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Vectorise.Generic.PAMethods.buildToArrPReprs was building an expression like
pvoids# (lengthSels2# sels)
which does not satisfy the let/app invariant. It should be more like
case lengthSels2# sels of l -> pvoids# l
This was caught by Core Lint (once it was taught to check for the invariant)
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The issue here is described in Note [Binding scoped type variables] in
TcPat. When implementing this fix I was able to make things quite a
bit simpler:
* The type variables in a type signature now never unify
with each other, and so can be straightfoward skolems.
* We only need the SigTv stuff for signatures in patterns,
and for kind variables.
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Roles are a solution to the GeneralizedNewtypeDeriving type-safety
problem.
Roles were first described in the "Generative type abstraction" paper,
by Stephanie Weirich, Dimitrios Vytiniotis, Simon PJ, and Steve Zdancewic.
The implementation is a little different than that paper. For a quick
primer, check out Note [Roles] in Coercion. Also see
http://ghc.haskell.org/trac/ghc/wiki/Roles
and
http://ghc.haskell.org/trac/ghc/wiki/RolesImplementation
For a more formal treatment, check out docs/core-spec/core-spec.pdf.
This fixes Trac #1496, #4846, #7148.
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This commit changes the syntax and story around overlapping type
family instances. Before, we had "unbranched" instances and
"branched" instances. Now, we have closed type families and
open ones.
The behavior of open families is completely unchanged. In particular,
coincident overlap of open type family instances still works, despite
emails to the contrary.
A closed type family is declared like this:
> type family F a where
> F Int = Bool
> F a = Char
The equations are tried in order, from top to bottom, subject to
certain constraints, as described in the user manual. It is not
allowed to declare an instance of a closed family.
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This is a long-standing regression (Trac #7797), which meant that in
particular the Eq [Char] instance does not get specialised.
(The *methods* do, but the dictionary itself doesn't.) So when you
call a function
f :: Eq a => blah
on a string type (ie a=[Char]), 7.6 passes a dictionary of un-specialised
methods.
This only matters when calling an overloaded function from a
specialised context, but that does matter in some programs. I
remember (though I cannot find the details) that Nick Frisby discovered
this to be the source of some pretty solid performanc regresisons.
Anyway it works now. The key change is that a DFunUnfolding now takes
a form that is both simpler than before (the DFunArg type is eliminated)
and more general:
data Unfolding
= ...
| DFunUnfolding { -- The Unfolding of a DFunId
-- See Note [DFun unfoldings]
-- df = /\a1..am. \d1..dn. MkD t1 .. tk
-- (op1 a1..am d1..dn)
-- (op2 a1..am d1..dn)
df_bndrs :: [Var], -- The bound variables [a1..m],[d1..dn]
df_con :: DataCon, -- The dictionary data constructor (never a newtype datacon)
df_args :: [CoreExpr] -- Args of the data con: types, superclasses and methods,
} -- in positional order
That in turn allowed me to re-enable the DFunUnfolding specialisation in
DsBinds. Lots of details here in TcInstDcls:
Note [SPECIALISE instance pragmas]
I also did some refactoring, in particular to pass the InScopeSet to
exprIsConApp_maybe (which in turn means it has to go to a RuleFun).
NB: Interface file format has changed!
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* Move tidyType and friends from TcType to TypeRep
(It was always wrong to have it in TcType.)
* Move mkCoAxBranch and friends from FamInst to Coercion
* Move pprCoAxBranch and friends from FamInstEnv to Coercion
No change in functionality, though there might be a little
wibble in error message output, because I combined two different
functions both called pprCoAxBranch!
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Quite a bit of tidying up here; the fix to #7524 is actually
only a small part.
* Be fully clear that the cab_tvs in a CoAxBranch are not
fresh. See Note [CoAxBranch type variables] in CoAxiom.
* Use CoAxBranch to replace the ATDfeault type in Class.
CoAxBranch is perfect here. This change allowed me to
delete quite a bit of boilerplate code, including the
corresponding IfaceSynType.
* Tidy up the construction of CoAxBranches, and when FamIntBranch is
freshened. The latter onw happens only in FamInst.newFamInst.
* Tidy the tyvars of a CoAxBranch when we build them, done in
FamInst.mkCoAxBranch. See Note [Tidy axioms when we build them]
in that module. This is what fixes #7524.
Much niceer now.
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It should be the case that either an entire mutually recursive
group of data type declarations can be promoted, or none of them.
It's really odd to promote some data constructors of a type but
not others. Eg
data T a = T1 a | T2 Int
Here T1 is sort-of-promotable but T2 isn't (becuase Int isn't
promotable).
This patch makes it all-or-nothing. At the same time I've made
the TyCon point to its promoted cousin (via the tcPromoted field
of an AlgTyCon), as well as vice versa (via the ty_con field of
PromotedTyCon).
The inference for the group is done in TcTyDecls, the same place
that infers which data types are recursive, another global question.
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This commit mirrors work done in the commit for ClsInsts, 5efe9b...
Specifically:
- All FamInsts have *fresh* type variables. So, no more freshness work
in addLocalFamInst
Also:
- Some pretty-printing code around FamInsts was cleaned up a bit
This caused location information to be added to CoAxioms and index
information to be added to FamInstBranches.
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Conflicts:
compiler/basicTypes/MkId.lhs
compiler/iface/IfaceSyn.lhs
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An ordered, overlapping type family instance is introduced by 'type
instance
where', followed by equations. See the new section in the user manual
(7.7.2.2) for details. The canonical example is Boolean equality at the
type
level:
type family Equals (a :: k) (b :: k) :: Bool
type instance where
Equals a a = True
Equals a b = False
A branched family instance, such as this one, checks its equations in
order
and applies only the first the matches. As explained in the note
[Instance
checking within groups] in FamInstEnv.lhs, we must be careful not to
simplify,
say, (Equals Int b) to False, because b might later unify with Int.
This commit includes all of the commits on the overlapping-tyfams
branch. SPJ
requested that I combine all my commits over the past several months
into one
monolithic commit. The following GHC repos are affected: ghc, testsuite,
utils/haddock, libraries/template-haskell, and libraries/dph.
Here are some details for the interested:
- The definition of CoAxiom has been moved from TyCon.lhs to a
new file CoAxiom.lhs. I made this decision because of the
number of definitions necessary to support BranchList.
- BranchList is a GADT whose type tracks whether it is a
singleton list or not-necessarily-a-singleton-list. The reason
I introduced this type is to increase static checking of places
where GHC code assumes that a FamInst or CoAxiom is indeed a
singleton. This assumption takes place roughly 10 times
throughout the code. I was worried that a future change to GHC
would invalidate the assumption, and GHC might subtly fail to
do the right thing. By explicitly labeling CoAxioms and
FamInsts as being Unbranched (singleton) or
Branched (not-necessarily-singleton), we make this assumption
explicit and checkable. Furthermore, to enforce the accuracy of
this label, the list of branches of a CoAxiom or FamInst is
stored using a BranchList, whose constructors constrain its
type index appropriately.
I think that the decision to use BranchList is probably the most
controversial decision I made from a code design point of view.
Although I provide conversions to/from ordinary lists, it is more
efficient to use the brList... functions provided in CoAxiom than
always to convert. The use of these functions does not wander far
from the core CoAxiom/FamInst logic.
BranchLists are motivated and explained in the note [Branched axioms] in
CoAxiom.lhs.
- The CoAxiom type has changed significantly. You can see the new
type in CoAxiom.lhs. It uses a CoAxBranch type to track
branches of the CoAxiom. Correspondingly various functions
producing and consuming CoAxioms had to change, including the
binary layout of interface files.
- To get branched axioms to work correctly, it is important to have a
notion
of type "apartness": two types are apart if they cannot unify, and no
substitution of variables can ever get them to unify, even after type
family
simplification. (This is different than the normal failure to unify
because
of the type family bit.) This notion in encoded in tcApartTys, in
Unify.lhs.
Because apartness is finer-grained than unification, the tcUnifyTys
now
calls tcApartTys.
- CoreLinting axioms has been updated, both to reflect the new
form of CoAxiom and to enforce the apartness rules of branch
application. The formalization of the new rules is in
docs/core-spec/core-spec.pdf.
- The FamInst type (in types/FamInstEnv.lhs) has changed
significantly, paralleling the changes to CoAxiom. Of course,
this forced minor changes in many files.
- There are several new Notes in FamInstEnv.lhs, including one
discussing confluent overlap and why we're not doing it.
- lookupFamInstEnv, lookupFamInstEnvConflicts, and
lookup_fam_inst_env' (the function that actually does the work)
have all been more-or-less completely rewritten. There is a
Note [lookup_fam_inst_env' implementation] describing the
implementation. One of the changes that affects other files is
to change the type of matches from a pair of (FamInst, [Type])
to a new datatype (which now includes the index of the matching
branch). This seemed a better design.
- The TySynInstD constructor in Template Haskell was updated to
use the new datatype TySynEqn. I also bumped the TH version
number, requiring changes to DPH cabal files. (That's why the
DPH repo has an overlapping-tyfams branch.)
- As SPJ requested, I refactored some of the code in HsDecls:
* splitting up TyDecl into SynDecl and DataDecl, correspondingly
changing HsTyDefn to HsDataDefn (with only one constructor)
* splitting FamInstD into TyFamInstD and DataFamInstD and
splitting FamInstDecl into DataFamInstDecl and TyFamInstDecl
* making the ClsInstD take a ClsInstDecl, for parallelism with
InstDecl's other constructors
* changing constructor TyFamily into FamDecl
* creating a FamilyDecl type that stores the details for a family
declaration; this is useful because FamilyDecls can appear in classes
but
other decls cannot
* restricting the associated types and associated type defaults for a
* class
to be the new, more restrictive types
* splitting cid_fam_insts into cid_tyfam_insts and cid_datafam_insts,
according to the new types
* perhaps one or two more that I'm overlooking
None of these changes has far-reaching implications.
- The user manual, section 7.7.2.2, is updated to describe the new type
family
instances.
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This fixes most of Trac #3990. Consider
data family D a
data instance D Double = CD Int Int
data T = T {-# UNPACK #-} !(D Double)
Then we want the (D Double unpacked).
To do this we need to construct a suitable coercion, and it's much
safer to record that coercion in the interface file, lest the in-scope
instances differ somehow. That in turn means elaborating the HsBang
type to include a coercion.
To do that I moved HsBang from BasicTypes to DataCon, which caused
quite a few minor knock-on changes.
Interface-file format has changed!
Still to do: need to do knot-tying to allow instances to take effect
within the same module.
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Silent superclass parameters solve the problem that
the superclasses of a dicionary construction can easily
turn out to be (wrongly) bottom. The problem and solution
are described in
Note [Silent superclass arguments] in TcInstDcls
I first implemented this fix (with Dimitrios) in Dec 2010, but removed
it again in Jun 2011 becuase we thought it wasn't necessary any
more. (The reason we thought it wasn't necessary is that we'd stopped
generating derived superclass constraints for *wanteds*. But we were
wrong; that didn't solve the superclass-loop problem.)
So we have to re-implement it. It's not hard. Main features:
* The IdDetails for a DFunId says how many silent arguments it has
* A DFunUnfolding describes which dictionary args are
just parameters (DFunLamArg) and which are a function to apply
to the parameters (DFunPolyArg). This adds the DFunArg type
to CoreSyn
* Consequential changes to IfaceSyn. (Binary hi file format changes
slightly.)
* TcInstDcls changes to generate the right dfuns
* CoreSubst.exprIsConApp_maybe handles the new DFunUnfolding
The thing taht is *not* done yet is to alter the vectoriser to
pass the relevant extra argument when building a PA dictionary.
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For now, the syntax is
type {-# CTYPE "some C type" #-} Foo = ...
newtype {-# CTYPE "some C type" #-} Foo = ...
data {-# CTYPE "some C type" #-} Foo = ...
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This patch should have no user-visible effect. It implements a
significant internal refactoring of the way that FC axioms are
handled. The ultimate goal is to put us in a position to implement
"pattern-matching axioms". But the changes here are only does
refactoring; there is no change in functionality.
Specifically:
* We now treat data/type family instance declarations very,
very similarly to types class instance declarations:
- Renamed InstEnv.Instance as InstEnv.ClsInst, for symmetry with
FamInstEnv.FamInst. This change does affect the GHC API, but
for the better I think.
- Previously, each family type/data instance declaration gave rise
to a *TyCon*; typechecking a type/data instance decl produced
that TyCon. Now, each type/data instance gives rise to
a *FamInst*, by direct analogy with each class instance
declaration giving rise to a ClsInst.
- Just as each ClsInst contains its evidence, a DFunId, so each FamInst
contains its evidence, a CoAxiom. See Note [FamInsts and CoAxioms]
in FamInstEnv. The CoAxiom is a System-FC thing, and can relate any
two types, whereas the FamInst relates directly to the Haskell source
language construct, and always has a function (F tys) on the LHS.
- Just as a DFunId has its own declaration in an interface file, so now
do CoAxioms (see IfaceSyn.IfaceAxiom).
These changes give rise to almost all the refactoring.
* We used to have a hack whereby a type family instance produced a dummy
type synonym, thus
type instance F Int = Bool -> Bool
translated to
axiom FInt :: F Int ~ R:FInt
type R:FInt = Bool -> Bool
This was always a hack, and now it's gone. Instead the type instance
declaration produces a FamInst, whose axiom has kind
axiom FInt :: F Int ~ Bool -> Bool
just as you'd expect.
* Newtypes are done just as before; they generate a CoAxiom. These
CoAxioms are "implicit" (do not generate an IfaceAxiom declaration),
unlike the ones coming from family instance declarations. See
Note [Implicit axioms] in TyCon
On the whole the code gets significantly nicer. There were consequential
tidy-ups in the vectoriser, but I think I got them right.
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Also give a civilised error message when the data type to vectorised contains no data.
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