Data.Unique.Tag

Documentation

data Tag s a

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 Tags and values of type DSum (Tag s) can be tested for equality even when their types are not known to be equal.

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.

Instances

GEq (Tag s)

GCompare (Tag s)

GShow (Tag RealWorld)

Eq (Tag s a)

Ord (Tag s a)

Show (Tag RealWorld a)

newTag :: PrimMonad m => m (Tag (PrimState m) a)

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.)

data RealWorld

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)

There is only one RealWorld, so this instance is sound (unlike the general unsafeShowsPrecUniq). Note that there is no particular relationship between Uniq values (or the strings show turns them into) created in different executions of a program. The value they render to should be considered completely arbitrary, and the Show instance only even exists for convenience when testing code that uses Uniqs.

GShow (Tag RealWorld)

Show (Tag RealWorld a)

data a := b where

A GADT witnessing equality of two types. Its only inhabitant is Refl.

Constructors

Refl :: := a a

Instances

Typeable2 :=

GEq (:= a)

GCompare (:= a)

GShow (:= a)

GRead (:= a)

Eq (:= a b)

Ord (:= a b)

Read (:= a a)

Show (:= a b)

class GEq f where

A class for type-contexts which contain enough information to (at least in some cases) decide the equality of types occurring within them.

Methods

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)

Instances

GEq (:= a)

GEq (Tag s)

data GOrdering a b 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.

Constructors

GLT :: GOrdering a b

GEQ :: GOrdering t t

GGT :: GOrdering a b

Instances

Typeable2 GOrdering

GShow (GOrdering a)

GRead (GOrdering a)

Eq (GOrdering a b)

Ord (GOrdering a b)

Show (GOrdering a b)

class GEq f => GCompare f where

Type class for orderable GADT-like structures. When 2 things are equal, must return a witness that their parameter types are equal as well (GEQ). |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).

Methods

gcompare :: f a -> f b -> GOrdering a b

Instances

GCompare (:= a)

GCompare (Tag s)