patch applied (ghc): Simon's big boxy-type commit
Simon Peyton Jones
simonpj at microsoft.com
Wed Jan 25 11:31:12 EST 2006
Wed Jan 25 08:28:32 PST 2006 simonpj at microsoft.com
* Simon's big boxy-type commit
This very large commit adds impredicativity to GHC, plus
numerous other small things.
*** WARNING: I have compiled all the libraries, and
*** a stage-2 compiler, and everything seems
*** fine. But don't grab this patch if you
*** can't tolerate a hiccup if something is
The big picture is this:
a) GHC handles impredicative polymorphism, as described in the
"Boxy types: type inference for higher-rank types and
b) GHC handles GADTs in the new simplified (and very sligtly less
epxrssive) way described in the
"Simple unification-based type inference for GADTs" paper
But there are lots of smaller changes, and since it was pre-Darcs
they are not individually recorded.
Some things to watch out for:
c) The story on lexically-scoped type variables has changed, as per
my email. I append the story below for completeness, but I
am still not happy with it, and it may change again. In particular,
the new story does not allow a pattern-bound scoped type variable
to be wobbly, so (\(x::[a]) -> ...) is usually rejected. This is
more restrictive than before, and we might loosen up again.
d) A consequence of adding impredicativity is that GHC is a bit less
gung ho about converting automatically between
(ty1 -> forall a. ty2) and (forall a. ty1 -> ty2)
In particular, you may need to eta-expand some functions to make
typechecking work again.
Furthermore, functions are now invariant in their argument types,
rather than being contravariant. Again, the main consequence is
that you may occasionally need to eta-expand function arguments when
using higher-rank polymorphism.
Please test, and let me know of any hiccups
Scoped type variables in GHC
A *pattern binding* is of the form
pat = rhs
A *function binding* is of the form
f pat1 .. patn = rhs
A binding of the formm
var = rhs
is treated as a (degenerate) *function binding*.
A *declaration type signature* is a separate type signature for a
let-bound or where-bound variable:
f :: Int -> Int
A *pattern type signature* is a signature in a pattern:
\(x::a) -> x
f (x::a) = x
A *result type signature* is a signature on the result of a
f :: forall a. [a] -> a
head (x:xs) :: a = x
x :: a = rhs
is treated as a (degnerate) function binding with a result
type signature, not as a pattern binding.
1) The main invariants:
A) A lexically-scoped type variable always names a (rigid)
type variable (not an arbitrary type). THIS IS A CHANGE.
Previously, a scoped type variable named an arbitrary *type*.
B) A type signature always describes a rigid type (since
its free (scoped) type variables name rigid type variables).
This is also a change, a consequence of (A).
C) Distinct lexically-scoped type variables name distinct
rigid type variables. This choice is open;
2(a) If a declaration type signature has an explicit forall, those type
variables are brought into scope in the right hand side of the
corresponding binding (plus, for function bindings, the patterns on
f :: forall a. a -> [a]
f (x::a) = [x :: a, x]
Both occurences of 'a' in the second line are bound by
the 'forall a' in the first line
A declaration type signature *without* an explicit top-level forall
is implicitly quantified over all the type variables that are
mentioned in the type but not already in scope. GHC's current
rule is that this implicit quantification does *not* bring into scope
any new scoped type variables.
f :: a -> a
f x = ...('a' is not in scope here)...
This gives compatibility with Haskell 98
2(b) A pattern type signature implicitly brings into scope any type
variables mentioned in the type that are not already into scope.
These are called *pattern-bound type variables*.
g :: a -> a -> [a]
g (x::a) (y::a) = [y :: a, x]
The pattern type signature (x::a) brings 'a' into scope.
The 'a' in the pattern (y::a) is bound, as is the occurrence on
A pattern type siganture is the only way you can bring existentials
data T where
MkT :: forall a. a -> (a->Int) -> T
f x = case x of
MkT (x::a) f -> f (x::a)
class C a where
op :: forall b. b->a->a
instance C (T p q) where
op = <rhs>
Clearly p,q are in scope in <rhs>, but is 'b'? Not at the moment.
Nor can you add a type signature for op in the instance decl.
You'd have to say this:
instance C (T p q) where
op = let op' :: forall b. ...
op' = <rhs>
3) A pattern-bound type variable is allowed only if the pattern's
expected type is rigid. Otherwise we don't know exactly *which*
skolem the scoped type variable should be bound to, and that means
we can't do GADT refinement. This is invariant (A), and it is a
big change from the current situation.
f (x::a) = x -- NO; pattern type is wobbly
g1 :: b -> b
g1 (x::b) = x -- YES, because the pattern type is rigid
g2 :: b -> b
g2 (x::c) = x -- YES, same reason
h :: forall b. b -> b
h (x::b) = x -- YES, but the inner b is bound
k :: forall b. b -> b
k (x::c) = x -- NO, it can't be both b and c
3a) You cannot give different names for the same type variable in the same scope
f1 :: p -> p -> p -- NO; because 'a' and 'b' would be
f1 (x::a) (y::b) = (x::a) -- bound to the same type variable
f2 :: p -> p -> p -- OK; 'a' is bound to the type variable
f2 (x::a) (y::a) = (x::a) -- over which f2 is quantified
-- NB: 'p' is not lexically scoped
f3 :: forall p. p -> p -> p -- NO: 'p' is now scoped, and is bound to
f3 (x::a) (y::a) = (x::a) -- to the same type varialble as 'a'
f4 :: forall p. p -> p -> p -- OK: 'p' is now scoped, and its occurences
f4 (x::p) (y::p) = (x::p) -- in the patterns are bound by the forall
3b) You can give a different name to the same type variable in different
disjoint scopes, just as you can (if you want) give diferent names to
the same value parameter
g :: a -> Bool -> Maybe a
g (x::p) True = Just x :: Maybe p
g (y::q) False = Nothing :: Maybe q
3c) Scoped type variables respect alpha renaming. For example,
function f2 from (3a) above could also be written:
f2' :: p -> p -> p
f2' (x::b) (y::b) = x::b
where the scoped type variable is called 'b' instead of 'a'.
4) Result type signatures obey the same rules as pattern types signatures.
In particular, they can bind a type variable only if the result type is rigid
f x :: a = x -- NO
g :: b -> b
g x :: b = x -- YES; binds b in rhs
5) A *pattern type signature* in a *pattern binding* cannot bind a
scoped type variable
(x::a, y) = ... -- Legal only if 'a' is already in scope
Reason: in type checking, the "expected type" of the LHS pattern is
always wobbly, so we can't bind a rigid type variable. (The exception
would be for an existential type variable, but existentials are not
allowed in pattern bindings either.)
Even this is illegal
f :: forall a. a -> a
f x = let ((y::b)::a, z) = ...
Here it looks as if 'b' might get a rigid binding; but you can't bind
it to the same skolem as a.
6) Explicitly-forall'd type variables in the *declaration type signature(s)*
for a *pattern binding* do not scope AT ALL.
x :: forall a. a->a -- NO; the forall a does
Just (x::a->a) = Just id -- not scope at all
y :: forall a. a->a
Just y = Just (id :: a->a) -- NO; same reason
THIS IS A CHANGE, but one I bet that very few people will notice.
strange :: forall b. (b->b,b->b)
strange = (id,id)
x1 :: forall a. a->a
y1 :: forall b. b->b
(x1,y1) = strange
This is legal Haskell 98 (modulo the forall). If both 'a' and 'b'
both scoped over the RHS, they'd get unified and so cannot stand
for distinct type variables. One could *imagine* allowing this:
x2 :: forall a. a->a
y2 :: forall a. a->a
(x2,y2) = strange
using the very same type variable 'a' in both signatures, so that
a single 'a' scopes over the RHS. That seems defensible, but odd,
because though there are two type signatures, they introduce just
*one* scoped type variable, a.
7) Possible extension. We might consider allowing
\(x :: [ _ ]) -> <expr>
where "_" is a wild card, to mean "x has type list of something", without
naming the something.
M ./ghc/compiler/basicTypes/BasicTypes.lhs -1 +5
M ./ghc/compiler/basicTypes/Id.lhs -3 +9
M ./ghc/compiler/basicTypes/Module.lhs -1
M ./ghc/compiler/basicTypes/Name.lhs -3 +7
M ./ghc/compiler/basicTypes/SrcLoc.lhs -1 +1
M ./ghc/compiler/basicTypes/Var.lhs -9 +11
M ./ghc/compiler/basicTypes/VarEnv.lhs -1 +3
M ./ghc/compiler/coreSyn/PprCore.lhs -13 +26
M ./ghc/compiler/deSugar/Check.lhs -1 +1
M ./ghc/compiler/deSugar/DsBinds.lhs -4 +35
M ./ghc/compiler/deSugar/DsExpr.lhs -3 +6
M ./ghc/compiler/deSugar/DsMeta.hs -6 +7
M ./ghc/compiler/deSugar/DsUtils.lhs -3 +6
M ./ghc/compiler/hsSyn/Convert.lhs -4 +4
M ./ghc/compiler/hsSyn/HsBinds.lhs -46 +104
M ./ghc/compiler/hsSyn/HsExpr.lhs -2 +15
M ./ghc/compiler/hsSyn/HsTypes.lhs -1 +1
M ./ghc/compiler/hsSyn/HsUtils.lhs -35 +13
M ./ghc/compiler/iface/IfaceSyn.lhs -3 +3
M ./ghc/compiler/iface/IfaceType.lhs -1 +1
M ./ghc/compiler/iface/TcIface.lhs -18 +4
M ./ghc/compiler/main/GHC.hs -2 +2
M ./ghc/compiler/main/HscMain.lhs -1 +1
M ./ghc/compiler/main/HscStats.lhs -3 +3
M ./ghc/compiler/main/PprTyThing.hs -1 +1
M ./ghc/compiler/parser/Parser.y.pp -7 +7
M ./ghc/compiler/parser/RdrHsSyn.lhs -16 +17
M ./ghc/compiler/rename/RnBinds.lhs -13 +14
M ./ghc/compiler/typecheck/Inst.lhs -123 +65
M ./ghc/compiler/typecheck/TcArrows.lhs -45 +38
M ./ghc/compiler/typecheck/TcBinds.lhs -151 +275
M ./ghc/compiler/typecheck/TcClassDcl.lhs -34 +37
M ./ghc/compiler/typecheck/TcEnv.lhs -19 +52
M ./ghc/compiler/typecheck/TcExpr.lhs -547 +584
M ./ghc/compiler/typecheck/TcExpr.lhs-boot -10 +5
M ./ghc/compiler/typecheck/TcForeign.lhs -2 +2
M ./ghc/compiler/typecheck/TcGenDeriv.lhs -2 +2
M ./ghc/compiler/typecheck/TcHsSyn.lhs -62 +48
M ./ghc/compiler/typecheck/TcHsType.lhs -131 +128
M ./ghc/compiler/typecheck/TcMType.lhs -299 +251
M ./ghc/compiler/typecheck/TcMatches.lhs -295 +158
M ./ghc/compiler/typecheck/TcMatches.lhs-boot -5 +5
M ./ghc/compiler/typecheck/TcPat.lhs -250 +440
M ./ghc/compiler/typecheck/TcRnDriver.lhs -8 +9
M ./ghc/compiler/typecheck/TcRnMonad.lhs -28 +44
M ./ghc/compiler/typecheck/TcRnTypes.lhs -36 +29
M ./ghc/compiler/typecheck/TcRules.lhs -10 +16
M ./ghc/compiler/typecheck/TcSimplify.lhs -6 +7
M ./ghc/compiler/typecheck/TcSplice.lhs -17 +17
M ./ghc/compiler/typecheck/TcSplice.lhs-boot -3 +3
M ./ghc/compiler/typecheck/TcTyClsDecls.lhs -24 +5
M ./ghc/compiler/typecheck/TcTyDecls.lhs -2 +2
M ./ghc/compiler/typecheck/TcType.hi-boot-6 +1
M ./ghc/compiler/typecheck/TcType.lhs -175 +339
M ./ghc/compiler/typecheck/TcType.lhs-boot +2
M ./ghc/compiler/typecheck/TcUnify.hi-boot-6 -1 +2
M ./ghc/compiler/typecheck/TcUnify.lhs -697 +851
M ./ghc/compiler/typecheck/TcUnify.lhs-boot -2 +3
M ./ghc/compiler/types/Generics.lhs -7 +4
M ./ghc/compiler/types/TyCon.lhs -4 +7
M ./ghc/compiler/types/Type.lhs -64 +18
M ./ghc/compiler/types/Unify.lhs -11 +23
M ./ghc/compiler/utils/IOEnv.hs -4 +4
M ./ghc/compiler/utils/UniqFM.lhs -5 +5
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