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PR: https://github.com/ghc/ghc/pull/184
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GHC 8.4.1 is out, so now GHC's support window only extends
back to GHC 8.2. This means we can delete gobs of code that were
only used for GHC 8.0 support. Hooray!
Test Plan: ./validate
Reviewers: bgamari, erikd, dfeuer
Reviewed By: bgamari, dfeuer
Subscribers: alexbiehl, dfeuer, rwbarton, thomie, carter
Differential Revision: https://phabricator.haskell.org/D4492
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I forgot to fold these in to the patch merged earlier.
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This at long last realizes the ideas for type-indexed Typeable discussed in A
Reflection on Types (#11011). The general sketch of the project is described on
the Wiki (Typeable/BenGamari). The general idea is that we are adding a type
index to `TypeRep`,
data TypeRep (a :: k)
This index allows the typechecker to reason about the type represented by the `TypeRep`.
This index representation mechanism is exposed as `Type.Reflection`, which also provides
a number of patterns for inspecting `TypeRep`s,
```lang=haskell
pattern TRFun :: forall k (fun :: k). ()
=> forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
(arg :: TYPE r1) (res :: TYPE r2).
(k ~ Type, fun ~~ (arg -> res))
=> TypeRep arg
-> TypeRep res
-> TypeRep fun
pattern TRApp :: forall k2 (t :: k2). ()
=> forall k1 (a :: k1 -> k2) (b :: k1). (t ~ a b)
=> TypeRep a -> TypeRep b -> TypeRep t
-- | Pattern match on a type constructor.
pattern TRCon :: forall k (a :: k). TyCon -> TypeRep a
-- | Pattern match on a type constructor including its instantiated kind
-- variables.
pattern TRCon' :: forall k (a :: k). TyCon -> [SomeTypeRep] -> TypeRep a
```
In addition, we give the user access to the kind of a `TypeRep` (#10343),
typeRepKind :: TypeRep (a :: k) -> TypeRep k
Moreover, all of this plays nicely with 8.2's levity polymorphism, including the
newly levity polymorphic (->) type constructor.
Library changes
---------------
The primary change here is the introduction of a Type.Reflection module to base.
This module provides access to the new type-indexed TypeRep introduced in this
patch. We also continue to provide the unindexed Data.Typeable interface, which
is simply a type synonym for the existentially quantified SomeTypeRep,
data SomeTypeRep where SomeTypeRep :: TypeRep a -> SomeTypeRep
Naturally, this change also touched Data.Dynamic, which can now export the
Dynamic data constructor. Moreover, I removed a blanket reexport of
Data.Typeable from Data.Dynamic (which itself doesn't even import Data.Typeable
now).
We also add a kind heterogeneous type equality type, (:~~:), to
Data.Type.Equality.
Implementation
--------------
The implementation strategy is described in Note [Grand plan for Typeable] in
TcTypeable. None of it was difficult, but it did exercise a number of parts of
the new levity polymorphism story which had not yet been exercised, which took
some sorting out.
The rough idea is that we augment the TyCon produced for each type constructor
with information about the constructor's kind (which we call a KindRep). This
allows us to reconstruct the monomorphic result kind of an particular
instantiation of a type constructor given its kind arguments.
Unfortunately all of this takes a fair amount of work to generate and send
through the compilation pipeline. In particular, the KindReps can unfortunately
get quite large. Moreover, the simplifier will float out various pieces of them,
resulting in numerous top-level bindings. Consequently we mark the KindRep
bindings as noinline, ensuring that the float-outs don't make it into the
interface file. This is important since there is generally little benefit to
inlining KindReps and they would otherwise strongly affect compiler performance.
Performance
-----------
Initially I was hoping to also clear up the remaining holes in Typeable's
coverage by adding support for both unboxed tuples (#12409) and unboxed sums
(#13276). While the former was fairly straightforward, the latter ended up being
quite difficult: while the implementation can support them easily, enabling this
support causes thousands of Typeable bindings to be emitted to the GHC.Types as
each arity-N sum tycon brings with it N promoted datacons, each of which has a
KindRep whose size which itself scales with N. Doing this was simply too
expensive to be practical; consequently I've disabled support for the time
being.
Even after disabling sums this change regresses compiler performance far more
than I would like. In particular there are several testcases in the testsuite
which consist mostly of types which regress by over 30% in compiler allocations.
These include (considering the "bytes allocated" metric),
* T1969: +10%
* T10858: +23%
* T3294: +19%
* T5631: +41%
* T6048: +23%
* T9675: +20%
* T9872a: +5.2%
* T9872d: +12%
* T9233: +10%
* T10370: +34%
* T12425: +30%
* T12234: +16%
* 13035: +17%
* T4029: +6.1%
I've spent quite some time chasing down the source of this regression and while
I was able to make som improvements, I think this approach of generating
Typeable bindings at time of type definition is doomed to give us unnecessarily
large compile-time overhead.
In the future I think we should consider moving some of all of the Typeable
binding generation logic back to the solver (where it was prior to
91c6b1f54aea658b0056caec45655475897f1972). I've opened #13261 documenting this
proposal.
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GHC 7.8 / base-4.7 is the oldest GHC/base version compatible with this
package, so let's declare this properly in the cabal file.
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Summary:
(Apologies for the size of this patch, I couldn't make a smaller one
that was validate-clean and also made sense independently)
(Some of this code is derived from GHCJS.)
This commit adds support for running interpreted code (for GHCi and
TemplateHaskell) in a separate process. The functionality is
experimental, so for now it is off by default and enabled by the flag
-fexternal-interpreter.
Reaosns we want this:
* compiling Template Haskell code with -prof does not require
building the code without -prof first
* when GHC itself is profiled, it can interpret unprofiled code, and
the same applies to dynamic linking. We would no longer need to
force -dynamic-too with TemplateHaskell, and we can load ordinary
objects into a dynamically-linked GHCi (and vice versa).
* An unprofiled GHCi can load and run profiled code, which means it
can use the stack-trace functionality provided by profiling without
taking the performance hit on the compiler that profiling would
entail.
Amongst other things; see
https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi for more details.
Notes on the implementation are in Note [Remote GHCi] in the new
module compiler/ghci/GHCi.hs. It probably needs more documenting,
feel free to suggest things I could elaborate on.
Things that are not currently implemented for -fexternal-interpreter:
* The GHCi debugger
* :set prog, :set args in GHCi
* `recover` in Template Haskell
* Redirecting stdin/stdout for the external process
These are all doable, I just wanted to get to a working validate-clean
patch first.
I also haven't done any benchmarking yet. I expect there to be slight hit
to link times for byte code and some penalty due to having to
serialize/deserialize TH syntax, but I don't expect it to be a serious
problem. There's also lots of low-hanging fruit in the byte code
generator/linker that we could exploit to speed things up.
Test Plan:
* validate
* I've run parts of the test suite with
EXTRA_HC_OPTS=-fexternal-interpreter, notably tests/ghci and tests/th.
There are a few failures due to the things not currently implemented
(see above).
Reviewers: simonpj, goldfire, ezyang, austin, alanz, hvr, niteria, bgamari, gibiansky, luite
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D1562
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