing', which is also a typecheckable -- thing but in the *local* context. See 'TcEnv' for how to retrieve -- a 'TyThing' given a 'Name'. data TyThing = AnId Id | AConLike ConLike | ATyCon TyCon -- TyCons and classes; see Note [ATyCon for classes] | ACoAxiom (CoAxiom Branched) instance Outputable TyThing where ppr = pprShortTyThing instance NamedThing TyThing where -- Can't put this with the type getName (AnId id) = getName id -- decl, because the DataCon instance getName (ATyCon tc) = getName tc -- isn't visible there getName (ACoAxiom cc) = getName cc getName (AConLike cl) = conLikeName cl pprShortTyThing :: TyThing -> SDoc -- c.f. PprTyThing.pprTyThing, which prints all the details pprShortTyThing thing = pprTyThingCategory thing <+> quotes (ppr (getName thing)) pprTyThingCategory :: TyThing -> SDoc pprTyThingCategory = text . capitalise . tyThingCategory tyThingCategory :: TyThing -> String tyThingCategory (ATyCon tc) | isClassTyCon tc = "class" | otherwise = "type constructor" tyThingCategory (ACoAxiom _) = "coercion axiom" tyThingCategory (AnId _) = "identifier" tyThingCategory (AConLike (RealDataCon _)) = "data constructor" tyThingCategory (AConLike (PatSynCon _)) = "pattern synonym" {- ********************************************************************** * * Type * * ********************************************************************** -} -- | The key representation of types within the compiler type KindOrType = Type -- See Note [Arguments to type constructors] -- | The key type representing kinds in the compiler. type Kind = Type -- If you edit this type, you may need to update the GHC formalism -- See Note [GHC Formalism] in coreSyn/CoreLint.hs data Type -- See Note [Non-trivial definitional equality] = TyVarTy Var -- ^ Vanilla type or kind variable (*never* a coercion variable) | AppTy Type Type -- ^ Type application to something other than a 'TyCon'. Parameters: -- -- 1) Function: must /not/ be a 'TyConApp' or 'CastTy', -- must be another 'AppTy', or 'TyVarTy' -- See Note [Respecting definitional equality] (EQ1) about the -- no 'CastTy' requirement -- -- 2) Argument type | TyConApp TyCon [KindOrType] -- ^ Application of a 'TyCon', including newtypes /and/ synonyms. -- Invariant: saturated applications of 'FunTyCon' must -- use 'FunTy' and saturated synonyms must use their own -- constructors. However, /unsaturated/ 'FunTyCon's -- do appear as 'TyConApp's. -- Parameters: -- -- 1) Type constructor being applied to. -- -- 2) Type arguments. Might not have enough type arguments -- here to saturate the constructor. -- Even type synonyms are not necessarily saturated; -- for example unsaturated type synonyms -- can appear as the right hand side of a type synonym. | ForAllTy {-# UNPACK #-} !TyCoVarBinder Type -- ^ A Π type. | FunTy -- ^ t1 -> t2 Very common, so an important special case -- See Note [Function types] { ft_af :: AnonArgFlag -- Is this (->) or (=>)? , ft_arg :: Type -- Argument type , ft_res :: Type } -- Result type | LitTy TyLit -- ^ Type literals are similar to type constructors. | CastTy Type KindCoercion -- ^ A kind cast. The coercion is always nominal. -- INVARIANT: The cast is never refl. -- INVARIANT: The Type is not a CastTy (use TransCo instead) -- See Note [Respecting definitional equality] (EQ2) and (EQ3) | CoercionTy Coercion -- ^ Injection of a Coercion into a type -- This should only ever be used in the RHS of an AppTy, -- in the list of a TyConApp, when applying a promoted -- GADT data constructor deriving Data.Data instance Outputable Type where ppr = pprType -- NOTE: Other parts of the code assume that type literals do not contain -- types or type variables. data TyLit = NumTyLit Integer | StrTyLit FastString deriving (Eq, Ord, Data.Data) instance Outputable TyLit where ppr = pprTyLit {- Note [Function types] ~~~~~~~~~~~~~~~~~~~~~~~~ FFunTy is the constructor for a function type. Lots of things to say about it! * FFunTy is the data constructor, meaning "full function type". * The function type constructor (->) has kind (->) :: forall r1 r2. TYPE r1 -> TYPE r2 -> Type LiftedRep mkTyConApp ensure that we convert a saturated application TyConApp (->) [r1,r2,t1,t2] into FunTy t1 t2 dropping the 'r1' and 'r2' arguments; they are easily recovered from 't1' and 't2'. * The ft_af field says whether or not this is an invisible argument VisArg: t1 -> t2 Ordinary function type InvisArg: t1 => t2 t1 is guaranteed to be a predicate type, i.e. t1 :: Constraint See Note [Types for coercions, predicates, and evidence] This visibility info makes no difference in Core; it matters only when we regard the type as a Haskell source type. * FunTy is a (unidirectional) pattern synonym that allows positional pattern matching (FunTy arg res), ignoring the ArgFlag. -} {- ----------------------- Commented out until the pattern match checker can handle it; see #16185 For now we use the CPP macro #define FunTy FFunTy _ (see HsVersions.h) to allow pattern matching on a (positional) FunTy constructor. {-# COMPLETE FunTy, TyVarTy, AppTy, TyConApp , ForAllTy, LitTy, CastTy, CoercionTy :: Type #-} -- | 'FunTy' is a (uni-directional) pattern synonym for the common -- case where we want to match on the argument/result type, but -- ignoring the AnonArgFlag pattern FunTy :: Type -> Type -> Type pattern FunTy arg res <- FFunTy { ft_arg = arg, ft_res = res } End of commented out block ---------------------------------- -} {- Note [Types for coercions, predicates, and evidence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We treat differently: (a) Predicate types Test: isPredTy Binders: DictIds Kind: Constraint Examples: (Eq a), and (a ~ b) (b) Coercion types are primitive, unboxed equalities Test: isCoVarTy Binders: CoVars (can appear in coercions) Kind: TYPE (TupleRep []) Examples: (t1 ~# t2) or (t1 ~R# t2) (c) Evidence types is the type of evidence manipulated by the type constraint solver. Test: isEvVarType Binders: EvVars Kind: Constraint or TYPE (TupleRep []) Examples: all coercion types and predicate types Coercion types and predicate types are mutually exclusive, but evidence types are a superset of both. When treated as a user type, - Predicates (of kind Constraint) are invisible and are implicitly instantiated - Coercion types, and non-pred evidence types (i.e. not of kind Constrain), are just regular old types, are visible, and are not implicitly instantiated. In a FunTy { ft_af = InvisArg }, the argument type is always a Predicate type. Note [Constraints in kinds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do we allow a type constructor to have a kind like S :: Eq a => a -> Type No, we do not. Doing so would mean would need a TyConApp like S @k @(d :: Eq k) (ty :: k) and we have no way to build, or decompose, evidence like (d :: Eq k) at the type level. But we admit one exception: equality. We /do/ allow, say, MkT :: (a ~ b) => a -> b -> Type a b Why? Because we can, without much difficulty. Moreover we can promote a GADT data constructor (see TyCon Note [Promoted data constructors]), like data GT a b where MkGT : a -> a -> GT a a so programmers might reasonably expect to be able to promote MkT as well. How does this work? * In TcValidity.checkConstraintsOK we reject kinds that have constraints other than (a~b) and (a~~b). * In Inst.tcInstInvisibleTyBinder we instantiate a call of MkT by emitting [W] co :: alpha ~# beta and producing the elaborated term MkT @alpha @beta (Eq# alpha beta co) We don't generate a boxed "Wanted"; we generate only a regular old /unboxed/ primitive-equality Wanted, and build the box on the spot. * How can we get such a MkT? By promoting a GADT-style data constructor data T a b where MkT :: (a~b) => a -> b -> T a b See DataCon.mkPromotedDataCon and Note [Promoted data constructors] in TyCon * We support both homogeneous (~) and heterogeneous (~~) equality. (See Note [The equality types story] in TysPrim for a primer on these equality types.) * How do we prevent a MkT having an illegal constraint like Eq a? We check for this at use-sites; see TcHsType.tcTyVar, specifically dc_theta_illegal_constraint. * Notice that nothing special happens if K :: (a ~# b) => blah because (a ~# b) is not a predicate type, and is never implicitly instantiated. (Mind you, it's not clear how you could creates a type constructor with such a kind.) See Note [Types for coercions, predicates, and evidence] * The existence of promoted MkT with an equality-constraint argument is the (only) reason that the AnonTCB constructor of TyConBndrVis carries an AnonArgFlag (VisArg/InvisArg). For example, when we promote the data constructor MkT :: forall a b. (a~b) => a -> b -> T a b we get a PromotedDataCon with tyConBinders Bndr (a :: Type) (NamedTCB Inferred) Bndr (b :: Type) (NamedTCB Inferred) Bndr (_ :: a ~ b) (AnonTCB InvisArg) Bndr (_ :: a) (AnonTCB VisArg)) Bndr (_ :: b) (AnonTCB VisArg)) * One might reasonably wonder who *unpacks* these boxes once they are made. After all, there is no type-level `case` construct. The surprising answer is that no one ever does. Instead, if a GADT constructor is used on the left-hand side of a type family equation, that occurrence forces GHC to unify the types in question. For example: data G a where MkG :: G Bool type family F (x :: G a) :: a where F MkG = False When checking the LHS `F MkG`, GHC sees the MkG constructor and then must unify F's implicit parameter `a` with Bool. This succeeds, making the equation F Bool (MkG @Bool <Bool>) = False Note that we never need unpack the coercion. This is because type family equations are *not* parametric in their kind variables. That is, we could have just said type family H (x :: G a) :: a where H _ = False The presence of False on the RHS also forces `a` to become Bool, giving us H Bool _ = False The fact that any of this works stems from the lack of phase separation between types and kinds (unlike the very present phase separation between terms and types). Once we have the ability to pattern-match on types below top-level, this will no longer cut it, but it seems fine for now. Note [Arguments to type constructors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Because of kind polymorphism, in addition to type application we now have kind instantiation. We reuse the same notations to do so. For example: Just (* -> *) Maybe Right * Nat Zero are represented by: TyConApp (PromotedDataCon Just) [* -> *, Maybe] TyConApp (PromotedDataCon Right) [*, Nat, (PromotedDataCon Zero)] Important note: Nat is used as a *kind* and not as a type. This can be confusing, since type-level Nat and kind-level Nat are identical. We use the kind of (PromotedDataCon Right) to know if its arguments are kinds or types. This kind instantiation only happens in TyConApp currently. Note [Non-trivial definitional equality] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Is Int |> <*> the same as Int? YES! In order to reduce headaches, we decide that any reflexive casts in types are just ignored. (Indeed they must be. See Note [Respecting definitional equality].) More generally, the `eqType` function, which defines Core's type equality relation, ignores casts and coercion arguments, as long as the two types have the same kind. This allows us to be a little sloppier in keeping track of coercions, which is a good thing. It also means that eqType does not depend on eqCoercion, which is also a good thing. Why is this sensible? That is, why is something different than α-equivalence appropriate for the implementation of eqType? Anything smaller than ~ and homogeneous is an appropriate definition for equality. The type safety of FC depends only on ~. Let's say η : τ ~ σ. Any expression of type τ can be transmuted to one of type σ at any point by casting. The same is true of expressions of type σ. So in some sense, τ and σ are interchangeable. But let's be more precise. If we examine the typing rules of FC (say, those in https://cs.brynmawr.edu/~rae/papers/2015/equalities/equalities.pdf) there are several places where the same metavariable is used in two different premises to a rule. (For example, see Ty_App.) There is an implicit equality check here. What definition of equality should we use? By convention, we use α-equivalence. Take any rule with one (or more) of these implicit equality checks. Then there is an admissible rule that uses ~ instead of the implicit check, adding in casts as appropriate. The only problem here is that ~ is heterogeneous. To make the kinds work out in the admissible rule that uses ~, it is necessary to homogenize the coercions. That is, if we have η : (τ : κ1) ~ (σ : κ2), then we don't use η; we use η |> kind η, which is homogeneous. The effect of this all is that eqType, the implementation of the implicit equality check, can use any homogeneous relation that is smaller than ~, as those rules must also be admissible. A more drawn out argument around all of this is presented in Section 7.2 of Richard E's thesis (http://cs.brynmawr.edu/~rae/papers/2016/thesis/eisenberg-thesis.pdf). What would go wrong if we insisted on the casts matching? See the beginning of Section 8 in the unpublished paper above. Theoretically, nothing at all goes wrong. But in practical terms, getting the coercions right proved to be nightmarish. And types would explode: during kind-checking, we often produce reflexive kind coercions. When we try to cast by these, mkCastTy just discards them. But if we used an eqType that distinguished between Int and Int |> <*>, then we couldn't discard -- the output of kind-checking would be enormous, and we would need enormous casts with lots of CoherenceCo's to straighten them out. Would anything go wrong if eqType respected type families? No, not at all. But that makes eqType rather hard to implement. Thus, the guideline for eqType is that it should be the largest easy-to-implement relation that is still smaller than ~ and homogeneous. The precise choice of relation is somewhat incidental, as long as the smart constructors and destructors in Type respect whatever relation is chosen. Another helpful principle with eqType is this: (EQ) If (t1 `eqType` t2) then I can replace t1 by t2 anywhere. This principle also tells us that eqType must relate only types with the same kinds. Note [Respecting definitional equality] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Note [Non-trivial definitional equality] introduces the property (EQ). How is this upheld? Any function that pattern matches on all the constructors will have to consider the possibility of CastTy. Presumably, those functions will handle CastTy appropriately and we'll be OK. More dangerous are the splitXXX functions. Let's focus on splitTyConApp. We don't want it to fail on (T a b c |> co). Happily, if we have (T a b c |> co) `eqType` (T d e f) then co must be reflexive. Why? eqType checks that the kinds are equal, as well as checking that (a `eqType` d), (b `eqType` e), and (c `eqType` f). By the kind check, we know that (T a b c |> co) and (T d e f) have the same kind. So the only way that co could be non-reflexive is for (T a b c) to have a different kind than (T d e f). But because T's kind is closed (all tycon kinds are closed), the only way for this to happen is that one of the arguments has to differ, leading to a contradiction. Thus, co is reflexive. Accordingly, by eliminating reflexive casts, splitTyConApp need not worry about outermost casts to uphold (EQ). Eliminating reflexive casts is done in mkCastTy. Unforunately, that's not the end of the story. Consider comparing (T a b c) =? (T a b |> (co -> <Type>)) (c |> co) These two types have the same kind (Type), but the left type is a TyConApp while the right type is not. To handle this case, we say that the right-hand type is ill-formed, requiring an AppTy never to have a casted TyConApp on its left. It is easy enough to pull around the coercions to maintain this invariant, as done in Type.mkAppTy. In the example above, trying to form the right-hand type will instead yield (T a b (c |> co |> sym co) |> <Type>). Both the casts there are reflexive and will be dropped. Huzzah. This idea of pulling coercions to the right works for splitAppTy as well. However, there is one hiccup: it's possible that a coercion doesn't relate two Pi-types. For example, if we have @type family Fun a b where Fun a b = a -> b@, then we might have (T :: Fun Type Type) and (T |> axFun) Int. That axFun can't be pulled to the right. But we don't need to pull it: (T |> axFun) Int is not `eqType` to any proper TyConApp -- thus, leaving it where it is doesn't violate our (EQ) property. Lastly, in order to detect reflexive casts reliably, we must make sure not to have nested casts: we update (t |> co1 |> co2) to (t |> (co1 `TransCo` co2)). In sum, in order to uphold (EQ), we need the following three invariants: (EQ1) No decomposable CastTy to the left of an AppTy, where a decomposable cast is one that relates either a FunTy to a FunTy or a ForAllTy to a ForAllTy. (EQ2) No reflexive casts in CastTy. (EQ3) No nested CastTys. (EQ4) No CastTy over (ForAllTy (Bndr tyvar vis) body). See Note [Weird typing rule for ForAllTy] in Type. These invariants are all documented above, in the declaration for Type. Note [Unused coercion variable in ForAllTy] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have \(co:t1 ~ t2). e What type should we give to this expression? (1) forall (co:t1 ~ t2) -> t (2) (t1 ~ t2) -> t If co is used in t, (1) should be the right choice. if co is not used in t, we would like to have (1) and (2) equivalent. However, we want to keep eqType simple and don't want eqType (1) (2) to return True in any case. We decide to always construct (2) if co is not used in t. Thus in mkLamType, we check whether the variable is a coercion variable (of type (t1 ~# t2), and whether it is un-used in the body. If so, it returns a FunTy instead of a ForAllTy. There are cases we want to skip the check. For example, the check is unnecessary when it is known from the context that the input variable is a type variable. In those cases, we use mkForAllTy. -} -- | A type labeled 'KnotTied' might have knot-tied tycons in it. See -- Note [Type checking recursive type and class declarations] in -- TcTyClsDecls type KnotTied ty = ty {- ********************************************************************** * * TyCoBinder and ArgFlag * * ********************************************************************** -} -- | A 'TyCoBinder' represents an argument to a function. TyCoBinders can be -- dependent ('Named') or nondependent ('Anon'). They may also be visible or -- not. See Note [TyCoBinders] data TyCoBinder = Named TyCoVarBinder -- A type-lambda binder | Anon AnonArgFlag Type -- A term-lambda binder. Type here can be CoercionTy. -- Visibility is determined by the AnonArgFlag deriving Data.Data instance Outputable TyCoBinder where ppr (Anon af ty) = ppr af <+> ppr ty ppr (Named (Bndr v Required)) = ppr v ppr (Named (Bndr v Specified)) = char '@' <> ppr v ppr (Named (Bndr v Inferred)) = braces (ppr v) -- | 'TyBinder' is like 'TyCoBinder', but there can only be 'TyVarBinder' -- in the 'Named' field. type TyBinder = TyCoBinder -- | Remove the binder's variable from the set, if the binder has -- a variable. delBinderVar :: VarSet -> TyCoVarBinder -> VarSet delBinderVar vars (Bndr tv _) = vars `delVarSet` tv -- | Does this binder bind an invisible argument? isInvisibleBinder :: TyCoBinder -> Bool isInvisibleBinder (Named (Bndr _ vis)) = isInvisibleArgFlag vis isInvisibleBinder (Anon InvisArg _) = True isInvisibleBinder (Anon VisArg _) = False -- | Does this binder bind a visible argument? isVisibleBinder :: TyCoBinder -> Bool isVisibleBinder = not . isInvisibleBinder isNamedBinder :: TyCoBinder -> Bool isNamedBinder (Named {}) = True isNamedBinder (Anon {}) = False -- | If its a named binder, is the binder a tyvar? -- Returns True for nondependent binder. -- This check that we're really returning a *Ty*Binder (as opposed to a -- coercion binder). That way, if/when we allow coercion quantification -- in more places, we'll know we missed updating some function. isTyBinder :: TyCoBinder -> Bool isTyBinder (Named bnd) = isTyVarBinder bnd isTyBinder _ = True {- Note [TyCoBinders] ~~~~~~~~~~~~~~~~~~~ A ForAllTy contains a TyCoVarBinder. But a type can be decomposed to a telescope consisting of a [TyCoBinder] A TyCoBinder represents the type of binders -- that is, the type of an argument to a Pi-type. GHC Core currently supports two different Pi-types: * A non-dependent function type, written with ->, e.g. ty1 -> ty2 represented as FunTy ty1 ty2. These are lifted to Coercions with the corresponding FunCo. * A dependent compile-time-only polytype, written with forall, e.g. forall (a:*). ty represented as ForAllTy (Bndr a v) ty Both Pi-types classify terms/types that take an argument. In other words, if `x` is either a function or a polytype, `x arg` makes sense (for an appropriate `arg`). Note [VarBndrs, TyCoVarBinders, TyConBinders, and visibility] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * A ForAllTy (used for both types and kinds) contains a TyCoVarBinder. Each TyCoVarBinder Bndr a tvis is equipped with tvis::ArgFlag, which says whether or not arguments for this binder should be visible (explicit) in source Haskell. * A TyCon contains a list of TyConBinders. Each TyConBinder Bndr a cvis is equipped with cvis::TyConBndrVis, which says whether or not type and kind arguments for this TyCon should be visible (explicit) in source Haskell. This table summarises the visibility rules: --------------------------------------------------------------------------------------- | Occurrences look like this | GHC displays type as in Haskell source code |-------------------------------------------------------------------------------------- | Bndr a tvis :: TyCoVarBinder, in the binder of ForAllTy for a term | tvis :: ArgFlag | tvis = Inferred: f :: forall {a}. type Arg not allowed: f f :: forall {co}. type Arg not allowed: f | tvis = Specified: f :: forall a. type Arg optional: f or f @Int | tvis = Required: T :: forall k -> type Arg required: T * | This last form is illegal in terms: See Note [No Required TyCoBinder in terms] | | Bndr k cvis :: TyConBinder, in the TyConBinders of a TyCon | cvis :: TyConBndrVis | cvis = AnonTCB: T :: kind -> kind Required: T * | cvis = NamedTCB Inferred: T :: forall {k}. kind Arg not allowed: T | T :: forall {co}. kind Arg not allowed: T | cvis = NamedTCB Specified: T :: forall k. kind Arg not allowed[1]: T | cvis = NamedTCB Required: T :: forall k -> kind Required: T * --------------------------------------------------------------------------------------- [1] In types, in the Specified case, it would make sense to allow optional kind applications, thus (T @*), but we have not yet implemented that ---- In term declarations ---- * Inferred. Function defn, with no signature: f1 x = x We infer f1 :: forall {a}. a -> a, with 'a' Inferred It's Inferred because it doesn't appear in any user-written signature for f1 * Specified. Function defn, with signature (implicit forall): f2 :: a -> a; f2 x = x So f2 gets the type f2 :: forall a. a -> a, with 'a' Specified even though 'a' is not bound in the source code by an explicit forall * Specified. Function defn, with signature (explicit forall): f3 :: forall a. a -> a; f3 x = x So f3 gets the type f3 :: forall a. a -> a, with 'a' Specified * Inferred/Specified. Function signature with inferred kind polymorphism. f4 :: a b -> Int So 'f4' gets the type f4 :: forall {k} (a:k->*) (b:k). a b -> Int Here 'k' is Inferred (it's not mentioned in the type), but 'a' and 'b' are Specified. * Specified. Function signature with explicit kind polymorphism f5 :: a (b :: k) -> Int This time 'k' is Specified, because it is mentioned explicitly, so we get f5 :: forall (k:*) (a:k->*) (b:k). a b -> Int * Similarly pattern synonyms: Inferred - from inferred types (e.g. no pattern type signature) - or from inferred kind polymorphism ---- In type declarations ---- * Inferred (k) data T1 a b = MkT1 (a b) Here T1's kind is T1 :: forall {k:*}. (k->*) -> k -> * The kind variable 'k' is Inferred, since it is not mentioned Note that 'a' and 'b' correspond to /Anon/ TyCoBinders in T1's kind, and Anon binders don't have a visibility flag. (Or you could think of Anon having an implicit Required flag.) * Specified (k) data T2 (a::k->*) b = MkT (a b) Here T's kind is T :: forall (k:*). (k->*) -> k -> * The kind variable 'k' is Specified, since it is mentioned in the signature. * Required (k) data T k (a::k->*) b = MkT (a b) Here T's kind is T :: forall k:* -> (k->*) -> k -> * The kind is Required, since it bound in a positional way in T's declaration Every use of T must be explicitly applied to a kind * Inferred (k1), Specified (k) data T a b (c :: k) = MkT (a b) (Proxy c) Here T's kind is T :: forall {k1:*} (k:*). (k1->*) -> k1 -> k -> * So 'k' is Specified, because it appears explicitly, but 'k1' is Inferred, because it does not Generally, in the list of TyConBinders for a TyCon, * Inferred arguments always come first * Specified, Anon and Required can be mixed e.g. data Foo (a :: Type) :: forall b. (a -> b -> Type) -> Type where ... Here Foo's TyConBinders are [Required 'a', Specified 'b', Anon] and its kind prints as Foo :: forall a -> forall b. (a -> b -> Type) -> Type See also Note [Required, Specified, and Inferred for types] in TcTyClsDecls ---- Printing ----- We print forall types with enough syntax to tell you their visibility flag. But this is not source Haskell, and these types may not all be parsable. Specified: a list of Specified binders is written between `forall` and `.`: const :: forall a b. a -> b -> a Inferred: with -fprint-explicit-foralls, Inferred binders are written in braces: f :: forall {k} (a:k). S k a -> Int Otherwise, they are printed like Specified binders. Required: binders are put between `forall` and `->`: T :: forall k -> * ---- Other points ----- * In classic Haskell, all named binders (that is, the type variables in a polymorphic function type f :: forall a. a -> a) have been Inferred. * Inferred variables correspond to "generalized" variables from the Visible Type Applications paper (ESOP'16). Note [No Required TyCoBinder in terms] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We don't allow Required foralls for term variables, including pattern synonyms and data constructors. Why? Because then an application would need a /compulsory/ type argument (possibly without an "@"?), thus (f Int); and we don't have concrete syntax for that. We could change this decision, but Required, Named TyCoBinders are rare anyway. (Most are Anons.) However the type of a term can (just about) have a required quantifier; see Note [Required quantifiers in the type of a term] in TcExpr. -} {- ********************************************************************** * * PredType * * ********************************************************************** -} -- | A type of the form @p@ of constraint kind represents a value whose type is -- the Haskell predicate @p@, where a predicate is what occurs before -- the @=>@ in a Haskell type. -- -- We use 'PredType' as documentation to mark those types that we guarantee to -- have this kind. -- -- It can be expanded into its representation, but: -- -- * The type checker must treat it as opaque -- -- * The rest of the compiler treats it as transparent -- -- Consider these examples: -- -- > f :: (Eq a) => a -> Int -- > g :: (?x :: Int -> Int) => a -> Int -- > h :: (r\l) => {r} => {l::Int | r} -- -- Here the @Eq a@ and @?x :: Int -> Int@ and @r\l@ are all called \"predicates\" type PredType = Type -- | A collection of 'PredType's type ThetaType = [PredType] {- (We don't support TREX records yet, but the setup is designed to expand to allow them.) A Haskell qualified type, such as that for f,g,h above, is represented using * a FunTy for the double arrow * with a type of kind Constraint as the function argument The predicate really does turn into a real extra argument to the function. If the argument has type (p :: Constraint) then the predicate p is represented by evidence of type p. %************************************************************************ %* * Simple constructors %* * %************************************************************************ These functions are here so that they can be used by TysPrim, which in turn is imported by Type -} mkTyVarTy :: TyVar -> Type mkTyVarTy v = ASSERT2( isTyVar v, ppr v <+> dcolon <+> ppr (tyVarKind v) ) TyVarTy v mkTyVarTys :: [TyVar] -> [Type] mkTyVarTys = map mkTyVarTy -- a common use of mkTyVarTy mkTyCoVarTy :: TyCoVar -> Type mkTyCoVarTy v | isTyVar v = TyVarTy v | otherwise = CoercionTy (CoVarCo v) mkTyCoVarTys :: [TyCoVar] -> [Type] mkTyCoVarTys = map mkTyCoVarTy infixr 3 `mkFunTy`, `mkVisFunTy`, `mkInvisFunTy` -- Associates to the right mkFunTy :: AnonArgFlag -> Type -> Type -> Type mkFunTy af arg res = FunTy { ft_af = af, ft_arg = arg, ft_res = res } mkVisFunTy, mkInvisFunTy :: Type -> Type -> Type mkVisFunTy = mkFunTy VisArg mkInvisFunTy = mkFunTy InvisArg -- | Make nested arrow types mkVisFunTys, mkInvisFunTys :: [Type] -> Type -> Type mkVisFunTys tys ty = foldr mkVisFunTy ty tys mkInvisFunTys tys ty = foldr mkInvisFunTy ty tys -- | Like 'mkTyCoForAllTy', but does not check the occurrence of the binder -- See Note [Unused coercion variable in ForAllTy] mkForAllTy :: TyCoVar -> ArgFlag -> Type -> Type mkForAllTy tv vis ty = ForAllTy (Bndr tv vis) ty -- | Wraps foralls over the type using the provided 'TyCoVar's from left to right mkForAllTys :: [TyCoVarBinder] -> Type -> Type mkForAllTys tyvars ty = foldr ForAllTy ty tyvars mkPiTy:: TyCoBinder -> Type -> Type mkPiTy (Anon af ty1) ty2 = FunTy { ft_af = af, ft_arg = ty1, ft_res = ty2 } mkPiTy (Named (Bndr tv vis)) ty = mkForAllTy tv vis ty mkPiTys :: [TyCoBinder] -> Type -> Type mkPiTys tbs ty = foldr mkPiTy ty tbs -- | Create the plain type constructor type which has been applied to no type arguments at all. mkTyConTy :: TyCon -> Type mkTyConTy tycon = TyConApp tycon [] {- %************************************************************************ %* * Coercions %* * %************************************************************************ -} -- | A 'Coercion' is concrete evidence of the equality/convertibility -- of two types. -- If you edit this type, you may need to update the GHC formalism -- See Note [GHC Formalism] in coreSyn/CoreLint.hs data Coercion -- Each constructor has a "role signature", indicating the way roles are -- propagated through coercions. -- - P, N, and R stand for coercions of the given role -- - e stands for a coercion of a specific unknown role -- (think "role polymorphism") -- - "e" stands for an explicit role parameter indicating role e. -- - _ stands for a parameter that is not a Role or Coercion. -- These ones mirror the shape of types = -- Refl :: _ -> N Refl Type -- See Note [Refl invariant] -- Invariant: applications of (Refl T) to a bunch of identity coercions -- always show up as Refl. -- For example (Refl T) (Refl a) (Refl b) shows up as (Refl (T a b)). -- Applications of (Refl T) to some coercions, at least one of -- which is NOT the identity, show up as TyConAppCo. -- (They may not be fully saturated however.) -- ConAppCo coercions (like all coercions other than Refl) -- are NEVER the identity. -- Use (GRefl Representational ty MRefl), not (SubCo (Refl ty)) -- GRefl :: "e" -> _ -> Maybe N -> e -- See Note [Generalized reflexive coercion] | GRefl Role Type MCoercionN -- See Note [Refl invariant] -- Use (Refl ty), not (GRefl Nominal ty MRefl) -- Use (GRefl Representational _ _), not (SubCo (GRefl Nominal _ _)) -- These ones simply lift the correspondingly-named -- Type constructors into Coercions -- TyConAppCo :: "e" -> _ -> ?? -> e -- See Note [TyConAppCo roles] | TyConAppCo Role TyCon [Coercion] -- lift TyConApp -- The TyCon is never a synonym; -- we expand synonyms eagerly -- But it can be a type function | AppCo Coercion CoercionN -- lift AppTy -- AppCo :: e -> N -> e -- See Note [Forall coercions] | ForAllCo TyCoVar KindCoercion Coercion -- ForAllCo :: _ -> N -> e -> e | FunCo Role Coercion Coercion -- lift FunTy -- FunCo :: "e" -> e -> e -> e -- Note: why doesn't FunCo have a AnonArgFlag, like FunTy? -- Because the AnonArgFlag has no impact on Core; it is only -- there to guide implicit instantiation of Haskell source -- types, and that is irrelevant for coercions, which are -- Core-only. -- These are special | CoVarCo CoVar -- :: _ -> (N or R) -- result role depends on the tycon of the variable's type -- AxiomInstCo :: e -> _ -> ?? -> e | AxiomInstCo (CoAxiom Branched) BranchIndex [Coercion] -- See also [CoAxiom index] -- The coercion arguments always *precisely* saturate -- arity of (that branch of) the CoAxiom. If there are -- any left over, we use AppCo. -- See [Coercion axioms applied to coercions] -- The roles of the argument coercions are determined -- by the cab_roles field of the relevant branch of the CoAxiom | AxiomRuleCo CoAxiomRule [Coercion] -- AxiomRuleCo is very like AxiomInstCo, but for a CoAxiomRule -- The number coercions should match exactly the expectations -- of the CoAxiomRule (i.e., the rule is fully saturated). | UnivCo UnivCoProvenance Role Type Type -- :: _ -> "e" -> _ -> _ -> e | SymCo Coercion -- :: e -> e | TransCo Coercion Coercion -- :: e -> e -> e | NthCo Role Int Coercion -- Zero-indexed; decomposes (T t0 ... tn) -- :: "e" -> _ -> e0 -> e (inverse of TyConAppCo, see Note [TyConAppCo roles]) -- Using NthCo on a ForAllCo gives an N coercion always -- See Note [NthCo and newtypes] -- -- Invariant: (NthCo r i co), it is always the case that r = role of (Nth i co) -- That is: the role of the entire coercion is redundantly cached here. -- See Note [NthCo Cached Roles] | LRCo LeftOrRight CoercionN -- Decomposes (t_left t_right) -- :: _ -> N -> N | InstCo Coercion CoercionN -- :: e -> N -> e -- See Note [InstCo roles] -- Extract a kind coercion from a (heterogeneous) type coercion -- NB: all kind coercions are Nominal | KindCo Coercion -- :: e -> N | SubCo CoercionN -- Turns a ~N into a ~R -- :: N -> R | HoleCo CoercionHole -- ^ See Note [Coercion holes] -- Only present during typechecking deriving Data.Data type CoercionN = Coercion -- always nominal type CoercionR = Coercion -- always representational type CoercionP = Coercion -- always phantom type KindCoercion = CoercionN -- always nominal instance Outputable Coercion where ppr = pprCo -- | A semantically more meaningful type to represent what may or may not be a -- useful 'Coercion'. data MCoercion = MRefl -- A trivial Reflexivity coercion | MCo Coercion -- Other coercions deriving Data.Data type MCoercionR = MCoercion type MCoercionN = MCoercion instance Outputable MCoercion where ppr MRefl = text "MRefl" ppr (MCo co) = text "MCo" <+> ppr co {- Note [Refl invariant] ~~~~~~~~~~~~~~~~~~~~~ Invariant 1: Coercions have the following invariant Refl (similar for GRefl r ty MRefl) is always lifted as far as possible. You might think that a consequencs is: Every identity coercions has Refl at the root But that's not quite true because of coercion variables. Consider g where g :: Int~Int Left h where h :: Maybe Int ~ Maybe Int etc. So the consequence is only true of coercions that have no coercion variables. Note [Generalized reflexive coercion] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ GRefl is a generalized reflexive coercion (see #15192). It wraps a kind coercion, which might be reflexive (MRefl) or any coercion (MCo co). The typing rules for GRefl: ty : k1 ------------------------------------ GRefl r ty MRefl: ty ~r ty ty : k1 co :: k1 ~ k2 ------------------------------------ GRefl r ty (MCo co) : ty ~r ty |> co Consider we have g1 :: s ~r t s :: k1 g2 :: k1 ~ k2 and we want to construct a coercions co which has type (s |> g2) ~r t We can define co = Sym (GRefl r s g2) ; g1 It is easy to see that Refl == GRefl Nominal ty MRefl :: ty ~n ty A nominal reflexive coercion is quite common, so we keep the special form Refl to save allocation. Note [Coercion axioms applied to coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The reason coercion axioms can be applied to coercions and not just types is to allow for better optimization. There are some cases where we need to be able to "push transitivity inside" an axiom in order to expose further opportunities for optimization. For example, suppose we have C a : t[a] ~ F a g : b ~ c and we want to optimize sym (C b) ; t[g] ; C c which has the kind F b ~ F c (stopping through t[b] and t[c] along the way). We'd like to optimize this to just F g -- but how? The key is that we need to allow axioms to be instantiated by *coercions*, not just by types. Then we can (in certain cases) push transitivity inside the axiom instantiations, and then react opposite-polarity instantiations of the same axiom. In this case, e.g., we match t[g] against the LHS of (C c)'s kind, to obtain the substitution a |-> g (note this operation is sort of the dual of lifting!) and hence end up with C g : t[b] ~ F c which indeed has the same kind as t[g] ; C c. Now we have sym (C b) ; C g which can be optimized to F g. Note [CoAxiom index] ~~~~~~~~~~~~~~~~~~~~ A CoAxiom has 1 or more branches. Each branch has contains a list of the free type variables in that branch, the LHS type patterns, and the RHS type for that branch. When we apply an axiom to a list of coercions, we must choose which branch of the axiom we wish to use, as the different branches may have different numbers of free type variables. (The number of type patterns is always the same among branches, but that doesn't quite concern us here.) The Int in the AxiomInstCo constructor is the 0-indexed number of the chosen branch. Note [Forall coercions] ~~~~~~~~~~~~~~~~~~~~~~~ Constructing coercions between forall-types can be a bit tricky, because the kinds of the bound tyvars can be different. The typing rule is: kind_co : k1 ~ k2 tv1:k1 |- co : t1 ~ t2 ------------------------------------------------------------------- ForAllCo tv1 kind_co co : all tv1:k1. t1 ~ all tv1:k2. (t2[tv1 |-> tv1 |> sym kind_co]) First, the TyCoVar stored in a ForAllCo is really an optimisation: this field should be a Name, as its kind is redundant. Thinking of the field as a Name is helpful in understanding what a ForAllCo means. The kind of TyCoVar always matches the left-hand kind of the coercion. The idea is that kind_co gives the two kinds of the tyvar. See how, in the conclusion, tv1 is assigned kind k1 on the left but kind k2 on the right. Of course, a type variable can't have different kinds at the same time. So, we arbitrarily prefer the first kind when using tv1 in the inner coercion co, which shows that t1 equals t2. The last wrinkle is that we need to fix the kinds in the conclusion. In t2, tv1 is assumed to have kind k1, but it has kind k2 in the conclusion of the rule. So we do a kind-fixing substitution, replacing (tv1:k1) with (tv1:k2) |> sym kind_co. This substitution is slightly bizarre, because it mentions the same name with different kinds, but it *is* well-kinded, noting that `(tv1:k2) |> sym kind_co` has kind k1. This all really would work storing just a Name in the ForAllCo. But we can't add Names to, e.g., VarSets, and there generally is just an impedance mismatch in a bunch of places. So we use tv1. When we need tv2, we can use setTyVarKind. Note [Predicate coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have g :: a~b How can we coerce between types ([c]~a) => [a] -> c and ([c]~b) => [b] -> c where the equality predicate *itself* differs? Answer: we simply treat (~) as an ordinary type constructor, so these types really look like ((~) [c] a) -> [a] -> c ((~) [c] b) -> [b] -> c So the coercion between the two is obviously ((~) [c] g) -> [g] -> c Another way to see this to say that we simply collapse predicates to their representation type (see Type.coreView and Type.predTypeRep). This collapse is done by mkPredCo; there is no PredCo constructor in Coercion. This is important because we need Nth to work on predicates too: Nth 1 ((~) [c] g) = g See Simplify.simplCoercionF, which generates such selections. Note [Roles] ~~~~~~~~~~~~ Roles are a solution to the GeneralizedNewtypeDeriving problem, articulated in #1496. The full story is in docs/core-spec/core-spec.pdf. Also, see https://gitlab.haskell.org/ghc/ghc/wikis/roles-implementation Here is one way to phrase the problem: Given: newtype Age = MkAge Int type family F x type instance F Age = Bool type instance F Int = Char This compiles down to: axAge :: Age ~ Int axF1 :: F Age ~ Bool axF2 :: F Int ~ Char Then, we can make: (sym (axF1) ; F axAge ; axF2) :: Bool ~ Char Yikes! The solution is _roles_, as articulated in "Generative Type Abstraction and Type-level Computation" (POPL 2010), available at http://www.seas.upenn.edu/~sweirich/papers/popl163af-weirich.pdf The specification for roles has evolved somewhat since that paper. For the current full details, see the documentation in docs/core-spec. Here are some highlights. We label every equality with a notion of type equivalence, of which there are three options: Nominal, Representational, and Phantom. A ground type is nominally equivalent only with itself. A newtype (which is considered a ground type in Haskell) is representationally equivalent to its representation. Anything is "phantomly" equivalent to anything else. We use "N", "R", and "P" to denote the equivalences. The axioms above would be: axAge :: Age ~R Int axF1 :: F Age ~N Bool axF2 :: F Age ~N Char Then, because transitivity applies only to coercions proving the same notion of equivalence, the above construction is impossible. However, there is still an escape hatch: we know that any two types that are nominally equivalent are representationally equivalent as well. This is what the form SubCo proves -- it "demotes" a nominal equivalence into a representational equivalence. So, it would seem the following is possible: sub (sym axF1) ; F axAge ; sub axF2 :: Bool ~R Char -- WRONG What saves us here is that the arguments to a type function F, lifted into a coercion, *must* prove nominal equivalence. So, (F axAge) is ill-formed, and we are safe. Roles are attached to parameters to TyCons. When lifting a TyCon into a coercion (through TyConAppCo), we need to ensure that the arguments to the TyCon respect their roles. For example: data T a b = MkT a (F b) If we know that a1 ~R a2, then we know (T a1 b) ~R (T a2 b). But, if we know that b1 ~R b2, we know nothing about (T a b1) and (T a b2)! This is because the type function F branches on b's *name*, not representation. So, we say that 'a' has role Representational and 'b' has role Nominal. The third role, Phantom, is for parameters not used in the type's definition. Given the following definition data Q a = MkQ Int the Phantom role allows us to say that (Q Bool) ~R (Q Char), because we can construct the coercion Bool ~P Char (using UnivCo). See the paper cited above for more examples and information. Note [TyConAppCo roles] ~~~~~~~~~~~~~~~~~~~~~~~ The TyConAppCo constructor has a role parameter, indicating the role at which the coercion proves equality. The choice of this parameter affects the required roles of the arguments of the TyConAppCo. To help explain it, assume the following definition: type instance F Int = Bool -- Axiom axF : F Int ~N Bool newtype Age = MkAge Int -- Axiom axAge : Age ~R Int data Foo a = MkFoo a -- Role on Foo's parameter is Representational TyConAppCo Nominal Foo axF : Foo (F Int) ~N Foo Bool For (TyConAppCo Nominal) all arguments must have role Nominal. Why? So that Foo Age ~N Foo Int does *not* hold. TyConAppCo Representational Foo (SubCo axF) : Foo (F Int) ~R Foo Bool TyConAppCo Representational Foo axAge : Foo Age ~R Foo Int For (TyConAppCo Representational), all arguments must have the roles corresponding to the result of tyConRoles on the TyCon. This is the whole point of having roles on the TyCon to begin with. So, we can have Foo Age ~R Foo Int, if Foo's parameter has role R. If a Representational TyConAppCo is over-saturated (which is otherwise fine), the spill-over arguments must all be at Nominal. This corresponds to the behavior for AppCo. TyConAppCo Phantom Foo (UnivCo Phantom Int Bool) : Foo Int ~P Foo Bool All arguments must have role Phantom. This one isn't strictly necessary for soundness, but this choice removes ambiguity. The rules here dictate the roles of the parameters to mkTyConAppCo (should be checked by Lint). Note [NthCo and newtypes] ~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have newtype N a = MkN Int type role N representational This yields axiom NTCo:N :: forall a. N a ~R Int We can then build co :: forall a b. N a ~R N b co = NTCo:N a ; sym (NTCo:N b) for any `a` and `b`. Because of the role annotation on N, if we use NthCo, we'll get out a representational coercion. That is: NthCo r 0 co :: forall a b. a ~R b Yikes! Clearly, this is terrible. The solution is simple: forbid NthCo to be used on newtypes if the internal coercion is representational. This is not just some corner case discovered by a segfault somewhere; it was discovered in the proof of soundness of roles and described in the "Safe Coercions" paper (ICFP '14). Note [NthCo Cached Roles] ~~~~~~~~~~~~~~~~~~~~~~~~~ Why do we cache the role of NthCo in the NthCo constructor? Because computing role(Nth i co) involves figuring out that co :: T tys1 ~ T tys2 using coercionKind, and finding (coercionRole co), and then looking at the tyConRoles of T. Avoiding bad asymptotic behaviour here means we have to compute the kind and role of a coercion simultaneously, which makes the code complicated and inefficient. This only happens for NthCo. Caching the role solves the problem, and allows coercionKind and coercionRole to be simple. See #11735 Note [InstCo roles] ~~~~~~~~~~~~~~~~~~~ Here is (essentially) the typing rule for InstCo: g :: (forall a. t1) ~r (forall a. t2) w :: s1 ~N s2 ------------------------------- InstCo InstCo g w :: (t1 [a |-> s1]) ~r (t2 [a |-> s2]) Note that the Coercion w *must* be nominal. This is necessary because the variable a might be used in a "nominal position" (that is, a place where role inference would require a nominal role) in t1 or t2. If we allowed w to be representational, we could get bogus equalities. A more nuanced treatment might be able to relax this condition somewhat, by checking if t1 and/or t2 use their bound variables in nominal ways. If not, having w be representational is OK. %************************************************************************ %* * UnivCoProvenance %* * %************************************************************************ A UnivCo is a coercion whose proof does not directly express its role and kind (indeed for some UnivCos, like UnsafeCoerceProv, there /is/ no proof). The different kinds of UnivCo are described by UnivCoProvenance. Really each is entirely separate, but they all share the need to represent their role and kind, which is done in the UnivCo constructor. -} -- | For simplicity, we have just one UnivCo that represents a coercion from -- some type to some other type, with (in general) no restrictions on the -- type. The UnivCoProvenance specifies more exactly what the coercion really -- is and why a program should (or shouldn't!) trust the coercion. -- It is reasonable to consider each constructor of 'UnivCoProvenance' -- as a totally independent coercion form; their only commonality is -- that they don't tell you what types they coercion between. (That info -- is in the 'UnivCo' constructor of 'Coercion'. data UnivCoProvenance = UnsafeCoerceProv -- ^ From @unsafeCoerce#@. These are unsound. | PhantomProv KindCoercion -- ^ See Note [Phantom coercions]. Only in Phantom -- roled coercions | ProofIrrelProv KindCoercion -- ^ From the fact that any two coercions are -- considered equivalent. See Note [ProofIrrelProv]. -- Can be used in Nominal or Representational coercions | PluginProv String -- ^ From a plugin, which asserts that this coercion -- is sound. The string is for the use of the plugin. deriving Data.Data instance Outputable UnivCoProvenance where ppr UnsafeCoerceProv = text "(unsafeCoerce#)" ppr (PhantomProv _) = text "(phantom)" ppr (ProofIrrelProv _) = text "(proof irrel.)" ppr (PluginProv str) = parens (text "plugin" <+> brackets (text str)) -- | A coercion to be filled in by the type-checker. See Note [Coercion holes] data CoercionHole = CoercionHole { ch_co_var :: CoVar -- See Note [CoercionHoles and coercion free variables] , ch_ref :: IORef (Maybe Coercion) } coHoleCoVar :: CoercionHole -> CoVar coHoleCoVar = ch_co_var setCoHoleCoVar :: CoercionHole -> CoVar -> CoercionHole setCoHoleCoVar h cv = h { ch_co_var = cv } instance Data.Data CoercionHole where -- don't traverse? toConstr _ = abstractConstr "CoercionHole" gunfold _ _ = error "gunfold" dataTypeOf _ = mkNoRepType "CoercionHole" instance Outputable CoercionHole where ppr (CoercionHole { ch_co_var = cv }) = braces (ppr cv) {- Note [Phantom coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T a = T1 | T2 Then we have T s ~R T t for any old s,t. The witness for this is (TyConAppCo T Rep co), where (co :: s ~P t) is a phantom coercion built with PhantomProv. The role of the UnivCo is always Phantom. The Coercion stored is the (nominal) kind coercion between the types kind(s) ~N kind (t) Note [Coercion holes] ~~~~~~~~~~~~~~~~~~~~~~~~ During typechecking, constraint solving for type classes works by - Generate an evidence Id, d7 :: Num a - Wrap it in a Wanted constraint, [W] d7 :: Num a - Use the evidence Id where the evidence is needed - Solve the constraint later - When solved, add an enclosing let-binding let d7 = .... in .... which actually binds d7 to the (Num a) evidence For equality constraints we use a different strategy. See Note [The equality types story] in TysPrim for background on equality constraints. - For /boxed/ equality constraints, (t1 ~N t2) and (t1 ~R t2), it's just like type classes above. (Indeed, boxed equality constraints *are* classes.) - But for /unboxed/ equality constraints (t1 ~R# t2) and (t1 ~N# t2) we use a different plan For unboxed equalities: - Generate a CoercionHole, a mutable variable just like a unification variable - Wrap the CoercionHole in a Wanted constraint; see TcRnTypes.TcEvDest - Use the CoercionHole in a Coercion, via HoleCo - Solve the constraint later - When solved, fill in the CoercionHole by side effect, instead of doing the let-binding thing The main reason for all this is that there may be no good place to let-bind the evidence for unboxed equalities: - We emit constraints for kind coercions, to be used to cast a type's kind. These coercions then must be used in types. Because they might appear in a top-level type, there is no place to bind these (unlifted) coercions in the usual way. - A coercion for (forall a. t1) ~ (forall a. t2) will look like forall a. (coercion for t1~t2) But the coercion for (t1~t2) may mention 'a', and we don't have let-bindings within coercions. We could add them, but coercion holes are easier. - Moreover, nothing is lost from the lack of let-bindings. For dicionaries want to achieve sharing to avoid recomoputing the dictionary. But coercions are entirely erased, so there's little benefit to sharing. Indeed, even if we had a let-binding, we always inline types and coercions at every use site and drop the binding. Other notes about HoleCo: * INVARIANT: CoercionHole and HoleCo are used only during type checking, and should never appear in Core. Just like unification variables; a Type can contain a TcTyVar, but only during type checking. If, one day, we use type-level information to separate out forms that can appear during type-checking vs forms that can appear in core proper, holes in Core will be ruled out. * See Note [CoercionHoles and coercion free variables] * Coercion holes can be compared for equality like other coercions: by looking at the types coerced. Note [CoercionHoles and coercion free variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Why does a CoercionHole contain a CoVar, as well as reference to fill in? Because we want to treat that CoVar as a free variable of the coercion. See #14584, and Note [What prevents a constraint from floating] in TcSimplify, item (4): forall k. [W] co1 :: t1 ~# t2 |> co2 [W] co2 :: k ~# * Here co2 is a CoercionHole. But we /must/ know that it is free in co1, because that's all that stops it floating outside the implication. Note [ProofIrrelProv] ~~~~~~~~~~~~~~~~~~~~~ A ProofIrrelProv is a coercion between coercions. For example: data G a where MkG :: G Bool In core, we get G :: * -> * MkG :: forall (a :: *). (a ~ Bool) -> G a Now, consider 'MkG -- that is, MkG used in a type -- and suppose we want a proof that ('MkG a1 co1) ~ ('MkG a2 co2). This will have to be TyConAppCo Nominal MkG [co3, co4] where co3 :: co1 ~ co2 co4 :: a1 ~ a2 Note that co1 :: a1 ~ Bool co2 :: a2 ~ Bool Here, co3 = UnivCo (ProofIrrelProv co5) Nominal (CoercionTy co1) (CoercionTy co2) where co5 :: (a1 ~ Bool) ~ (a2 ~ Bool) co5 = TyConAppCo Nominal (~#) [<*>, <*>, co4, <Bool>] -} {- ********************************************************************* * * typeSize, coercionSize * * ********************************************************************* -} -- NB: We put typeSize/coercionSize here because they are mutually -- recursive, and have the CPR property. If we have mutual -- recursion across a hi-boot file, we don't get the CPR property -- and these functions allocate a tremendous amount of rubbish. -- It's not critical (because typeSize is really only used in -- debug mode, but I tripped over an example (T5642) in which -- typeSize was one of the biggest single allocators in all of GHC. -- And it's easy to fix, so I did. -- NB: typeSize does not respect `eqType`, in that two types that -- are `eqType` may return different sizes. This is OK, because this -- function is used only in reporting, not decision-making. typeSize :: Type -> Int typeSize (LitTy {}) = 1 typeSize (TyVarTy {}) = 1 typeSize (AppTy t1 t2) = typeSize t1 + typeSize t2 typeSize (FunTy _ t1 t2) = typeSize t1 + typeSize t2 typeSize (ForAllTy (Bndr tv _) t) = typeSize (varType tv) + typeSize t typeSize (TyConApp _ ts) = 1 + sum (map typeSize ts) typeSize (CastTy ty co) = typeSize ty + coercionSize co typeSize (CoercionTy co) = coercionSize co coercionSize :: Coercion -> Int coercionSize (Refl ty) = typeSize ty coercionSize (GRefl _ ty MRefl) = typeSize ty coercionSize (GRefl _ ty (MCo co)) = 1 + typeSize ty + coercionSize co coercionSize (TyConAppCo _ _ args) = 1 + sum (map coercionSize args) coercionSize (AppCo co arg) = coercionSize co + coercionSize arg coercionSize (ForAllCo _ h co) = 1 + coercionSize co + coercionSize h coercionSize (FunCo _ co1 co2) = 1 + coercionSize co1 + coercionSize co2 coercionSize (CoVarCo _) = 1 coercionSize (HoleCo _) = 1 coercionSize (AxiomInstCo _ _ args) = 1 + sum (map coercionSize args) coercionSize (UnivCo p _ t1 t2) = 1 + provSize p + typeSize t1 + typeSize t2 coercionSize (SymCo co) = 1 + coercionSize co coercionSize (TransCo co1 co2) = 1 + coercionSize co1 + coercionSize co2 coercionSize (NthCo _ _ co) = 1 + coercionSize co coercionSize (LRCo _ co) = 1 + coercionSize co coercionSize (InstCo co arg) = 1 + coercionSize co + coercionSize arg coercionSize (KindCo co) = 1 + coercionSize co coercionSize (SubCo co) = 1 + coercionSize co coercionSize (AxiomRuleCo _ cs) = 1 + sum (map coercionSize cs) provSize :: UnivCoProvenance -> Int provSize UnsafeCoerceProv = 1 provSize (PhantomProv co) = 1 + coercionSize co provSize (ProofIrrelProv co) = 1 + coercionSize co provSize (PluginProv _) = 1