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 CmdLineOpts ( opt_UnboxStrictFields )
45 import Maybes ( orElse )
46 import ListSetOps ( assoc )
47 import Util ( zipEqual, zipWithEqual, notNull )
51 Data constructor representation
52 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
53 Consider the following Haskell data type declaration
55 data T = T !Int ![Int]
57 Using the strictness annotations, GHC will represent this as
61 That is, the Int has been unboxed. Furthermore, the Haskell source construction
71 That is, the first argument is unboxed, and the second is evaluated. Finally,
72 pattern matching is translated too:
74 case e of { T a b -> ... }
78 case e of { T a' b -> let a = I# a' in ... }
80 To keep ourselves sane, we name the different versions of the data constructor
81 differently, as follows.
84 Note [Data Constructor Naming]
85 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
86 Each data constructor C has two, and possibly three, Names associated with it:
88 OccName Name space Used for
89 ---------------------------------------------------------------------------
90 * The "source data con" C DataName The DataCon itself
91 * The "real data con" C VarName Its worker Id
92 * The "wrapper data con" $wC VarName Wrapper Id (optional)
94 Each of these three has a distinct Unique. The "source data con" name
95 appears in the output of the renamer, and names the Haskell-source
96 data constructor. The type checker translates it into either the wrapper Id
97 (if it exists) or worker Id (otherwise).
99 The data con has one or two Ids associated with it:
101 The "worker Id", is the actual data constructor.
102 Its type may be different to the Haskell source constructor
104 - useless dict args are dropped
105 - strict args may be flattened
106 The worker is very like a primop, in that it has no binding.
108 Newtypes currently do get a worker-Id, but it is never used.
111 The "wrapper Id", $wC, whose type is exactly what it looks like
112 in the source program. It is an ordinary function,
113 and it gets a top-level binding like any other function.
115 The wrapper Id isn't generated for a data type if the worker
116 and wrapper are identical. It's always generated for a newtype.
120 A note about the stupid context
121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
122 Data types can have a context:
124 data (Eq a, Ord b) => T a b = T1 a b | T2 a
126 and that makes the constructors have a context too
127 (notice that T2's context is "thinned"):
129 T1 :: (Eq a, Ord b) => a -> b -> T a b
130 T2 :: (Eq a) => a -> T a b
132 Furthermore, this context pops up when pattern matching
133 (though GHC hasn't implemented this, but it is in H98, and
134 I've fixed GHC so that it now does):
138 f :: Eq a => T a b -> a
140 I say the context is "stupid" because the dictionaries passed
141 are immediately discarded -- they do nothing and have no benefit.
142 It's a flaw in the language.
144 Up to now [March 2002] I have put this stupid context into the type of
145 the "wrapper" constructors functions, T1 and T2, but that turned out
146 to be jolly inconvenient for generics, and record update, and other
147 functions that build values of type T (because they don't have
148 suitable dictionaries available).
150 So now I've taken the stupid context out. I simply deal with it
151 separately in the type checker on occurrences of a constructor, either
152 in an expression or in a pattern.
156 %************************************************************************
158 \subsection{Data constructors}
160 %************************************************************************
164 = MkData { -- Used for data constructors only;
165 -- there *is* no constructor for a newtype
168 dcUnique :: Unique, -- Cached from Name
173 -- data Eq a => T a = forall b. Ord b => MkT a [b]
175 dcRepType :: Type, -- Type of the constructor
176 -- forall a b . Ord b => a -> [b] -> MkT a
177 -- (this is *not* of the constructor wrapper Id:
178 -- see notes after this data type declaration)
180 -- Notice that the existential type parameters come *second*.
181 -- Reason: in a case expression we may find:
182 -- case (e :: T t) of { MkT b (d:Ord b) (x:t) (xs:[b]) -> ... }
183 -- It's convenient to apply the rep-type of MkT to 't', to get
184 -- forall b. Ord b => ...
185 -- and use that to check the pattern. Mind you, this is really only
189 -- The next six fields express the type of the constructor, in pieces
195 -- dcExTheta = [Ord b]
196 -- dcOrigArgTys = [a,List b]
199 dcTyVars :: [TyVar], -- Type vars for the data type decl
200 -- These are ALWAYS THE SAME AS THE TYVARS
201 -- FOR THE PARENT TyCon. We occasionally rely on
202 -- this just to avoid redundant instantiation
204 dcStupidTheta :: ThetaType, -- This is a "thinned" version of the context of
206 -- "Thinned", because the Report says
207 -- to eliminate any constraints that don't mention
208 -- tyvars free in the arg types for this constructor
210 -- "Stupid", because the dictionaries aren't used for anything.
212 -- Indeed, [as of March 02] they are no
213 -- longer in the type of the dcWrapId, because
214 -- that makes it harder to use the wrap-id to rebuild
215 -- values after record selection or in generics.
217 dcExTyVars :: [TyVar], -- Ditto for the context of the constructor,
218 dcExTheta :: ThetaType, -- the existentially quantified stuff
220 dcOrigArgTys :: [Type], -- Original argument types
221 -- (before unboxing and flattening of
224 dcRepArgTys :: [Type], -- Final, representation argument types, after unboxing and flattening,
225 -- and including existential dictionaries
227 dcRepStrictness :: [StrictnessMark], -- One for each representation argument
229 dcTyCon :: TyCon, -- Result tycon
231 -- Now the strictness annotations and field labels of the constructor
232 dcStrictMarks :: [StrictnessMark],
233 -- Strictness annotations as deduced by the compiler.
234 -- Has no MarkedUserStrict; they have been changed to MarkedStrict
235 -- or MarkedUnboxed by the compiler.
236 -- *Includes the existential dictionaries*
237 -- length = length dcExTheta + dataConSourceArity dataCon
239 dcFields :: [FieldLabel],
240 -- Field labels for this constructor, in the
241 -- same order as the argument types;
242 -- length = 0 (if not a record) or dataConSourceArity.
244 -- Finally, the curried worker function that corresponds to the constructor
245 -- It doesn't have an unfolding; the code generator saturates these Ids
246 -- and allocates a real constructor when it finds one.
248 -- An entirely separate wrapper function is built in TcTyDecls
250 dcWorkId :: Id, -- The corresponding worker Id
251 -- Takes dcRepArgTys as its arguments
252 -- Perhaps this should be a 'Maybe'; not reqd for newtype constructors
254 dcWrapId :: Maybe Id -- The wrapper Id, if it's necessary
255 -- It's deemed unnecessary if it performs the
262 fIRST_TAG = 1 -- Tags allocated from here for real constructors
265 The dcRepType field contains the type of the representation of a contructor
266 This may differ from the type of the contructor *Id* (built
267 by MkId.mkDataConId) for two reasons:
268 a) the constructor Id may be overloaded, but the dictionary isn't stored
269 e.g. data Eq a => T a = MkT a a
271 b) the constructor may store an unboxed version of a strict field.
273 Here's an example illustrating both:
274 data Ord a => T a = MkT Int! a
276 T :: Ord a => Int -> a -> T a
278 Trep :: Int# -> a -> T a
279 Actually, the unboxed part isn't implemented yet!
282 %************************************************************************
284 \subsection{Instances}
286 %************************************************************************
289 instance Eq DataCon where
290 a == b = getUnique a == getUnique b
291 a /= b = getUnique a /= getUnique b
293 instance Ord DataCon where
294 a <= b = getUnique a <= getUnique b
295 a < b = getUnique a < getUnique b
296 a >= b = getUnique a >= getUnique b
297 a > b = getUnique a > getUnique b
298 compare a b = getUnique a `compare` getUnique b
300 instance Uniquable DataCon where
303 instance NamedThing DataCon where
306 instance Outputable DataCon where
307 ppr con = ppr (dataConName con)
309 instance Show DataCon where
310 showsPrec p con = showsPrecSDoc p (ppr con)
314 %************************************************************************
316 \subsection{Construction}
318 %************************************************************************
322 -> [StrictnessMark] -> [FieldLabel]
323 -> [TyVar] -> ThetaType
324 -> [TyVar] -> ThetaType
326 -> Id -> Maybe Id -- Worker and possible wrapper
328 -- Can get the tag from the TyCon
331 arg_stricts -- Use [] to mean 'all non-strict'
333 tyvars theta ex_tyvars ex_theta orig_arg_tys tycon
337 con = MkData {dcName = name,
338 dcUnique = nameUnique name,
339 dcTyVars = tyvars, dcStupidTheta = theta,
340 dcOrigArgTys = orig_arg_tys,
341 dcRepArgTys = rep_arg_tys,
342 dcExTyVars = ex_tyvars, dcExTheta = ex_theta,
343 dcStrictMarks = real_stricts, dcRepStrictness = rep_arg_stricts,
344 dcFields = fields, dcTag = tag, dcTyCon = tycon, dcRepType = ty,
345 dcWorkId = work_id, dcWrapId = wrap_id}
347 -- Strictness marks for source-args
348 -- *after unboxing choices*,
349 -- but *including existential dictionaries*
351 -- The 'arg_stricts' passed to mkDataCon are simply those for the
352 -- source-language arguments. We add extra ones for the
353 -- dictionary arguments right here.
354 ex_dict_tys = mkPredTys ex_theta
355 real_stricts = map mk_dict_strict_mark ex_dict_tys ++
356 zipWith (chooseBoxingStrategy tycon)
358 (arg_stricts ++ repeat NotMarkedStrict)
359 real_arg_tys = ex_dict_tys ++ orig_arg_tys
361 -- Representation arguments and demands
362 (rep_arg_stricts, rep_arg_tys) = computeRep real_stricts real_arg_tys
364 tag = assoc "mkDataCon" (tyConDataCons tycon `zip` [fIRST_TAG..]) con
365 ty = mkForAllTys (tyvars ++ ex_tyvars)
366 (mkFunTys rep_arg_tys result_ty)
367 -- NB: the existential dict args are already in rep_arg_tys
369 result_ty = mkTyConApp tycon (mkTyVarTys tyvars)
371 mk_dict_strict_mark ty | isStrictType ty = MarkedStrict
372 | otherwise = NotMarkedStrict
376 dataConName :: DataCon -> Name
379 dataConTag :: DataCon -> ConTag
382 dataConTyCon :: DataCon -> TyCon
383 dataConTyCon = dcTyCon
385 dataConRepType :: DataCon -> Type
386 dataConRepType = dcRepType
388 dataConWorkId :: DataCon -> Id
389 dataConWorkId = dcWorkId
391 dataConWrapId_maybe :: DataCon -> Maybe Id
392 dataConWrapId_maybe = dcWrapId
394 dataConWrapId :: DataCon -> Id
395 -- Returns an Id which looks like the Haskell-source constructor
396 -- If there is no dcWrapId it's because there is no need for a
397 -- wrapper, so the worker is the Right Thing
398 dataConWrapId dc = dcWrapId dc `orElse` dcWorkId dc
400 dataConFieldLabels :: DataCon -> [FieldLabel]
401 dataConFieldLabels = dcFields
403 dataConStrictMarks :: DataCon -> [StrictnessMark]
404 dataConStrictMarks = dcStrictMarks
406 -- Number of type-instantiation arguments
407 -- All the remaining arguments of the DataCon are (notionally)
408 -- stored in the DataCon, and are matched in a case expression
409 dataConNumInstArgs (MkData {dcTyVars = tyvars}) = length tyvars
411 dataConSourceArity :: DataCon -> Arity
412 -- Source-level arity of the data constructor
413 dataConSourceArity dc = length (dcOrigArgTys dc)
415 -- dataConRepArity gives the number of actual fields in the
416 -- {\em representation} of the data constructor. This may be more than appear
417 -- in the source code; the extra ones are the existentially quantified
419 dataConRepArity (MkData {dcRepArgTys = arg_tys}) = length arg_tys
421 isNullaryDataCon con = dataConRepArity con == 0
423 dataConRepStrictness :: DataCon -> [StrictnessMark]
424 -- Give the demands on the arguments of a
425 -- Core constructor application (Con dc args)
426 dataConRepStrictness dc = dcRepStrictness dc
428 dataConSig :: DataCon -> ([TyVar], ThetaType,
432 dataConSig (MkData {dcTyVars = tyvars, dcStupidTheta = theta,
433 dcExTyVars = ex_tyvars, dcExTheta = ex_theta,
434 dcOrigArgTys = arg_tys, dcTyCon = tycon})
435 = (tyvars, theta, ex_tyvars, ex_theta, arg_tys, tycon)
437 dataConArgTys :: DataCon
438 -> [Type] -- Instantiated at these types
439 -- NB: these INCLUDE the existentially quantified arg types
440 -> [Type] -- Needs arguments of these types
441 -- NB: these INCLUDE the existentially quantified dict args
442 -- but EXCLUDE the data-decl context which is discarded
443 -- It's all post-flattening etc; this is a representation type
445 dataConArgTys (MkData {dcRepArgTys = arg_tys, dcTyVars = tyvars,
446 dcExTyVars = ex_tyvars}) inst_tys
447 = map (substTyWith (tyvars ++ ex_tyvars) inst_tys) arg_tys
449 dataConTheta :: DataCon -> ThetaType
450 dataConTheta dc = dcStupidTheta dc
452 dataConExistentialTyVars :: DataCon -> [TyVar]
453 dataConExistentialTyVars dc = dcExTyVars dc
455 -- And the same deal for the original arg tys:
457 dataConInstOrigArgTys :: DataCon -> [Type] -> [Type]
458 dataConInstOrigArgTys (MkData {dcOrigArgTys = arg_tys, dcTyVars = tyvars,
459 dcExTyVars = ex_tyvars}) inst_tys
460 = map (substTyWith (tyvars ++ ex_tyvars) inst_tys) arg_tys
463 These two functions get the real argument types of the constructor,
464 without substituting for any type variables.
466 dataConOrigArgTys returns the arg types of the wrapper, excluding all dictionary args.
468 dataConRepArgTys retuns the arg types of the worker, including all dictionaries, and
469 after any flattening has been done.
472 dataConOrigArgTys :: DataCon -> [Type]
473 dataConOrigArgTys dc = dcOrigArgTys dc
475 dataConRepArgTys :: DataCon -> [Type]
476 dataConRepArgTys dc = dcRepArgTys dc
481 isTupleCon :: DataCon -> Bool
482 isTupleCon (MkData {dcTyCon = tc}) = isTupleTyCon tc
484 isUnboxedTupleCon :: DataCon -> Bool
485 isUnboxedTupleCon (MkData {dcTyCon = tc}) = isUnboxedTupleTyCon tc
487 isExistentialDataCon :: DataCon -> Bool
488 isExistentialDataCon (MkData {dcExTyVars = tvs}) = notNull tvs
493 classDataCon :: Class -> DataCon
494 classDataCon clas = case tyConDataCons (classTyCon clas) of
495 (dict_constr:no_more) -> ASSERT( null no_more ) dict_constr
498 %************************************************************************
500 \subsection{Splitting products}
502 %************************************************************************
505 splitProductType_maybe
506 :: Type -- A product type, perhaps
507 -> Maybe (TyCon, -- The type constructor
508 [Type], -- Type args of the tycon
509 DataCon, -- The data constructor
510 [Type]) -- Its *representation* arg types
512 -- Returns (Just ...) for any
513 -- concrete (i.e. constructors visible)
514 -- single-constructor
515 -- not existentially quantified
516 -- type whether a data type or a new type
518 -- Rejecing existentials is conservative. Maybe some things
519 -- could be made to work with them, but I'm not going to sweat
520 -- it through till someone finds it's important.
522 splitProductType_maybe ty
523 = case splitTyConApp_maybe ty of
525 | isProductTyCon tycon -- Includes check for non-existential,
526 -- and for constructors visible
527 -> Just (tycon, ty_args, data_con, dataConArgTys data_con ty_args)
529 data_con = head (tyConDataCons tycon)
532 splitProductType str ty
533 = case splitProductType_maybe ty of
535 Nothing -> pprPanic (str ++ ": not a product") (pprType ty)
537 -- We attempt to unbox/unpack a strict field when either:
538 -- (i) The tycon is imported, and the field is marked '! !', or
539 -- (ii) The tycon is defined in this module, the field is marked '!',
540 -- and the -funbox-strict-fields flag is on.
542 -- This ensures that if we compile some modules with -funbox-strict-fields and
543 -- some without, the compiler doesn't get confused about the constructor
546 chooseBoxingStrategy :: TyCon -> Type -> StrictnessMark -> StrictnessMark
547 -- Transforms any MarkedUserStricts into MarkUnboxed or MarkedStrict
548 chooseBoxingStrategy tycon arg_ty strict
551 | opt_UnboxStrictFields
552 && unbox arg_ty -> MarkedUnboxed
553 | otherwise -> MarkedStrict
556 -- beware: repType will go into a loop if we try this on a recursive
557 -- type (for reasons unknown...), hence the check for recursion below.
559 case splitTyConApp_maybe ty of
562 | isRecursiveTyCon arg_tycon -> False
564 case splitTyConApp_maybe (repType ty) of
566 Just (arg_tycon, _) -> isProductTyCon arg_tycon
568 computeRep :: [StrictnessMark] -- Original arg strictness
569 -- [after strategy choice; can't be MarkedUserStrict]
570 -> [Type] -- and types
571 -> ([StrictnessMark], -- Representation arg strictness
574 computeRep stricts tys
575 = unzip $ concat $ zipWithEqual "computeRep" unbox stricts tys
577 unbox NotMarkedStrict ty = [(NotMarkedStrict, ty)]
578 unbox MarkedStrict ty = [(MarkedStrict, ty)]
579 unbox MarkedUnboxed ty = zipEqual "computeRep" (dataConRepStrictness arg_dc) arg_tys
581 (_, _, arg_dc, arg_tys) = splitProductType "unbox_strict_arg_ty" (repType ty)