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dependent-map [](https://travis-ci.org/obsidiansystems/dependent-map) [](http://hackage.haskell.org/package/dependent-map)
==============
This library defines a dependently-typed finite map type. It is derived from `Data.Map.Map` in the `containers` package, but rather than (conceptually) storing pairs indexed by the first component, it stores `DSum`s (from the `dependent-sum` package) indexed by tag. For example
```haskell
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module Example where
import Data.Constraint.Extras.TH (deriveArgDict)
import Data.Dependent.Map (DMap, fromList, singleton, union, unionWithKey)
import Data.Dependent.Sum ((==>))
import Data.Functor.Identity (Identity(..))
import Data.GADT.Compare.TH (deriveGCompare, deriveGEq)
import Data.GADT.Show.TH (deriveGShow)
data Tag a where
StringKey :: Tag String
IntKey :: Tag Int
DoubleKey :: Tag Double
deriveGEq ''Tag
deriveGCompare ''Tag
deriveGShow ''Tag
deriveArgDict ''Tag
x :: DMap Tag Identity
x = fromList [DoubleKey ==> pi, StringKey ==> "hello there"]
y :: DMap Tag Identity
y = singleton IntKey (Identity 42)
z :: DMap Tag Identity
z = y `union` fromList [DoubleKey ==> -1.1415926535897931]
addFoo :: Tag v -> Identity v -> Identity v -> Identity v
addFoo IntKey (Identity x) (Identity y) = Identity $ x + y
addFoo DoubleKey (Identity x) (Identity y) = Identity $ x + y
addFoo _ x _ = x
main :: IO ()
main = mapM_ print
[ x, y, z
, unionWithKey addFoo x z
]
```
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