CoAxiom.lhs 19.5 KB
 eir@cis.upenn.edu committed Dec 21, 2012 1 2 3 4 5 6 % % (c) The University of Glasgow 2012 % \begin{code}  eir@cis.upenn.edu committed Jun 21, 2013 7 {-# LANGUAGE GADTs, ScopedTypeVariables #-}  eir@cis.upenn.edu committed Dec 21, 2012 8 9 10 11 12  -- | Module for coercion axioms, used to represent type family instances -- and newtypes module CoAxiom (  Simon Peyton Jones committed Jan 09, 2013 13  Branched, Unbranched, BranchIndex, BranchList(..),  eir@cis.upenn.edu committed Dec 21, 2012 14 15  toBranchList, fromBranchList, toBranchedList, toUnbranchedList,  eir@cis.upenn.edu committed Jun 21, 2013 16 17  brListLength, brListNth, brListMap, brListFoldr, brListMapM, brListFoldlM_, brListZipWith,  eir@cis.upenn.edu committed Dec 21, 2012 18   Simon Peyton Jones committed Jan 28, 2013 19  CoAxiom(..), CoAxBranch(..),  eir@cis.upenn.edu committed Dec 21, 2012 20 21 22  toBranchedAxiom, toUnbranchedAxiom, coAxiomName, coAxiomArity, coAxiomBranches,  eir@cis.upenn.edu committed Jun 21, 2013 23  coAxiomTyCon, isImplicitCoAxiom, coAxiomNumPats,  eir@cis.upenn.edu committed Aug 02, 2013 24 25 26 27 28  coAxiomNthBranch, coAxiomSingleBranch_maybe, coAxiomRole, coAxiomSingleBranch, coAxBranchTyVars, coAxBranchRoles, coAxBranchLHS, coAxBranchRHS, coAxBranchSpan, coAxBranchIncomps, placeHolderIncomps,  eir@cis.upenn.edu committed Sep 17, 2013 29  Role(..), fsFromRole,  Iavor S. Diatchki committed Sep 12, 2013 30   Iavor S. Diatchki committed Nov 14, 2013 31 32  CoAxiomRule(..), Eqn, BuiltInSynFamily(..), trivialBuiltInFamily  eir@cis.upenn.edu committed Dec 21, 2012 33 34 35 36 37  ) where import {-# SOURCE #-} TypeRep ( Type ) import {-# SOURCE #-} TyCon ( TyCon ) import Outputable  eir@cis.upenn.edu committed Aug 27, 2013 38 import FastString  eir@cis.upenn.edu committed Dec 21, 2012 39 40 41 42 import Name import Unique import Var import Util  eir@cis.upenn.edu committed Aug 02, 2013 43 import Binary  Iavor S. Diatchki committed Sep 12, 2013 44 import Pair  eir@cis.upenn.edu committed Dec 21, 2012 45 46 import BasicTypes import Data.Typeable ( Typeable )  eir@cis.upenn.edu committed Jan 05, 2013 47 import SrcLoc  eir@cis.upenn.edu committed Dec 21, 2012 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 import qualified Data.Data as Data #include "HsVersions.h" \end{code} Note [Coercion axiom branches] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In order to allow type family instance groups, an axiom needs to contain an ordered list of alternatives, called branches. The kind of the coercion built from an axiom is determined by which index is used when building the coercion from the axiom. For example, consider the axiom derived from the following declaration: type instance where F [Int] = Bool F [a] = Double F (a b) = Char This will give rise to this axiom: axF :: { F [Int] ~ Bool ; forall (a :: *). F [a] ~ Double ; forall (k :: BOX) (a :: k -> *) (b :: k). F (a b) ~ Char } The axiom is used with the AxiomInstCo constructor of Coercion. If we wish to have a coercion showing that F (Maybe Int) ~ Char, it will look like axF[2] <*> :: F (Maybe Int) ~ Char -- or, written using concrete-ish syntax -- AxiomInstCo axF 2 [Refl *, Refl Maybe, Refl Int] Note that the index is 0-based. For type-checking, it is also necessary to check that no previous pattern can unify with the supplied arguments. After all, it is possible that some of the type arguments are lambda-bound type variables whose instantiation may cause an earlier match among the branches. We wish to prohibit this behavior, so the type checker rules out the choice of a branch where a previous branch  eir@cis.upenn.edu committed Apr 24, 2013 89 can unify. See also [Branched instance checking] in FamInstEnv.hs.  eir@cis.upenn.edu committed Dec 21, 2012 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110  For example, the following is malformed, where 'a' is a lambda-bound type variable: axF[2] <*> :: F (a Bool) ~ Char Why? Because a might be instantiated with [], meaning that branch 1 should apply, not branch 2. This is a vital consistency check; without it, we could derive Int ~ Bool, and that is a Bad Thing. Note [Branched axioms] ~~~~~~~~~~~~~~~~~~~~~~~ Although a CoAxiom has the capacity to store many branches, in certain cases, we want only one. These cases are in data/newtype family instances, newtype coercions, and type family instances declared with "type instance ...", not "type instance where". Furthermore, these unbranched axioms are used in a variety of places throughout GHC, and it would difficult to generalize all of that code to deal with branched axioms, especially when the code can be sure of the fact that an axiom is indeed a singleton. At the same time, it seems dangerous to assume singlehood in various places through GHC.  eir@cis.upenn.edu committed Jun 21, 2013 111 112 113 114 The solution to this is to label a CoAxiom with a phantom type variable declaring whether it is known to be a singleton or not. The list of branches is stored using a special form of list, declared below, that ensures that the type variable is accurate.  eir@cis.upenn.edu committed Dec 21, 2012 115 116 117 118 119 120 121  As of this writing (Dec 2012), it would not be appropriate to use a promoted type as the phantom type, so we use empty datatypes. We wish to have GHC remain compilable with GHC 7.2.1. If you are revising this code and GHC no longer needs to remain compatible with GHC 7.2.x, then please update this code to use promoted types.  eir@cis.upenn.edu committed Jan 05, 2013 122 123 124 125 126 127 128  %************************************************************************ %* * Branch lists %* * %************************************************************************  eir@cis.upenn.edu committed Dec 21, 2012 129 \begin{code}  Simon Peyton Jones committed Jan 09, 2013 130 131 type BranchIndex = Int -- The index of the branch in the list of branches -- Counting from zero  eir@cis.upenn.edu committed Dec 21, 2012 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166  -- the phantom type labels data Unbranched deriving Typeable data Branched deriving Typeable data BranchList a br where FirstBranch :: a -> BranchList a br NextBranch :: a -> BranchList a br -> BranchList a Branched -- convert to/from lists toBranchList :: [a] -> BranchList a Branched toBranchList [] = pprPanic "toBranchList" empty toBranchList [b] = FirstBranch b toBranchList (h:t) = NextBranch h (toBranchList t) fromBranchList :: BranchList a br -> [a] fromBranchList (FirstBranch b) = [b] fromBranchList (NextBranch h t) = h : (fromBranchList t) -- convert from any BranchList to a Branched BranchList toBranchedList :: BranchList a br -> BranchList a Branched toBranchedList (FirstBranch b) = FirstBranch b toBranchedList (NextBranch h t) = NextBranch h t -- convert from any BranchList to an Unbranched BranchList toUnbranchedList :: BranchList a br -> BranchList a Unbranched toUnbranchedList (FirstBranch b) = FirstBranch b toUnbranchedList _ = pprPanic "toUnbranchedList" empty -- length brListLength :: BranchList a br -> Int brListLength (FirstBranch _) = 1 brListLength (NextBranch _ t) = 1 + brListLength t -- lookup  Simon Peyton Jones committed Jan 09, 2013 167 brListNth :: BranchList a br -> BranchIndex -> a  eir@cis.upenn.edu committed Dec 21, 2012 168 169 170 171 172 173 174 175 176 177 178 179 180 181 brListNth (FirstBranch b) 0 = b brListNth (NextBranch h _) 0 = h brListNth (NextBranch _ t) n = brListNth t (n-1) brListNth _ _ = pprPanic "brListNth" empty -- map, fold brListMap :: (a -> b) -> BranchList a br -> [b] brListMap f (FirstBranch b) = [f b] brListMap f (NextBranch h t) = f h : (brListMap f t) brListFoldr :: (a -> b -> b) -> b -> BranchList a br -> b brListFoldr f x (FirstBranch b) = f b x brListFoldr f x (NextBranch h t) = f h (brListFoldr f x t)  eir@cis.upenn.edu committed Jun 21, 2013 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 brListMapM :: Monad m => (a -> m b) -> BranchList a br -> m [b] brListMapM f (FirstBranch b) = f b >>= \fb -> return [fb] brListMapM f (NextBranch h t) = do { fh <- f h ; ft <- brListMapM f t ; return (fh : ft) } brListFoldlM_ :: forall a b m br. Monad m => (a -> b -> m a) -> a -> BranchList b br -> m () brListFoldlM_ f z brs = do { _ <- go z brs ; return () } where go :: forall br'. Monad m => a -> BranchList b br' -> m a go acc (FirstBranch b) = f acc b go acc (NextBranch h t) = do { fh <- f acc h ; go fh t }  eir@cis.upenn.edu committed Dec 21, 2012 197 198 199 200 201 202 203 204 205 206 207 -- zipWith brListZipWith :: (a -> b -> c) -> BranchList a br1 -> BranchList b br2 -> [c] brListZipWith f (FirstBranch a) (FirstBranch b) = [f a b] brListZipWith f (FirstBranch a) (NextBranch b _) = [f a b] brListZipWith f (NextBranch a _) (FirstBranch b) = [f a b] brListZipWith f (NextBranch a ta) (NextBranch b tb) = f a b : brListZipWith f ta tb -- pretty-printing instance Outputable a => Outputable (BranchList a br) where ppr = ppr . fromBranchList  eir@cis.upenn.edu committed Jan 05, 2013 208 209 210 211 212 213 214 \end{code} %************************************************************************ %* * Coercion axioms %* * %************************************************************************  eir@cis.upenn.edu committed Dec 21, 2012 215   eir@cis.upenn.edu committed Jun 21, 2013 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 Note [Storing compatibility] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ During axiom application, we need to be aware of which branches are compatible with which others. The full explanation is in Note [Compatibility] in FamInstEnv. (The code is placed there to avoid a dependency from CoAxiom on the unification algorithm.) Although we could theoretically compute compatibility on the fly, this is silly, so we store it in a CoAxiom. Specifically, each branch refers to all other branches with which it is incompatible. This list might well be empty, and it will always be for the first branch of any axiom. CoAxBranches that do not (yet) belong to a CoAxiom should have a panic thunk stored in cab_incomps. The incompatibilities are properly a property of the axiom as a whole, and they are computed only when the final axiom is built. During serialization, the list is converted into a list of the indices of the branches.  eir@cis.upenn.edu committed Jan 05, 2013 235 \begin{code}  eir@cis.upenn.edu committed Dec 21, 2012 236 237 238 239 240 241 242 243 -- | A 'CoAxiom' is a \"coercion constructor\", i.e. a named equality axiom. -- If you edit this type, you may need to update the GHC formalism -- See Note [GHC Formalism] in coreSyn/CoreLint.lhs data CoAxiom br = CoAxiom -- Type equality axiom. { co_ax_unique :: Unique -- unique identifier , co_ax_name :: Name -- name for pretty-printing  eir@cis.upenn.edu committed Aug 02, 2013 244  , co_ax_role :: Role -- role of the axiom's equality  eir@cis.upenn.edu committed Dec 21, 2012 245 246 247 248 249 250 251 252 253 254 255  , co_ax_tc :: TyCon -- the head of the LHS patterns , co_ax_branches :: BranchList CoAxBranch br -- the branches that form this axiom , co_ax_implicit :: Bool -- True <=> the axiom is "implicit" -- See Note [Implicit axioms] -- INVARIANT: co_ax_implicit == True implies length co_ax_branches == 1. } deriving Typeable data CoAxBranch = CoAxBranch  eir@cis.upenn.edu committed Jun 21, 2013 256 257 258 259  { cab_loc :: SrcSpan -- Location of the defining equation -- See Note [CoAxiom locations] , cab_tvs :: [TyVar] -- Bound type variables; not necessarily fresh -- See Note [CoAxBranch type variables]  eir@cis.upenn.edu committed Aug 02, 2013 260  , cab_roles :: [Role] -- See Note [CoAxBranch roles]  eir@cis.upenn.edu committed Jun 21, 2013 261 262 263 264  , cab_lhs :: [Type] -- Type patterns to match against , cab_rhs :: Type -- Right-hand side of the equality , cab_incomps :: [CoAxBranch] -- The previous incompatible branches -- See Note [Storing compatibility]  eir@cis.upenn.edu committed Dec 21, 2012 265 266 267 268  } deriving Typeable toBranchedAxiom :: CoAxiom br -> CoAxiom Branched  eir@cis.upenn.edu committed Aug 02, 2013 269 270 toBranchedAxiom (CoAxiom unique name role tc branches implicit) = CoAxiom unique name role tc (toBranchedList branches) implicit  eir@cis.upenn.edu committed Dec 21, 2012 271 272  toUnbranchedAxiom :: CoAxiom br -> CoAxiom Unbranched  eir@cis.upenn.edu committed Aug 02, 2013 273 274 toUnbranchedAxiom (CoAxiom unique name role tc branches implicit) = CoAxiom unique name role tc (toUnbranchedList branches) implicit  eir@cis.upenn.edu committed Dec 21, 2012 275   eir@cis.upenn.edu committed Jun 21, 2013 276 277 278 coAxiomNumPats :: CoAxiom br -> Int coAxiomNumPats = length . coAxBranchLHS . (flip coAxiomNthBranch 0)  Simon Peyton Jones committed Jan 09, 2013 279 280 281 coAxiomNthBranch :: CoAxiom br -> BranchIndex -> CoAxBranch coAxiomNthBranch (CoAxiom { co_ax_branches = bs }) index = brListNth bs index  eir@cis.upenn.edu committed Dec 21, 2012 282   Simon Peyton Jones committed Jan 09, 2013 283 coAxiomArity :: CoAxiom br -> BranchIndex -> Arity  eir@cis.upenn.edu committed Dec 21, 2012 284 285 286 287 288 289 coAxiomArity ax index = length $cab_tvs$ coAxiomNthBranch ax index coAxiomName :: CoAxiom br -> Name coAxiomName = co_ax_name  eir@cis.upenn.edu committed Aug 02, 2013 290 291 292 coAxiomRole :: CoAxiom br -> Role coAxiomRole = co_ax_role  eir@cis.upenn.edu committed Dec 21, 2012 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 coAxiomBranches :: CoAxiom br -> BranchList CoAxBranch br coAxiomBranches = co_ax_branches coAxiomSingleBranch_maybe :: CoAxiom br -> Maybe CoAxBranch coAxiomSingleBranch_maybe (CoAxiom { co_ax_branches = branches }) | FirstBranch br <- branches = Just br | otherwise = Nothing coAxiomSingleBranch :: CoAxiom Unbranched -> CoAxBranch coAxiomSingleBranch (CoAxiom { co_ax_branches = FirstBranch br }) = br coAxiomTyCon :: CoAxiom br -> TyCon coAxiomTyCon = co_ax_tc coAxBranchTyVars :: CoAxBranch -> [TyVar] coAxBranchTyVars = cab_tvs coAxBranchLHS :: CoAxBranch -> [Type] coAxBranchLHS = cab_lhs coAxBranchRHS :: CoAxBranch -> Type coAxBranchRHS = cab_rhs  eir@cis.upenn.edu committed Aug 02, 2013 318 319 320 coAxBranchRoles :: CoAxBranch -> [Role] coAxBranchRoles = cab_roles  eir@cis.upenn.edu committed Jan 05, 2013 321 322 323 coAxBranchSpan :: CoAxBranch -> SrcSpan coAxBranchSpan = cab_loc  eir@cis.upenn.edu committed Dec 21, 2012 324 325 isImplicitCoAxiom :: CoAxiom br -> Bool isImplicitCoAxiom = co_ax_implicit  eir@cis.upenn.edu committed Jan 05, 2013 326   eir@cis.upenn.edu committed Jun 21, 2013 327 328 329 coAxBranchIncomps :: CoAxBranch -> [CoAxBranch] coAxBranchIncomps = cab_incomps  eir@cis.upenn.edu committed Mar 22, 2014 330 -- See Note [Compatibility checking] in FamInstEnv  eir@cis.upenn.edu committed Jun 21, 2013 331 332 333 placeHolderIncomps :: [CoAxBranch] placeHolderIncomps = panic "placeHolderIncomps"  eir@cis.upenn.edu committed Dec 21, 2012 334 335 \end{code}  Simon Peyton Jones committed Jan 28, 2013 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 Note [CoAxBranch type variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the case of a CoAxBranch of an associated type-family instance, we use the *same* type variables (where possible) as the enclosing class or instance. Consider class C a b where type F x b type F [y] b = ... -- Second param must be b instance C Int [z] where type F Int [z] = ... -- Second param must be [z] In the CoAxBranch in the instance decl (F Int [z]) we use the same 'z', so that it's easy to check that that type is the same as that in the instance header. Similarly in the CoAxBranch for the default decl for F in the class decl, we use the same 'b' to make the same check easy. So, unlike FamInsts, there is no expectation that the cab_tvs are fresh wrt each other, or any other CoAxBranch.  eir@cis.upenn.edu committed Aug 02, 2013 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 Note [CoAxBranch roles] ~~~~~~~~~~~~~~~~~~~~~~~ Consider this code: newtype Age = MkAge Int newtype Wrap a = MkWrap a convert :: Wrap Age -> Int convert (MkWrap (MkAge i)) = i We want this to compile to: NTCo:Wrap :: forall a. Wrap a ~R a NTCo:Age :: Age ~R Int convert = \x -> x |> (NTCo:Wrap[0] NTCo:Age[0]) But, note that NTCo:Age is at role R. Thus, we need to be able to pass coercions at role R into axioms. However, we don't *always* want to be able to do this, as it would be disastrous with type families. The solution is to annotate the arguments to the axiom with roles, much like we annotate tycon tyvars. Where do these roles get set? Newtype axioms inherit their roles from the newtype tycon; family axioms are all at role N.  Simon Peyton Jones committed Jan 09, 2013 381 382 383 384 385 386 387 388 389 390 391 392 393 394 Note [CoAxiom locations] ~~~~~~~~~~~~~~~~~~~~~~~~ The source location of a CoAxiom is stored in two places in the datatype tree. * The first is in the location info buried in the Name of the CoAxiom. This span includes all of the branches of a branched CoAxiom. * The second is in the cab_loc fields of the CoAxBranches. In the case of a single branch, we can extract the source location of the branch from the name of the CoAxiom. In other cases, we need an explicit SrcSpan to correctly store the location of the equation giving rise to the FamInstBranch.  eir@cis.upenn.edu committed Dec 21, 2012 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 Note [Implicit axioms] ~~~~~~~~~~~~~~~~~~~~~~ See also Note [Implicit TyThings] in HscTypes * A CoAxiom arising from data/type family instances is not "implicit". That is, it has its own IfaceAxiom declaration in an interface file * The CoAxiom arising from a newtype declaration *is* "implicit". That is, it does not have its own IfaceAxiom declaration in an interface file; instead the CoAxiom is generated by type-checking the newtype declaration \begin{code} instance Eq (CoAxiom br) where a == b = case (a compare b) of { EQ -> True; _ -> False } a /= b = case (a compare b) of { EQ -> False; _ -> True } instance Ord (CoAxiom br) where a <= b = case (a compare b) of { LT -> True; EQ -> True; GT -> False } a < b = case (a compare b) of { LT -> True; EQ -> False; GT -> False } a >= b = case (a compare b) of { LT -> False; EQ -> True; GT -> True } a > b = case (a compare b) of { LT -> False; EQ -> False; GT -> True } compare a b = getUnique a compare getUnique b instance Uniquable (CoAxiom br) where getUnique = co_ax_unique instance Outputable (CoAxiom br) where ppr = ppr . getName instance NamedThing (CoAxiom br) where getName = co_ax_name instance Typeable br => Data.Data (CoAxiom br) where -- don't traverse? toConstr _ = abstractConstr "CoAxiom" gunfold _ _ = error "gunfold" dataTypeOf _ = mkNoRepType "CoAxiom"  eir@cis.upenn.edu committed Jan 05, 2013 432 433 \end{code}  eir@cis.upenn.edu committed Aug 02, 2013 434 435 436 437 438 439 %************************************************************************ %* * Roles %* * %************************************************************************  Simon Peyton Jones committed Sep 18, 2013 440 Roles are defined here to avoid circular dependencies.  eir@cis.upenn.edu committed Aug 02, 2013 441 442 443 444 445 446 447 448  \begin{code} -- See Note [Roles] in Coercion -- defined here to avoid cyclic dependency with Coercion data Role = Nominal | Representational | Phantom deriving (Eq, Data.Data, Data.Typeable)  eir@cis.upenn.edu committed Sep 17, 2013 449 450 451 452 453 454 455 456 -- These names are slurped into the parser code. Changing these strings -- will change the **surface syntax** that GHC accepts! If you want to -- change only the pretty-printing, do some replumbing. See -- mkRoleAnnotDecl in RdrHsSyn fsFromRole :: Role -> FastString fsFromRole Nominal = fsLit "nominal" fsFromRole Representational = fsLit "representational" fsFromRole Phantom = fsLit "phantom"  eir@cis.upenn.edu committed Aug 27, 2013 457   eir@cis.upenn.edu committed Aug 02, 2013 458 instance Outputable Role where  eir@cis.upenn.edu committed Sep 17, 2013 459  ppr = ftext . fsFromRole  eir@cis.upenn.edu committed Aug 02, 2013 460 461 462 463 464 465 466 467 468 469 470 471  instance Binary Role where put_ bh Nominal = putByte bh 1 put_ bh Representational = putByte bh 2 put_ bh Phantom = putByte bh 3 get bh = do tag <- getByte bh case tag of 1 -> return Nominal 2 -> return Representational 3 -> return Phantom _ -> panic ("get Role " ++ show tag)  Iavor S. Diatchki committed Sep 12, 2013 472 473 474 \end{code}  Simon Peyton Jones committed Sep 18, 2013 475 476 477 478 479 480 %************************************************************************ %* * CoAxiomRule Rules for building Evidence %* * %************************************************************************  Iavor S. Diatchki committed Sep 12, 2013 481   Simon Peyton Jones committed Sep 18, 2013 482 Conditional axioms. The general idea is that a CoAxiomRule looks like this:  Iavor S. Diatchki committed Sep 12, 2013 483 484 485  forall as. (r1 ~ r2, s1 ~ s2) => t1 ~ t2  Simon Peyton Jones committed Sep 18, 2013 486 My intention is to reuse these for both (~) and (~#).  Iavor S. Diatchki committed Sep 12, 2013 487 The short-term plan is to use this datatype to represent the type-nat axioms.  Simon Peyton Jones committed Sep 18, 2013 488 In the longer run, it may be good to unify this and CoAxiom,  Iavor S. Diatchki committed Sep 12, 2013 489 490 491 492 493 494 495 496 497 498 499 500 501 as CoAxiom is the special case when there are no assumptions. \begin{code} -- | A more explicit representation for t1 ~ t2. type Eqn = Pair Type -- | For now, we work only with nominal equality. data CoAxiomRule = CoAxiomRule { coaxrName :: FastString , coaxrTypeArity :: Int -- number of type argumentInts , coaxrAsmpRoles :: [Role] -- roles of parameter equations , coaxrRole :: Role -- role of resulting equation , coaxrProves :: [Type] -> [Eqn] -> Maybe Eqn  Simon Peyton Jones committed Sep 18, 2013 502 503 504 505  -- ^ coaxrProves returns @Nothing@ when it doesn't like -- the supplied arguments. When this happens in a coercion -- that means that the coercion is ill-formed, and Core Lint -- checks for that.  Iavor S. Diatchki committed Sep 12, 2013 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524  } deriving Typeable instance Data.Data CoAxiomRule where -- don't traverse? toConstr _ = abstractConstr "CoAxiomRule" gunfold _ _ = error "gunfold" dataTypeOf _ = mkNoRepType "CoAxiomRule" instance Uniquable CoAxiomRule where getUnique = getUnique . coaxrName instance Eq CoAxiomRule where x == y = coaxrName x == coaxrName y instance Ord CoAxiomRule where compare x y = compare (coaxrName x) (coaxrName y) instance Outputable CoAxiomRule where ppr = ppr . coaxrName  Iavor S. Diatchki committed Nov 14, 2013 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542  -- Type checking of built-in families data BuiltInSynFamily = BuiltInSynFamily { sfMatchFam :: [Type] -> Maybe (CoAxiomRule, [Type], Type) , sfInteractTop :: [Type] -> Type -> [Eqn] , sfInteractInert :: [Type] -> Type -> [Type] -> Type -> [Eqn] } -- Provides default implementations that do nothing. trivialBuiltInFamily :: BuiltInSynFamily trivialBuiltInFamily = BuiltInSynFamily { sfMatchFam = \_ -> Nothing , sfInteractTop = \_ _ -> [] , sfInteractInert = \_ _ _ _ -> [] }  Iavor S. Diatchki committed Sep 12, 2013 543 544 \end{code}