2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1999
6 Analysis functions over data types. Specficially, detecting recursive types.
8 This stuff is only used for source-code decls; it's recorded in interface
9 files for imported data types.
14 calcClassCycles, calcSynCycles
17 #include "HsVersions.h"
37 %************************************************************************
39 Cycles in class and type synonym declarations
41 %************************************************************************
43 Checking for class-decl loops is easy, because we don't allow class decls
46 We allow type synonyms in hi-boot files, but we *trust* hi-boot files,
47 so we don't check for loops that involve them. So we only look for synonym
48 loops in the module being compiled.
50 We check for type synonym and class cycles on the *source* code.
53 a) Otherwise we'd need a special function to extract type-synonym tycons
54 from a type, whereas we have extractHsTyNames already
56 b) If we checked for type synonym loops after building the TyCon, we
57 can't do a hoistForAllTys on the type synonym rhs, (else we fall into
58 a black hole) which seems unclean. Apart from anything else, it'd mean
59 that a type-synonym rhs could have for-alls to the right of an arrow,
60 which means adding new cases to the validity checker
62 Indeed, in general, checking for cycles beforehand means we need to
63 be less careful about black holes through synonym cycles.
65 The main disadvantage is that a cycle that goes via a type synonym in an
66 .hi-boot file can lead the compiler into a loop, because it assumes that cycles
67 only occur entirely within the source code of the module being compiled.
68 But hi-boot files are trusted anyway, so this isn't much worse than (say)
71 [ NOTE ----------------------------------------------
72 If we reverse this decision, this comment came from tcTyDecl1, and should
74 -- dsHsType, not tcHsKindedType, to avoid a loop. tcHsKindedType does hoisting,
75 -- which requires looking through synonyms... and therefore goes into a loop
76 -- on (erroneously) recursive synonyms.
77 -- Solution: do not hoist synonyms, because they'll be hoisted soon enough
78 -- when they are substituted
80 We'd also need to add back in this definition
82 synTyConsOfType :: Type -> [TyCon]
83 -- Does not look through type synonyms at all
84 -- Return a list of synonym tycons
88 go :: Type -> NameEnv TyCon -- The NameEnv does duplicate elim
89 go (TyVarTy v) = emptyNameEnv
90 go (TyConApp tc tys) = go_tc tc tys
91 go (AppTy a b) = go a `plusNameEnv` go b
92 go (FunTy a b) = go a `plusNameEnv` go b
93 go (PredTy (IParam _ ty)) = go ty
94 go (PredTy (ClassP cls tys)) = go_s tys -- Ignore class
95 go (NoteTy _ ty) = go ty
96 go (ForAllTy _ ty) = go ty
98 go_tc tc tys | isSynTyCon tc = extendNameEnv (go_s tys) (tyConName tc) tc
99 | otherwise = go_s tys
100 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys
101 ---------------------------------------- END NOTE ]
104 calcSynCycles :: [LTyClDecl Name] -> [SCC (LTyClDecl Name)]
106 = stronglyConnComp syn_edges
108 syn_edges = [ (ldecl, unLoc (tcdLName decl),
109 mk_syn_edges (tcdSynRhs decl))
110 | ldecl@(L _ decl) <- decls ]
112 mk_syn_edges rhs = [ tc | tc <- nameSetToList (extractHsTyNames rhs),
113 not (isTyVarName tc) ]
116 calcClassCycles :: [LTyClDecl Name] -> [[LTyClDecl Name]]
117 calcClassCycles decls
118 = [decls | CyclicSCC decls <- stronglyConnComp cls_edges]
120 cls_edges = [ (ldecl, unLoc (tcdLName decl),
121 mk_cls_edges (unLoc (tcdCtxt decl)))
122 | ldecl@(L _ decl) <- decls, isClassDecl decl ]
124 mk_cls_edges ctxt = [ cls | L _ (HsClassP cls _) <- ctxt ]
128 %************************************************************************
130 Deciding which type constructors are recursive
132 %************************************************************************
134 For newtypes, we label some as "recursive" such that
136 INVARIANT: there is no cycle of non-recursive newtypes
138 In any loop, only one newtype need be marked as recursive; it is
139 a "loop breaker". Labelling more than necessary as recursive is OK,
140 provided the invariant is maintained.
142 A newtype M.T is defined to be "recursive" iff
143 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
144 (b) it is declared in a source file, but that source file has a
145 companion hi-boot file which declares the type
146 or (c) one can get from T's rhs to T via type
147 synonyms, or non-recursive newtypes *in M*
148 e.g. newtype T = MkT (T -> Int)
150 (a) is conservative; declarations in hi-boot files are always
151 made loop breakers. That's why in (b) we can restrict attention
152 to tycons in M, because any loops through newtypes outside M
153 will be broken by those newtypes
154 (b) ensures that a newtype is not treated as a loop breaker in one place
155 and later as a non-loop-breaker. This matters in GHCi particularly, when
156 a newtype T might be embedded in many types in the environment, and then
157 T's source module is compiled. We don't want T's recursiveness to change.
159 The "recursive" flag for algebraic data types is irrelevant (never consulted)
160 for types with more than one constructor.
162 An algebraic data type M.T is "recursive" iff
163 it has just one constructor, and
164 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
165 (b) it is declared in a source file, but that source file has a
166 companion hi-boot file which declares the type
167 or (c) one can get from its arg types to T via type synonyms,
168 or by non-recursive newtypes or non-recursive product types in M
169 e.g. data T = MkT (T -> Int) Bool
170 Just like newtype in fact
172 A type synonym is recursive if one can get from its
173 right hand side back to it via type synonyms. (This is
174 reported as an error.)
176 A class is recursive if one can get from its superclasses
177 back to it. (This is an error too.)
181 A data type read from an hi-boot file will have an AbstractTyCon as its AlgTyConRhs
182 and will respond True to isHiBootTyCon. The idea is that we treat these as if one
183 could get from these types to anywhere. So when we see
186 import {-# SOURCE #-} Foo( T )
189 then we mark S as recursive, just in case. What that means is that if we see
194 then we don't need to look inside S to compute R's recursiveness. Since S is imported
195 (not from an hi-boot file), one cannot get from R back to S except via an hi-boot file,
196 and that means that some data type will be marked recursive along the way. So R is
197 unconditionly non-recursive (i.e. there'll be a loop breaker elsewhere if necessary)
199 This in turn means that we grovel through fewer interface files when computing
200 recursiveness, because we need only look at the type decls in the module being
201 compiled, plus the outer structure of directly-mentioned types.
204 calcRecFlags :: ModDetails -> [TyThing] -> (Name -> RecFlag)
205 -- The 'boot_names' are the things declared in M.hi-boot, if M is the current module.
206 -- Any type constructors in boot_names are automatically considered loop breakers
207 calcRecFlags boot_details tyclss
210 is_rec n | n `elemNameSet` rec_names = Recursive
211 | otherwise = NonRecursive
213 boot_name_set = availsToNameSet (md_exports boot_details)
214 rec_names = boot_name_set `unionNameSets`
215 nt_loop_breakers `unionNameSets`
218 all_tycons = [ tc | tycls <- tyclss,
219 -- Recursion of newtypes/data types can happen via
220 -- the class TyCon, so tyclss includes the class tycons
221 let tc = getTyCon tycls,
222 not (tyConName tc `elemNameSet` boot_name_set) ]
223 -- Remove the boot_name_set because they are going
224 -- to be loop breakers regardless.
226 -------------------------------------------------
228 -- These edge-construction loops rely on
229 -- every loop going via tyclss, the types and classes
230 -- in the module being compiled. Stuff in interface
231 -- files should be correctly marked. If not (e.g. a
232 -- type synonym in a hi-boot file) we can get an infinite
233 -- loop. We could program round this, but it'd make the code
234 -- rather less nice, so I'm not going to do that yet.
236 --------------- Newtypes ----------------------
237 new_tycons = filter isNewTyConAndNotOpen all_tycons
238 isNewTyConAndNotOpen tycon = isNewTyCon tycon && not (isOpenTyCon tycon)
239 nt_loop_breakers = mkNameSet (findLoopBreakers nt_edges)
240 is_rec_nt tc = tyConName tc `elemNameSet` nt_loop_breakers
241 -- is_rec_nt is a locally-used helper function
243 nt_edges = [(t, mk_nt_edges t) | t <- new_tycons]
245 mk_nt_edges nt -- Invariant: nt is a newtype
246 = concatMap (mk_nt_edges1 nt) (tcTyConsOfType (new_tc_rhs nt))
247 -- tyConsOfType looks through synonyms
250 | tc `elem` new_tycons = [tc] -- Loop
251 -- At this point we know that either it's a local *data* type,
252 -- or it's imported. Either way, it can't form part of a newtype cycle
255 --------------- Product types ----------------------
256 -- The "prod_tycons" are the non-newtype products
257 prod_tycons = [tc | tc <- all_tycons,
258 not (isNewTyCon tc), isProductTyCon tc]
259 prod_loop_breakers = mkNameSet (findLoopBreakers prod_edges)
261 prod_edges = [(tc, mk_prod_edges tc) | tc <- prod_tycons]
263 mk_prod_edges tc -- Invariant: tc is a product tycon
264 = concatMap (mk_prod_edges1 tc) (dataConOrigArgTys (head (tyConDataCons tc)))
266 mk_prod_edges1 ptc ty = concatMap (mk_prod_edges2 ptc) (tcTyConsOfType ty)
268 mk_prod_edges2 ptc tc
269 | tc `elem` prod_tycons = [tc] -- Local product
270 | tc `elem` new_tycons = if is_rec_nt tc -- Local newtype
272 else mk_prod_edges1 ptc (new_tc_rhs tc)
273 -- At this point we know that either it's a local non-product data type,
274 -- or it's imported. Either way, it can't form part of a cycle
277 new_tc_rhs tc = snd (newTyConRhs tc) -- Ignore the type variables
279 getTyCon (ATyCon tc) = tc
280 getTyCon (AClass cl) = classTyCon cl
281 getTyCon other = panic "getTyCon"
283 findLoopBreakers :: [(TyCon, [TyCon])] -> [Name]
284 -- Finds a set of tycons that cut all loops
285 findLoopBreakers deps
286 = go [(tc,tc,ds) | (tc,ds) <- deps]
289 | CyclicSCC ((tc,_,_) : edges') <- stronglyConnCompR edges,
290 name <- tyConName tc : go edges']
293 These two functions know about type representations, so they could be
294 in Type or TcType -- but they are very specialised to this module, so
295 I've chosen to put them here.
298 tcTyConsOfType :: Type -> [TyCon]
299 -- tcTyConsOfType looks through all synonyms, but not through any newtypes.
300 -- When it finds a Class, it returns the class TyCon. The reaons it's here
301 -- (not in Type.lhs) is because it is newtype-aware.
303 = nameEnvElts (go ty)
305 go :: Type -> NameEnv TyCon -- The NameEnv does duplicate elim
306 go ty | Just ty' <- tcView ty = go ty'
307 go (TyVarTy v) = emptyNameEnv
308 go (TyConApp tc tys) = go_tc tc tys
309 go (AppTy a b) = go a `plusNameEnv` go b
310 go (FunTy a b) = go a `plusNameEnv` go b
311 go (PredTy (IParam _ ty)) = go ty
312 go (PredTy (ClassP cls tys)) = go_tc (classTyCon cls) tys
313 go (ForAllTy _ ty) = go ty
314 go other = panic "tcTyConsOfType"
316 go_tc tc tys = extendNameEnv (go_s tys) (tyConName tc) tc
317 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys