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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 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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For instance decls we no longer store the SafeHaskell mode
in this data structure but instead store it as a bool field
in the overlap flag structure.
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OverlappingInstances in Safe modules can only overlap instances
defined in the same module.
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This patch, written by Max Bolingbroke, does two things
1. It adds a new CoreM monad (defined in simplCore/CoreMonad),
which is used as the top-level monad for all the Core-to-Core
transformations (starting at SimplCore). It supports
* I/O (for debug printing)
* Unique supply
* Statistics gathering
* Access to the HscEnv, RuleBase, Annotations, Module
The patch therefore refactors the top "skin" of every Core-to-Core
pass, but does not change their functionality.
2. It adds a completely new facility to GHC: Core "annotations".
The idea is that you can say
{#- ANN foo (Just "Hello") #-}
which adds the annotation (Just "Hello") to the top level function
foo. These annotations can be looked up in any Core-to-Core pass,
and are persisted into interface files. (Hence a Core-to-Core pass
can also query the annotations of imported things.) Furthermore,
a Core-to-Core pass can add new annotations (eg strictness info)
of its own, which can be queried by importing modules.
The design of the annotation system is somewhat in flux. It's
designed to work with the (upcoming) dynamic plug-ins mechanism,
but is meanwhile independently useful.
Do not merge to 6.10!
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- We need to keep pairs of (f, f_CC) in VectInfo as it is difficult
to obtain Names from OccNames (of imported modules) in Core passes.
- There is a choice of keeping Names or Vars in VectInfo. We go with Vars
for now; mainly to avoid converting between Names and Vars repeatedly for
the same VectInfo in other than one-shot mode.
Again goes to the HEAD straight away to avoid conflicts down the road.
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- Class and type family instances just got a lot more similar.
- FamInst, like Instance, now has a rough match signature. The idea is the
same: if the rough match doesn't match, there is no need to pull in the while
tycon describing the instance (from a lazily read iface).
- IfaceFamInst changes in a similar way and the list of all IFaceFamInsts is
now written into the binary iface (as for class instances), as deriving it
from the tycon (as before) would render the whole rough matching useless.
- As a result of this, the plumbing of class instances and type instances
through the various environments, ModIface, ModGuts, and ModDetails is now
almost the same. (The remaining difference are mostly because the dfun of a
class instance is an Id, but type instance refer to a TyCon, not an Id.)
*** WARNING: The interface file format changed! ***
*** Rebuild from scratch. ***
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This patch fixes Trac #909. The problem was that when compiling
the base package, the handling of wired-in things wasn't right;
in TcIface.tcWiredInTyCon it repeatedly loaded GHC.Base.hi into the
PIT, even though that was the very module it was compiling.
The main fix is by introducing TcIface.ifCheckWiredInThing.
But I did some minor refactoring as well.
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Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.
The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level. The build system now makes no
pretense at being multi-project, it is just the GHC build system.
No doubt this will break many things, and there will be a period of
instability while we fix the dependencies. A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.
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