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 (ForAllTy _ ty) = go ty
104 go_tc tc tys | isSynTyCon tc = extendNameEnv (go_s tys) (tyConName tc) tc
105 | otherwise = go_s tys
106 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys
107 ---------------------------------------- END NOTE ]
110 calcSynCycles :: [LTyClDecl Name] -> [SCC (LTyClDecl Name)]
112 = stronglyConnComp syn_edges
114 syn_edges = [ (ldecl, unLoc (tcdLName decl),
115 mk_syn_edges (tcdSynRhs decl))
116 | ldecl@(L _ decl) <- decls ]
118 mk_syn_edges rhs = [ tc | tc <- nameSetToList (extractHsTyNames rhs),
119 not (isTyVarName tc) ]
122 calcClassCycles :: [LTyClDecl Name] -> [[LTyClDecl Name]]
123 calcClassCycles decls
124 = [decls | CyclicSCC decls <- stronglyConnComp cls_edges]
126 cls_edges = [ (ldecl, unLoc (tcdLName decl),
127 mk_cls_edges (unLoc (tcdCtxt decl)))
128 | ldecl@(L _ decl) <- decls, isClassDecl decl ]
130 mk_cls_edges ctxt = [ cls | L _ (HsClassP cls _) <- ctxt ]
134 %************************************************************************
136 Deciding which type constructors are recursive
138 %************************************************************************
140 For newtypes, we label some as "recursive" such that
142 INVARIANT: there is no cycle of non-recursive newtypes
144 In any loop, only one newtype need be marked as recursive; it is
145 a "loop breaker". Labelling more than necessary as recursive is OK,
146 provided the invariant is maintained.
148 A newtype M.T is defined to be "recursive" iff
149 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
150 (b) it is declared in a source file, but that source file has a
151 companion hi-boot file which declares the type
152 or (c) one can get from T's rhs to T via type
153 synonyms, or non-recursive newtypes *in M*
154 e.g. newtype T = MkT (T -> Int)
156 (a) is conservative; declarations in hi-boot files are always
157 made loop breakers. That's why in (b) we can restrict attention
158 to tycons in M, because any loops through newtypes outside M
159 will be broken by those newtypes
160 (b) ensures that a newtype is not treated as a loop breaker in one place
161 and later as a non-loop-breaker. This matters in GHCi particularly, when
162 a newtype T might be embedded in many types in the environment, and then
163 T's source module is compiled. We don't want T's recursiveness to change.
165 The "recursive" flag for algebraic data types is irrelevant (never consulted)
166 for types with more than one constructor.
168 An algebraic data type M.T is "recursive" iff
169 it has just one constructor, and
170 (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl)
171 (b) it is declared in a source file, but that source file has a
172 companion hi-boot file which declares the type
173 or (c) one can get from its arg types to T via type synonyms,
174 or by non-recursive newtypes or non-recursive product types in M
175 e.g. data T = MkT (T -> Int) Bool
176 Just like newtype in fact
178 A type synonym is recursive if one can get from its
179 right hand side back to it via type synonyms. (This is
180 reported as an error.)
182 A class is recursive if one can get from its superclasses
183 back to it. (This is an error too.)
187 A data type read from an hi-boot file will have an AbstractTyCon as its AlgTyConRhs
188 and will respond True to isHiBootTyCon. The idea is that we treat these as if one
189 could get from these types to anywhere. So when we see
192 import {-# SOURCE #-} Foo( T )
195 then we mark S as recursive, just in case. What that means is that if we see
200 then we don't need to look inside S to compute R's recursiveness. Since S is imported
201 (not from an hi-boot file), one cannot get from R back to S except via an hi-boot file,
202 and that means that some data type will be marked recursive along the way. So R is
203 unconditionly non-recursive (i.e. there'll be a loop breaker elsewhere if necessary)
205 This in turn means that we grovel through fewer interface files when computing
206 recursiveness, because we need only look at the type decls in the module being
207 compiled, plus the outer structure of directly-mentioned types.
210 calcRecFlags :: ModDetails -> [TyThing] -> (Name -> RecFlag)
211 -- The 'boot_names' are the things declared in M.hi-boot, if M is the current module.
212 -- Any type constructors in boot_names are automatically considered loop breakers
213 calcRecFlags boot_details tyclss
216 is_rec n | n `elemNameSet` rec_names = Recursive
217 | otherwise = NonRecursive
219 boot_name_set = availsToNameSet (md_exports boot_details)
220 rec_names = boot_name_set `unionNameSets`
221 nt_loop_breakers `unionNameSets`
224 all_tycons = [ tc | tycls <- tyclss,
225 -- Recursion of newtypes/data types can happen via
226 -- the class TyCon, so tyclss includes the class tycons
227 let tc = getTyCon tycls,
228 not (tyConName tc `elemNameSet` boot_name_set) ]
229 -- Remove the boot_name_set because they are going
230 -- to be loop breakers regardless.
232 -------------------------------------------------
234 -- These edge-construction loops rely on
235 -- every loop going via tyclss, the types and classes
236 -- in the module being compiled. Stuff in interface
237 -- files should be correctly marked. If not (e.g. a
238 -- type synonym in a hi-boot file) we can get an infinite
239 -- loop. We could program round this, but it'd make the code
240 -- rather less nice, so I'm not going to do that yet.
242 --------------- Newtypes ----------------------
243 new_tycons = filter isNewTyConAndNotOpen all_tycons
244 isNewTyConAndNotOpen tycon = isNewTyCon tycon && not (isOpenTyCon tycon)
245 nt_loop_breakers = mkNameSet (findLoopBreakers nt_edges)
246 is_rec_nt tc = tyConName tc `elemNameSet` nt_loop_breakers
247 -- is_rec_nt is a locally-used helper function
249 nt_edges = [(t, mk_nt_edges t) | t <- new_tycons]
251 mk_nt_edges nt -- Invariant: nt is a newtype
252 = concatMap (mk_nt_edges1 nt) (tcTyConsOfType (new_tc_rhs nt))
253 -- tyConsOfType looks through synonyms
256 | tc `elem` new_tycons = [tc] -- Loop
257 -- At this point we know that either it's a local *data* type,
258 -- or it's imported. Either way, it can't form part of a newtype cycle
261 --------------- Product types ----------------------
262 -- The "prod_tycons" are the non-newtype products
263 prod_tycons = [tc | tc <- all_tycons,
264 not (isNewTyCon tc), isProductTyCon tc]
265 prod_loop_breakers = mkNameSet (findLoopBreakers prod_edges)
267 prod_edges = [(tc, mk_prod_edges tc) | tc <- prod_tycons]
269 mk_prod_edges tc -- Invariant: tc is a product tycon
270 = concatMap (mk_prod_edges1 tc) (dataConOrigArgTys (head (tyConDataCons tc)))
272 mk_prod_edges1 ptc ty = concatMap (mk_prod_edges2 ptc) (tcTyConsOfType ty)
274 mk_prod_edges2 ptc tc
275 | tc `elem` prod_tycons = [tc] -- Local product
276 | tc `elem` new_tycons = if is_rec_nt tc -- Local newtype
278 else mk_prod_edges1 ptc (new_tc_rhs tc)
279 -- At this point we know that either it's a local non-product data type,
280 -- or it's imported. Either way, it can't form part of a cycle
283 new_tc_rhs tc = snd (newTyConRhs tc) -- Ignore the type variables
285 getTyCon (ATyCon tc) = tc
286 getTyCon (AClass cl) = classTyCon cl
287 getTyCon other = panic "getTyCon"
289 findLoopBreakers :: [(TyCon, [TyCon])] -> [Name]
290 -- Finds a set of tycons that cut all loops
291 findLoopBreakers deps
292 = go [(tc,tc,ds) | (tc,ds) <- deps]
295 | CyclicSCC ((tc,_,_) : edges') <- stronglyConnCompR edges,
296 name <- tyConName tc : go edges']
299 These two functions know about type representations, so they could be
300 in Type or TcType -- but they are very specialised to this module, so
301 I've chosen to put them here.
304 tcTyConsOfType :: Type -> [TyCon]
305 -- tcTyConsOfType looks through all synonyms, but not through any newtypes.
306 -- When it finds a Class, it returns the class TyCon. The reaons it's here
307 -- (not in Type.lhs) is because it is newtype-aware.
309 = nameEnvElts (go ty)
311 go :: Type -> NameEnv TyCon -- The NameEnv does duplicate elim
312 go ty | Just ty' <- tcView ty = go ty'
313 go (TyVarTy v) = emptyNameEnv
314 go (TyConApp tc tys) = go_tc tc tys
315 go (AppTy a b) = go a `plusNameEnv` go b
316 go (FunTy a b) = go a `plusNameEnv` go b
317 go (PredTy (IParam _ ty)) = go ty
318 go (PredTy (ClassP cls tys)) = go_tc (classTyCon cls) tys
319 go (ForAllTy _ ty) = go ty
320 go other = panic "tcTyConsOfType"
322 go_tc tc tys = extendNameEnv (go_s tys) (tyConName tc) tc
323 go_s tys = foldr (plusNameEnv . go) emptyNameEnv tys