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