dependent-sum

This library defines a dependent sum type:

data DSum tag f = forall a. !(tag a) :=> f a

By analogy to the key => value construction for dictionary entries in many dynamic languages, we use :=> as the constructor for dependent sums. The key is a tag that specifies the type of the value; for example, think of a GADT such as:

data Tag a where
  StringKey :: Tag String
  IntKey    :: Tag Int

Then, we have the following valid expressions of type DSum Tag []:

StringKey :=> ["hello!"]
IntKey    :=> [42]

And we can write functions that consume DSum Tag values by matching, such as:

toString :: DSum Tag [] -> [String]
toString (StringKey :=> strs) = strs
toString (IntKey    :=> ints) = show ints

The :=> and ==> operators have very low precedence and bind to the right, so if the Tag GADT is extended with an additional constructor Rec :: Tag (DSum Tag Identity), then Rec ==> AnInt ==> 3 + 4 is parsed as would be expected (Rec ==> (AnInt ==> (3 + 4))) and has type DSum Identity Tag. Its precedence is just above that of $, so foo bar $ AString ==> "eep" is equivalent to foo bar (AString ==> "eep").

In order to support basic type classes from the Prelude for DSum, there are also several type classes defined for "tag" types:

• GEq tag is similar to an Eq instance for tag a except that with geq, values of types tag a and tag b may be compared, and in the case of equality, evidence that the types a and b are equal is provided.
• GCompare tag is similar to the above for Ord, and provides gcompare, giving a GOrdering that gives similar evidence of type equality when values match.
• GShow tag means that tag a has (the equivalent of) a Show instance.
• GRead tag means that tag a has (the equivalent of) a Read instance.

In order to be able to compare values of type DSum tag f for equality, in addition to having a GEq tag instance, we need to know that, given a value t :: tag a, we may obtain an instance Eq (f a), which is expressed by the use of the Has' Eq tag f constraint from the constraints-extras package, so we have the following instances:

(GEq tag, Has' Eq tag f) => Eq (DSum tag f)
(GCompare tag, Has' Eq tag f, Has' Ord tag f) => Ord (DSum tag f)
(GShow tag, Has' Show tag f) => Show (DSum tag f)
(GRead tag, Has' Read tag f) => Read (DSum tag f)

In order to satisfy the Has' constraints, you'll want to use deriveArgDict from constraints-extras, or less-commonly, write your own instance of the ArgDict class by hand, in addition to making sure that it's actually the case that for every value of your tag type, there will be a corresponding instance of Eq/Ord/Read/Show as appropriate.

For example implementations of these classes, see the generated Haddock docs or the code in the examples directory. There is a fair amount of boilerplate. A few of the more common classes (GEq, GCompare, and GShow) can be automatically derived by Template Haskell code in the dependent-sum-template package.