2 % (c) The GRASP/AQUA Project, Glasgow University, 1998
4 \section[DataCon]{@DataCon@: Data Constructors}
11 dataConRepType, dataConSig, dataConName, dataConTag, dataConTyCon,
12 dataConArgTys, dataConOrigArgTys, dataConInstOrigArgTys,
13 dataConRepArgTys, dataConTheta,
14 dataConFieldLabels, dataConStrictMarks,
15 dataConSourceArity, dataConRepArity,
17 dataConWorkId, dataConWrapId, dataConWrapId_maybe,
19 isNullaryDataCon, isTupleCon, isUnboxedTupleCon,
20 isExistentialDataCon, classDataCon, dataConExistentialTyVars,
22 splitProductType_maybe, splitProductType,
25 #include "HsVersions.h"
27 import {-# SOURCE #-} Subst( substTyWith )
28 import {-# SOURCE #-} PprType( pprType )
30 import Type ( Type, ThetaType,
31 mkForAllTys, mkFunTys, mkTyConApp,
32 mkTyVarTys, splitTyConApp_maybe, repType,
33 mkPredTys, isStrictType
35 import TyCon ( TyCon, tyConDataCons, tyConDataCons, isProductTyCon,
36 isTupleTyCon, isUnboxedTupleTyCon, isRecursiveTyCon )
37 import Class ( Class, classTyCon )
38 import Name ( Name, NamedThing(..), nameUnique )
39 import Var ( TyVar, Id )
40 import FieldLabel ( FieldLabel )
41 import BasicTypes ( Arity, StrictnessMark(..) )
43 import Unique ( Unique, Uniquable(..) )
44 import Maybes ( orElse )
45 import ListSetOps ( assoc )
46 import Util ( zipEqual, zipWithEqual, notNull )
50 Data constructor representation
51 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
52 Consider the following Haskell data type declaration
54 data T = T !Int ![Int]
56 Using the strictness annotations, GHC will represent this as
60 That is, the Int has been unboxed. Furthermore, the Haskell source construction
70 That is, the first argument is unboxed, and the second is evaluated. Finally,
71 pattern matching is translated too:
73 case e of { T a b -> ... }
77 case e of { T a' b -> let a = I# a' in ... }
79 To keep ourselves sane, we name the different versions of the data constructor
80 differently, as follows.
83 Note [Data Constructor Naming]
84 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
85 Each data constructor C has two, and possibly three, Names associated with it:
87 OccName Name space Used for
88 ---------------------------------------------------------------------------
89 * The "source data con" C DataName The DataCon itself
90 * The "real data con" C VarName Its worker Id
91 * The "wrapper data con" $wC VarName Wrapper Id (optional)
93 Each of these three has a distinct Unique. The "source data con" name
94 appears in the output of the renamer, and names the Haskell-source
95 data constructor. The type checker translates it into either the wrapper Id
96 (if it exists) or worker Id (otherwise).
98 The data con has one or two Ids associated with it:
100 The "worker Id", is the actual data constructor.
101 Its type may be different to the Haskell source constructor
103 - useless dict args are dropped
104 - strict args may be flattened
105 The worker is very like a primop, in that it has no binding.
107 Newtypes currently do get a worker-Id, but it is never used.
110 The "wrapper Id", $wC, whose type is exactly what it looks like
111 in the source program. It is an ordinary function,
112 and it gets a top-level binding like any other function.
114 The wrapper Id isn't generated for a data type if the worker
115 and wrapper are identical. It's always generated for a newtype.
119 A note about the stupid context
120 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
121 Data types can have a context:
123 data (Eq a, Ord b) => T a b = T1 a b | T2 a
125 and that makes the constructors have a context too
126 (notice that T2's context is "thinned"):
128 T1 :: (Eq a, Ord b) => a -> b -> T a b
129 T2 :: (Eq a) => a -> T a b
131 Furthermore, this context pops up when pattern matching
132 (though GHC hasn't implemented this, but it is in H98, and
133 I've fixed GHC so that it now does):
137 f :: Eq a => T a b -> a
139 I say the context is "stupid" because the dictionaries passed
140 are immediately discarded -- they do nothing and have no benefit.
141 It's a flaw in the language.
143 Up to now [March 2002] I have put this stupid context into the type of
144 the "wrapper" constructors functions, T1 and T2, but that turned out
145 to be jolly inconvenient for generics, and record update, and other
146 functions that build values of type T (because they don't have
147 suitable dictionaries available).
149 So now I've taken the stupid context out. I simply deal with it
150 separately in the type checker on occurrences of a constructor, either
151 in an expression or in a pattern.
153 [May 2003: actually I think this decision could evasily be reversed now,
154 and probably should be. Generics could be disabled for types with
155 a stupid context; record updates now (H98) needs the context too; etc.
156 It's an unforced change, so I'm leaving it for now --- but it does seem
157 odd that the wrapper doesn't include the stupid context.]
161 %************************************************************************
163 \subsection{Data constructors}
165 %************************************************************************
169 = MkData { -- Used for data constructors only;
170 -- there *is* no constructor for a newtype
172 dcName :: Name, -- This is the name of the *source data con*
173 -- (see "Note [Data Constructor Naming]" above)
175 dcUnique :: Unique, -- Cached from Name
180 -- data Eq a => T a = forall b. Ord b => MkT a [b]
182 dcRepType :: Type, -- Type of the constructor
183 -- forall a b . Ord b => a -> [b] -> MkT a
184 -- (this is *not* of the constructor wrapper Id:
185 -- see notes after this data type declaration)
187 -- Notice that the existential type parameters come *second*.
188 -- Reason: in a case expression we may find:
189 -- case (e :: T t) of { MkT b (d:Ord b) (x:t) (xs:[b]) -> ... }
190 -- It's convenient to apply the rep-type of MkT to 't', to get
191 -- forall b. Ord b => ...
192 -- and use that to check the pattern. Mind you, this is really only
196 -- The next six fields express the type of the constructor, in pieces
202 -- dcExTheta = [Ord b]
203 -- dcOrigArgTys = [a,List b]
206 dcTyVars :: [TyVar], -- Type vars for the data type decl
207 -- These are ALWAYS THE SAME AS THE TYVARS
208 -- FOR THE PARENT TyCon. We occasionally rely on
209 -- this just to avoid redundant instantiation
211 dcStupidTheta :: ThetaType, -- This is a "thinned" version of the context of
213 -- "Thinned", because the Report says
214 -- to eliminate any constraints that don't mention
215 -- tyvars free in the arg types for this constructor
217 -- "Stupid", because the dictionaries aren't used for anything.
219 -- Indeed, [as of March 02] they are no
220 -- longer in the type of the dcWrapId, because
221 -- that makes it harder to use the wrap-id to rebuild
222 -- values after record selection or in generics.
224 dcExTyVars :: [TyVar], -- Ditto for the context of the constructor,
225 dcExTheta :: ThetaType, -- the existentially quantified stuff
227 dcOrigArgTys :: [Type], -- Original argument types
228 -- (before unboxing and flattening of
231 dcRepArgTys :: [Type], -- Final, representation argument types, after unboxing and flattening,
232 -- and including existential dictionaries
234 dcRepStrictness :: [StrictnessMark], -- One for each representation argument
236 dcTyCon :: TyCon, -- Result tycon
238 -- Now the strictness annotations and field labels of the constructor
239 dcStrictMarks :: [StrictnessMark],
240 -- Strictness annotations as deduced by the compiler.
241 -- Has no MarkedUserStrict; they have been changed to MarkedStrict
242 -- or MarkedUnboxed by the compiler.
243 -- *Includes the existential dictionaries*
244 -- length = length dcExTheta + dataConSourceArity dataCon
246 dcFields :: [FieldLabel],
247 -- Field labels for this constructor, in the
248 -- same order as the argument types;
249 -- length = 0 (if not a record) or dataConSourceArity.
251 -- Finally, the curried worker function that corresponds to the constructor
252 -- It doesn't have an unfolding; the code generator saturates these Ids
253 -- and allocates a real constructor when it finds one.
255 -- An entirely separate wrapper function is built in TcTyDecls
257 dcWorkId :: Id, -- The corresponding worker Id
258 -- Takes dcRepArgTys as its arguments
259 -- Perhaps this should be a 'Maybe'; not reqd for newtype constructors
261 dcWrapId :: Maybe Id -- The wrapper Id, if it's necessary
262 -- It's deemed unnecessary if it performs the
269 fIRST_TAG = 1 -- Tags allocated from here for real constructors
272 The dcRepType field contains the type of the representation of a contructor
273 This may differ from the type of the contructor *Id* (built
274 by MkId.mkDataConId) for two reasons:
275 a) the constructor Id may be overloaded, but the dictionary isn't stored
276 e.g. data Eq a => T a = MkT a a
278 b) the constructor may store an unboxed version of a strict field.
280 Here's an example illustrating both:
281 data Ord a => T a = MkT Int! a
283 T :: Ord a => Int -> a -> T a
285 Trep :: Int# -> a -> T a
286 Actually, the unboxed part isn't implemented yet!
289 %************************************************************************
291 \subsection{Instances}
293 %************************************************************************
296 instance Eq DataCon where
297 a == b = getUnique a == getUnique b
298 a /= b = getUnique a /= getUnique b
300 instance Ord DataCon where
301 a <= b = getUnique a <= getUnique b
302 a < b = getUnique a < getUnique b
303 a >= b = getUnique a >= getUnique b
304 a > b = getUnique a > getUnique b
305 compare a b = getUnique a `compare` getUnique b
307 instance Uniquable DataCon where
310 instance NamedThing DataCon where
313 instance Outputable DataCon where
314 ppr con = ppr (dataConName con)
316 instance Show DataCon where
317 showsPrec p con = showsPrecSDoc p (ppr con)
321 %************************************************************************
323 \subsection{Construction}
325 %************************************************************************
329 -> [StrictnessMark] -> [FieldLabel]
330 -> [TyVar] -> ThetaType
331 -> [TyVar] -> ThetaType
333 -> Id -> Maybe Id -- Worker and possible wrapper
335 -- Can get the tag from the TyCon
338 arg_stricts -- Use [] to mean 'all non-strict'
340 tyvars theta ex_tyvars ex_theta orig_arg_tys tycon
344 con = MkData {dcName = name,
345 dcUnique = nameUnique name,
346 dcTyVars = tyvars, dcStupidTheta = theta,
347 dcOrigArgTys = orig_arg_tys,
348 dcRepArgTys = rep_arg_tys,
349 dcExTyVars = ex_tyvars, dcExTheta = ex_theta,
350 dcStrictMarks = real_stricts, dcRepStrictness = rep_arg_stricts,
351 dcFields = fields, dcTag = tag, dcTyCon = tycon, dcRepType = ty,
352 dcWorkId = work_id, dcWrapId = wrap_id}
354 -- Strictness marks for source-args
355 -- *after unboxing choices*,
356 -- but *including existential dictionaries*
358 -- The 'arg_stricts' passed to mkDataCon are simply those for the
359 -- source-language arguments. We add extra ones for the
360 -- dictionary arguments right here.
361 ex_dict_tys = mkPredTys ex_theta
362 real_stricts = map mk_dict_strict_mark ex_dict_tys ++
363 zipWith (chooseBoxingStrategy tycon)
365 (arg_stricts ++ repeat NotMarkedStrict)
366 real_arg_tys = ex_dict_tys ++ orig_arg_tys
368 -- Representation arguments and demands
369 (rep_arg_stricts, rep_arg_tys) = computeRep real_stricts real_arg_tys
371 tag = assoc "mkDataCon" (tyConDataCons tycon `zip` [fIRST_TAG..]) con
372 ty = mkForAllTys (tyvars ++ ex_tyvars)
373 (mkFunTys rep_arg_tys result_ty)
374 -- NB: the existential dict args are already in rep_arg_tys
376 result_ty = mkTyConApp tycon (mkTyVarTys tyvars)
378 mk_dict_strict_mark ty | isStrictType ty = MarkedStrict
379 | otherwise = NotMarkedStrict
383 dataConName :: DataCon -> Name
386 dataConTag :: DataCon -> ConTag
389 dataConTyCon :: DataCon -> TyCon
390 dataConTyCon = dcTyCon
392 dataConRepType :: DataCon -> Type
393 dataConRepType = dcRepType
395 dataConWorkId :: DataCon -> Id
396 dataConWorkId = dcWorkId
398 dataConWrapId_maybe :: DataCon -> Maybe Id
399 dataConWrapId_maybe = dcWrapId
401 dataConWrapId :: DataCon -> Id
402 -- Returns an Id which looks like the Haskell-source constructor
403 -- If there is no dcWrapId it's because there is no need for a
404 -- wrapper, so the worker is the Right Thing
405 dataConWrapId dc = dcWrapId dc `orElse` dcWorkId dc
407 dataConFieldLabels :: DataCon -> [FieldLabel]
408 dataConFieldLabels = dcFields
410 dataConStrictMarks :: DataCon -> [StrictnessMark]
411 dataConStrictMarks = dcStrictMarks
413 -- Number of type-instantiation arguments
414 -- All the remaining arguments of the DataCon are (notionally)
415 -- stored in the DataCon, and are matched in a case expression
416 dataConNumInstArgs (MkData {dcTyVars = tyvars}) = length tyvars
418 dataConSourceArity :: DataCon -> Arity
419 -- Source-level arity of the data constructor
420 dataConSourceArity dc = length (dcOrigArgTys dc)
422 -- dataConRepArity gives the number of actual fields in the
423 -- {\em representation} of the data constructor. This may be more than appear
424 -- in the source code; the extra ones are the existentially quantified
426 dataConRepArity (MkData {dcRepArgTys = arg_tys}) = length arg_tys
428 isNullaryDataCon con = dataConRepArity con == 0
430 dataConRepStrictness :: DataCon -> [StrictnessMark]
431 -- Give the demands on the arguments of a
432 -- Core constructor application (Con dc args)
433 dataConRepStrictness dc = dcRepStrictness dc
435 dataConSig :: DataCon -> ([TyVar], ThetaType,
439 dataConSig (MkData {dcTyVars = tyvars, dcStupidTheta = theta,
440 dcExTyVars = ex_tyvars, dcExTheta = ex_theta,
441 dcOrigArgTys = arg_tys, dcTyCon = tycon})
442 = (tyvars, theta, ex_tyvars, ex_theta, arg_tys, tycon)
444 dataConArgTys :: DataCon
445 -> [Type] -- Instantiated at these types
446 -- NB: these INCLUDE the existentially quantified arg types
447 -> [Type] -- Needs arguments of these types
448 -- NB: these INCLUDE the existentially quantified dict args
449 -- but EXCLUDE the data-decl context which is discarded
450 -- It's all post-flattening etc; this is a representation type
452 dataConArgTys (MkData {dcRepArgTys = arg_tys, dcTyVars = tyvars,
453 dcExTyVars = ex_tyvars}) inst_tys
454 = map (substTyWith (tyvars ++ ex_tyvars) inst_tys) arg_tys
456 dataConTheta :: DataCon -> ThetaType
457 dataConTheta dc = dcStupidTheta dc
459 dataConExistentialTyVars :: DataCon -> [TyVar]
460 dataConExistentialTyVars dc = dcExTyVars dc
462 -- And the same deal for the original arg tys:
464 dataConInstOrigArgTys :: DataCon -> [Type] -> [Type]
465 dataConInstOrigArgTys (MkData {dcOrigArgTys = arg_tys, dcTyVars = tyvars,
466 dcExTyVars = ex_tyvars}) inst_tys
467 = map (substTyWith (tyvars ++ ex_tyvars) inst_tys) arg_tys
470 These two functions get the real argument types of the constructor,
471 without substituting for any type variables.
473 dataConOrigArgTys returns the arg types of the wrapper, excluding all dictionary args.
475 dataConRepArgTys retuns the arg types of the worker, including all dictionaries, and
476 after any flattening has been done.
479 dataConOrigArgTys :: DataCon -> [Type]
480 dataConOrigArgTys dc = dcOrigArgTys dc
482 dataConRepArgTys :: DataCon -> [Type]
483 dataConRepArgTys dc = dcRepArgTys dc
488 isTupleCon :: DataCon -> Bool
489 isTupleCon (MkData {dcTyCon = tc}) = isTupleTyCon tc
491 isUnboxedTupleCon :: DataCon -> Bool
492 isUnboxedTupleCon (MkData {dcTyCon = tc}) = isUnboxedTupleTyCon tc
494 isExistentialDataCon :: DataCon -> Bool
495 isExistentialDataCon (MkData {dcExTyVars = tvs}) = notNull tvs
500 classDataCon :: Class -> DataCon
501 classDataCon clas = case tyConDataCons (classTyCon clas) of
502 (dict_constr:no_more) -> ASSERT( null no_more ) dict_constr
505 %************************************************************************
507 \subsection{Splitting products}
509 %************************************************************************
512 splitProductType_maybe
513 :: Type -- A product type, perhaps
514 -> Maybe (TyCon, -- The type constructor
515 [Type], -- Type args of the tycon
516 DataCon, -- The data constructor
517 [Type]) -- Its *representation* arg types
519 -- Returns (Just ...) for any
520 -- concrete (i.e. constructors visible)
521 -- single-constructor
522 -- not existentially quantified
523 -- type whether a data type or a new type
525 -- Rejecing existentials is conservative. Maybe some things
526 -- could be made to work with them, but I'm not going to sweat
527 -- it through till someone finds it's important.
529 splitProductType_maybe ty
530 = case splitTyConApp_maybe ty of
532 | isProductTyCon tycon -- Includes check for non-existential,
533 -- and for constructors visible
534 -> Just (tycon, ty_args, data_con, dataConArgTys data_con ty_args)
536 data_con = head (tyConDataCons tycon)
539 splitProductType str ty
540 = case splitProductType_maybe ty of
542 Nothing -> pprPanic (str ++ ": not a product") (pprType ty)
544 -- We attempt to unbox/unpack a strict field when either:
545 -- (i) The tycon is imported, and the field is marked '! !', or
546 -- (ii) The tycon is defined in this module, the field is marked '!',
547 -- and the -funbox-strict-fields flag is on.
549 -- This ensures that if we compile some modules with -funbox-strict-fields and
550 -- some without, the compiler doesn't get confused about the constructor
553 chooseBoxingStrategy :: TyCon -> Type -> StrictnessMark -> StrictnessMark
554 -- Transforms any MarkedUserStricts into MarkUnboxed or MarkedStrict
555 chooseBoxingStrategy tycon arg_ty strict
557 MarkedUserStrict -> MarkedStrict
559 | can_unbox -> MarkedUnboxed
560 | otherwise -> MarkedStrict
563 can_unbox = unbox arg_ty
564 -- beware: repType will go into a loop if we try this on a recursive
565 -- type (for reasons unknown...), hence the check for recursion below.
567 case splitTyConApp_maybe ty of
570 | isRecursiveTyCon arg_tycon -> False
572 case splitTyConApp_maybe (repType ty) of
574 Just (arg_tycon, _) -> isProductTyCon arg_tycon
576 computeRep :: [StrictnessMark] -- Original arg strictness
577 -- [after strategy choice; can't be MarkedUserStrict]
578 -> [Type] -- and types
579 -> ([StrictnessMark], -- Representation arg strictness
582 computeRep stricts tys
583 = unzip $ concat $ zipWithEqual "computeRep" unbox stricts tys
585 unbox NotMarkedStrict ty = [(NotMarkedStrict, ty)]
586 unbox MarkedStrict ty = [(MarkedStrict, ty)]
587 unbox MarkedUnboxed ty = zipEqual "computeRep" (dataConRepStrictness arg_dc) arg_tys
589 (_, _, arg_dc, arg_tys) = splitProductType "unbox_strict_arg_ty" (repType ty)