a3ad4588
by sheaf at 2026-09-17T02:39:14-04:00
Use a type family for boxing unboxed types
This commit overhauls the way in which unboxed types are boxed, as per
the updated Note [Boxing constructors] in GHC.Builtin.WiredIn.Types.Box.
The main change is to allow boxing to be parametrised by representation,
e.g. making 'Box ty' a valid type. We achieve this by implementing 'Box'
using a closed type family:
newtype Box (a :: TYPE r) = Box (BoxTF r)
type BoxTF :: RuntimeRep -> Type
type family BoxTF r where
BoxTF IntRep = BoxInt
BoxTF Int8Rep = BoxInt8
BoxTF WordRep = BoxWord
BoxTF LiftedRep = Any @Type
BoxTF (TupleRep [r1, r2]) = (BoxTF r1, BoxTF r2)
...
BoxTF (SumRep rs) = BoxSum rs
This new design solves the following problems:
- It allows boxing a type whose 'RuntimeRep' has not yet been
determined (e.g. because we only learn the 'RuntimeRep' after
constraint solving). That is, 'Box @r a' is a perfectly valid type
even if 'r' is an unfilled metavariable. This is a requirement
to fix #25065 (in which we need to box a type whose 'RuntimeRep'
is only filled in during constraint solving).
- It handles boxing of unboxed tuples and unboxed sums, via recursive
boxing. See Note [Boxing TupleRep and SumRep] in
GHC.Builtin.WiredIn.Types.Box.
'Box', 'BoxTF' and all of the boxing data constructors are defined in
a new 'ghc-internal' module, 'GHC.Internal.Box'. This module contains
nothing but wired-in declarations, and GHC never loads its interface on
account of a wired-in thing (GHC.Builtin.Modules.isWiredInOnlyModule);
see Note [Loading instances for wired-in things] in GHC.Iface.Load.
The individual boxing data constructors are deliberately left as an
internal implementation detail, and not exported. The intended API is
via the two new magic 'Id's:
box :: forall {r} (a :: TYPE r). a -> Box a
unbox :: forall {r} (a :: TYPE r). Box a -> a
exported from 'GHC.PrimOps' (in 'ghc-experimental'). The old boxing
data types (IntBox, WordBox, FloatBox, DoubleBox, DictBox) are no longer
exported from base (CLC proposal #421).
We use this new design to consistently desugar:
- parallel comprehensions (GHC.HsToCore.ListComp.{deListComp,dsMcStmt})
- then/group/using statements (GHC.HsToCore.ListComp.{dsTransStmt,dsMcStmt})
- recursive do blocks (mdo/rec) (GHC.HsToCore.Expr.dsDo)
- arrows (GHC.HsToCore.Arrows)
by consistently using 'box'/'unbox' to deal with unboxed types being put
into big tuples. We also adapt 'GHC.Tc.Gen.Match.{tcMcStmt,tcDoStmt}'
to create fresh /representation-polymorphic/ type variables for the
binders that we are going to put into a big tuple (previously, GHC only
allowed lifted types in that position).
These desugaring of box and unbox is explained in
Note [Desugaring box & unbox] in GHC.Core.Make.Box. Roughly speaking,
the desugaring looks like:
box @r @ty e ==> K e
unbox @r @ty e ==> case e of K x -> x
where 'K' is the boxing data constructor at RuntimeRep 'r'. It's
slightly more complicated than that in practice: we first need to
insert a cast (see Note [The canonical type of a RuntimeRep]) using the
new 'CanonicalProv' 'UnivCoProvenance', and unboxed tuples/sums need
recursive logic explained in Note [box/unbox for unboxed tuples and sums],
including a specific 'BoxSum' datatypes for unboxed sum (which also
needed a wired-in type-level list indexing type family (!!)).
The desugaring happens in two ways:
- Occurrences of 'box'/'unbox' emitted by the typechecker are
eliminated in the 'ds_app_var' case of GHC.HsToCore.Expr.
- Other code that needs to box/unbox emits the appropriate code inline.
This includes the big tuple machinery of GHC.Core.Make used by the
desugarer (GHC.HsToCore.{Utils,Binds,Arrows,ListComp}) as well as
the float-out pass (GHC.Core.Opt.SetLevels).
Core Lint checks that no occurrence of 'box'/'unbox' survives desugaring.
On the way, we overhaul the litany of big tuple functions by introducing
data BigTupleLayout = BoxedElements | BareElements
to distinguish the two kinds of "big tuples" that GHC routinely deals
with; for example 'mkBigCoreVarTupSolo' becomes 'mkBigCoreVarTup BareElements'.
All of this code is moved to the new module GHC.Core.Make.BigTuple.
Fixes #20864 #22336 #22473 #25065 #27331
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Metric Decrease:
T9872a
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