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.
13 -- The above warning supression flag is a temporary kludge.
14 -- While working on this module you are encouraged to remove it and fix
15 -- any warnings in the module. See
16 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
21 calcClassCycles, calcSynCycles
24 #include "HsVersions.h"
44 %************************************************************************
46 Cycles in class and type synonym declarations
48 %************************************************************************
50 Checking for class-decl loops is easy, because we don't allow class decls
53 We allow type synonyms in hi-boot files, but we *trust* hi-boot files,
54 so we don't check for loops that involve them. So we only look for synonym
55 loops in the module being compiled.
57 We check for type synonym and class cycles on the *source* code.
60 a) Otherwise we'd need a special function to extract type-synonym tycons
61 from a type, whereas we have extractHsTyNames already
63 b) If we checked for type synonym loops after building the TyCon, we
64 can't do a hoistForAllTys on the type synonym rhs, (else we fall into
65 a black hole) which seems unclean. Apart from anything else, it'd mean
66 that a type-synonym rhs could have for-alls to the right of an arrow,
67 which means adding new cases to the validity checker
69 Indeed, in general, checking for cycles beforehand means we need to
70 be less careful about black holes through synonym cycles.
72 The main disadvantage is that a cycle that goes via a type synonym in an
73 .hi-boot file can lead the compiler into a loop, because it assumes that cycles
74 only occur entirely within the source code of the module being compiled.
75 But hi-boot files are trusted anyway, so this isn't much worse than (say)
78 [ NOTE ----------------------------------------------
79 If we reverse this decision, this comment came from tcTyDecl1, and should
81 -- dsHsType, not tcHsKindedType, to avoid a loop. tcHsKindedType does hoisting,
82 -- which requires looking through synonyms... and therefore goes into a loop
83 -- on (erroneously) recursive synonyms.
84 -- Solution: do not hoist synonyms, because they'll be hoisted soon enough
85 -- when they are substituted
87 We'd also need to add back in this definition
89 synTyConsOfType :: Type -> [TyCon]
90 -- Does not look through type synonyms at all
91 -- Return a list of synonym tycons
95 go :: Type -> NameEnv TyCon -- The NameEnv does duplicate elim
96 go (TyVarTy v) = emptyNameEnv
97 go (TyConApp tc tys) = go_tc tc tys
98 go (AppTy a b) = go a `plusNameEnv` go b
99 go (FunTy a b) = go a `plusNameEnv` go b
100 go (PredTy (IParam _ ty)) = go ty
101 go (PredTy (ClassP cls tys)) = go_s tys -- Ignore class
102 go (NoteTy _ ty) = go ty
103 go (ForAllTy _ ty) = go ty
105 go_tc tc tys | isSynTyCon tc = extendNameEnv (go_s tys) (tyConName tc) tc
106 | otherwise = go_s tys
107 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys
108 ---------------------------------------- END NOTE ]
111 calcSynCycles :: [LTyClDecl Name] -> [SCC (LTyClDecl Name)]
113 = stronglyConnComp syn_edges
115 syn_edges = [ (ldecl, unLoc (tcdLName decl),
116 mk_syn_edges (tcdSynRhs decl))
117 | ldecl@(L _ decl) <- decls ]
119 mk_syn_edges rhs = [ tc | tc <- nameSetToList (extractHsTyNames rhs),
120 not (isTyVarName tc) ]
123 calcClassCycles :: [LTyClDecl Name] -> [[LTyClDecl Name]]
124 calcClassCycles decls
125 = [decls | CyclicSCC decls <- stronglyConnComp cls_edges]
127 cls_edges = [ (ldecl, unLoc (tcdLName decl),
128 mk_cls_edges (unLoc (tcdCtxt decl)))
129 | ldecl@(L _ decl) <- decls, isClassDecl decl ]
131 mk_cls_edges ctxt = [ cls | L _ (HsClassP cls _) <- ctxt ]
135 %************************************************************************
137 Deciding which type constructors are recursive
139 %************************************************************************
141 For newtypes, we label some as "recursive" such that
143 INVARIANT: there is no cycle of non-recursive newtypes
145 In any loop, only one newtype need be marked as recursive; it is
146 a "loop breaker". Labelling more than necessary as recursive is OK,
147 provided the invariant is maintained.
149 A newtype M.T is defined to be "recursive" iff
150 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
151 (b) it is declared in a source file, but that source file has a
152 companion hi-boot file which declares the type
153 or (c) one can get from T's rhs to T via type
154 synonyms, or non-recursive newtypes *in M*
155 e.g. newtype T = MkT (T -> Int)
157 (a) is conservative; declarations in hi-boot files are always
158 made loop breakers. That's why in (b) we can restrict attention
159 to tycons in M, because any loops through newtypes outside M
160 will be broken by those newtypes
161 (b) ensures that a newtype is not treated as a loop breaker in one place
162 and later as a non-loop-breaker. This matters in GHCi particularly, when
163 a newtype T might be embedded in many types in the environment, and then
164 T's source module is compiled. We don't want T's recursiveness to change.
166 The "recursive" flag for algebraic data types is irrelevant (never consulted)
167 for types with more than one constructor.
169 An algebraic data type M.T is "recursive" iff
170 it has just one constructor, and
171 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
172 (b) it is declared in a source file, but that source file has a
173 companion hi-boot file which declares the type
174 or (c) one can get from its arg types to T via type synonyms,
175 or by non-recursive newtypes or non-recursive product types in M
176 e.g. data T = MkT (T -> Int) Bool
177 Just like newtype in fact
179 A type synonym is recursive if one can get from its
180 right hand side back to it via type synonyms. (This is
181 reported as an error.)
183 A class is recursive if one can get from its superclasses
184 back to it. (This is an error too.)
188 A data type read from an hi-boot file will have an AbstractTyCon as its AlgTyConRhs
189 and will respond True to isHiBootTyCon. The idea is that we treat these as if one
190 could get from these types to anywhere. So when we see
193 import {-# SOURCE #-} Foo( T )
196 then we mark S as recursive, just in case. What that means is that if we see
201 then we don't need to look inside S to compute R's recursiveness. Since S is imported
202 (not from an hi-boot file), one cannot get from R back to S except via an hi-boot file,
203 and that means that some data type will be marked recursive along the way. So R is
204 unconditionly non-recursive (i.e. there'll be a loop breaker elsewhere if necessary)
206 This in turn means that we grovel through fewer interface files when computing
207 recursiveness, because we need only look at the type decls in the module being
208 compiled, plus the outer structure of directly-mentioned types.
211 calcRecFlags :: ModDetails -> [TyThing] -> (Name -> RecFlag)
212 -- The 'boot_names' are the things declared in M.hi-boot, if M is the current module.
213 -- Any type constructors in boot_names are automatically considered loop breakers
214 calcRecFlags boot_details tyclss
217 is_rec n | n `elemNameSet` rec_names = Recursive
218 | otherwise = NonRecursive
220 boot_name_set = availsToNameSet (md_exports boot_details)
221 rec_names = boot_name_set `unionNameSets`
222 nt_loop_breakers `unionNameSets`
225 all_tycons = [ tc | tycls <- tyclss,
226 -- Recursion of newtypes/data types can happen via
227 -- the class TyCon, so tyclss includes the class tycons
228 let tc = getTyCon tycls,
229 not (tyConName tc `elemNameSet` boot_name_set) ]
230 -- Remove the boot_name_set because they are going
231 -- to be loop breakers regardless.
233 -------------------------------------------------
235 -- These edge-construction loops rely on
236 -- every loop going via tyclss, the types and classes
237 -- in the module being compiled. Stuff in interface
238 -- files should be correctly marked. If not (e.g. a
239 -- type synonym in a hi-boot file) we can get an infinite
240 -- loop. We could program round this, but it'd make the code
241 -- rather less nice, so I'm not going to do that yet.
243 --------------- Newtypes ----------------------
244 new_tycons = filter isNewTyConAndNotOpen all_tycons
245 isNewTyConAndNotOpen tycon = isNewTyCon tycon && not (isOpenTyCon tycon)
246 nt_loop_breakers = mkNameSet (findLoopBreakers nt_edges)
247 is_rec_nt tc = tyConName tc `elemNameSet` nt_loop_breakers
248 -- is_rec_nt is a locally-used helper function
250 nt_edges = [(t, mk_nt_edges t) | t <- new_tycons]
252 mk_nt_edges nt -- Invariant: nt is a newtype
253 = concatMap (mk_nt_edges1 nt) (tcTyConsOfType (new_tc_rhs nt))
254 -- tyConsOfType looks through synonyms
257 | tc `elem` new_tycons = [tc] -- Loop
258 -- At this point we know that either it's a local *data* type,
259 -- or it's imported. Either way, it can't form part of a newtype cycle
262 --------------- Product types ----------------------
263 -- The "prod_tycons" are the non-newtype products
264 prod_tycons = [tc | tc <- all_tycons,
265 not (isNewTyCon tc), isProductTyCon tc]
266 prod_loop_breakers = mkNameSet (findLoopBreakers prod_edges)
268 prod_edges = [(tc, mk_prod_edges tc) | tc <- prod_tycons]
270 mk_prod_edges tc -- Invariant: tc is a product tycon
271 = concatMap (mk_prod_edges1 tc) (dataConOrigArgTys (head (tyConDataCons tc)))
273 mk_prod_edges1 ptc ty = concatMap (mk_prod_edges2 ptc) (tcTyConsOfType ty)
275 mk_prod_edges2 ptc tc
276 | tc `elem` prod_tycons = [tc] -- Local product
277 | tc `elem` new_tycons = if is_rec_nt tc -- Local newtype
279 else mk_prod_edges1 ptc (new_tc_rhs tc)
280 -- At this point we know that either it's a local non-product data type,
281 -- or it's imported. Either way, it can't form part of a cycle
284 new_tc_rhs tc = snd (newTyConRhs tc) -- Ignore the type variables
286 getTyCon (ATyCon tc) = tc
287 getTyCon (AClass cl) = classTyCon cl
288 getTyCon other = panic "getTyCon"
290 findLoopBreakers :: [(TyCon, [TyCon])] -> [Name]
291 -- Finds a set of tycons that cut all loops
292 findLoopBreakers deps
293 = go [(tc,tc,ds) | (tc,ds) <- deps]
296 | CyclicSCC ((tc,_,_) : edges') <- stronglyConnCompR edges,
297 name <- tyConName tc : go edges']
300 These two functions know about type representations, so they could be
301 in Type or TcType -- but they are very specialised to this module, so
302 I've chosen to put them here.
305 tcTyConsOfType :: Type -> [TyCon]
306 -- tcTyConsOfType looks through all synonyms, but not through any newtypes.
307 -- When it finds a Class, it returns the class TyCon. The reaons it's here
308 -- (not in Type.lhs) is because it is newtype-aware.
310 = nameEnvElts (go ty)
312 go :: Type -> NameEnv TyCon -- The NameEnv does duplicate elim
313 go ty | Just ty' <- tcView ty = go ty'
314 go (TyVarTy v) = emptyNameEnv
315 go (TyConApp tc tys) = go_tc tc tys
316 go (AppTy a b) = go a `plusNameEnv` go b
317 go (FunTy a b) = go a `plusNameEnv` go b
318 go (PredTy (IParam _ ty)) = go ty
319 go (PredTy (ClassP cls tys)) = go_tc (classTyCon cls) tys
320 go (ForAllTy _ ty) = go ty
321 go other = panic "tcTyConsOfType"
323 go_tc tc tys = extendNameEnv (go_s tys) (tyConName tc) tc
324 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys