Safe Haskell | Trustworthy |
---|---|
Language | Haskell98 |
Documentation
The Tag
type is like an ad-hoc GADT allowing runtime creation of new
constructors. Specifically, it is like a GADT "enumeration" with one
phantom type.
A Tag
constructor can be generated in any primitive monad (but only tags
from the same one can be compared). Every tag is equal to itself and to
no other. The GOrdering
class allows rediscovery of a tag's phantom type,
so that Tag
s and values of type
can be tested for
equality even when their types are not known to be equal.DSum
(Tag
s)
Tag
uses a Uniq
as a witness of type equality, which is sound as long
as the Uniq
is truly unique and only one Tag
is ever constructed from
any given Uniq
. The type of newTag
enforces these conditions.
veryUnsafeMkTag
provides a way for adventurous (or malicious!) users to
assert that they know better than the type system.
newTag :: PrimMonad m => m (Tag (PrimState m) a) Source #
Create a new tag witnessing a type a
. The GEq
or GOrdering
instance
can be used to discover type equality of two occurrences of the same tag.
(I'm not sure whether the recovery is sound if a
is instantiated as a
polymorphic type, so I'd advise caution if you intend to try it. I suspect
it is, but I have not thought through it very deeply and certainly have not
proved it.)
RealWorld
is deeply magical. It is primitive, but it is not
unlifted (hence ptrArg
). We never manipulate values of type
RealWorld
; it's only used in the type system, to parameterise State#
.
Instances
Show (Uniq RealWorld) Source # | There is only one |
GShow (Tag RealWorld) Source # | |
Defined in Unsafe.Unique.Tag | |
Show (Tag RealWorld a) Source # | |
data (a :: k) :~: (b :: k) :: forall k. k -> k -> Type where infix 4 #
Propositional equality. If a :~: b
is inhabited by some terminating
value, then the type a
is the same as the type b
. To use this equality
in practice, pattern-match on the a :~: b
to get out the Refl
constructor;
in the body of the pattern-match, the compiler knows that a ~ b
.
Since: base-4.7.0.0
Instances
TestEquality ((:~:) a :: k -> Type) | Since: base-4.7.0.0 |
Defined in Data.Type.Equality | |
GShow ((:~:) a :: k -> Type) | |
Defined in Data.GADT.Internal gshowsPrec :: Int -> (a :~: a0) -> ShowS # | |
GRead ((:~:) a :: k -> Type) | |
Defined in Data.GADT.Internal greadsPrec :: Int -> GReadS ((:~:) a) # | |
GEq ((:~:) a :: k -> Type) | |
GCompare ((:~:) a :: k -> Type) | |
a ~ b => Bounded (a :~: b) | Since: base-4.7.0.0 |
a ~ b => Enum (a :~: b) | Since: base-4.7.0.0 |
Defined in Data.Type.Equality | |
Eq (a :~: b) | Since: base-4.7.0.0 |
Ord (a :~: b) | Since: base-4.7.0.0 |
Defined in Data.Type.Equality | |
a ~ b => Read (a :~: b) | Since: base-4.7.0.0 |
Show (a :~: b) | Since: base-4.7.0.0 |
class GEq (f :: k -> Type) where #
A class for type-contexts which contain enough information to (at least in some cases) decide the equality of types occurring within them.
geq :: f a -> f b -> Maybe (a :~: b) #
Produce a witness of type-equality, if one exists.
A handy idiom for using this would be to pattern-bind in the Maybe monad, eg.:
extract :: GEq tag => tag a -> DSum tag -> Maybe a extract t1 (t2 :=> x) = do Refl <- geq t1 t2 return x
Or in a list comprehension:
extractMany :: GEq tag => tag a -> [DSum tag] -> [a] extractMany t1 things = [ x | (t2 :=> x) <- things, Refl <- maybeToList (geq t1 t2)]
(Making use of the DSum
type from Data.Dependent.Sum in both examples)
data GOrdering (a :: k) (b :: k) :: forall k. k -> k -> Type where #
A type for the result of comparing GADT constructors; the type parameters of the GADT values being compared are included so that in the case where they are equal their parameter types can be unified.
GLT :: forall k (a :: k) (b :: k). GOrdering a b | |
GEQ :: forall k (a :: k) (b :: k). GOrdering a a | |
GGT :: forall k (a :: k) (b :: k). GOrdering a b |
Instances
GShow (GOrdering a :: k -> Type) | |
Defined in Data.GADT.Internal gshowsPrec :: Int -> GOrdering a a0 -> ShowS # | |
GRead (GOrdering a :: k -> Type) | |
Defined in Data.GADT.Internal greadsPrec :: Int -> GReadS (GOrdering a) # | |
Eq (GOrdering a b) | |
Ord (GOrdering a b) | |
Defined in Data.GADT.Internal compare :: GOrdering a b -> GOrdering a b -> Ordering # (<) :: GOrdering a b -> GOrdering a b -> Bool # (<=) :: GOrdering a b -> GOrdering a b -> Bool # (>) :: GOrdering a b -> GOrdering a b -> Bool # (>=) :: GOrdering a b -> GOrdering a b -> Bool # | |
Show (GOrdering a b) | |
class GEq f => GCompare (f :: k -> Type) where #
Type class for comparable GADT-like structures. When 2 things are equal,
must return a witness that their parameter types are equal as well (GEQ
).