dependent-map-0.3: README.md

dependent-map [![Build Status](https://travis-ci.org/mokus0/dependent-map.svg)](https://travis-ci.org/mokus0/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 (using the types from the `dependent-sum` package's `FooGADT` example):

    {-# LANGUAGE GADTs #-}
    import FooGADT
    import Data.Dependent.Map
    
    x = fromList [Foo :=> pi, Baz :=> "hello there"]
    y = singleton Bar 42
    z = union y (read "fromList [Foo :=> (-1.1415926535897931)]")
    
    addFoo :: Foo v -> v -> v -> v
    addFoo Foo x y = x + y
    addFoo _   x _ = x
    
    main = mapM_ print
        [ x, y, z
        , unionWithKey addFoo x z
        ]

Which prints:

    fromList [Foo :=> 3.141592653589793,Baz :=> "hello there"]
    fromList [Bar :=> 42]
    fromList [Foo :=> -1.1415926535897931,Bar :=> 42]
    fromList [Foo :=> 2.0,Bar :=> 42,Baz :=> "hello there"]

This library can be found on Hackage: https://hackage.haskell.org/package/dependent-map