How to represent "Data.Map Text Text" in Dhall?

Question

If I have a type in Haskell like this:

data MyType = MyType
  { env :: Map Text Text
  }

How can I represent a value of MyType in Dhall?

{ env = ???
}

What I want to do is to write values of MyType in Dhall and then read it in from Haskell and unmarshal it into MyType, like this:

main :: IO ()
main = do
    x <- input auto "./config"
    print (x :: MyType)

I'm coming from Data.Aeson and YAML where you can represent maps like this:

env:
  KEY1: "foo"
  KEY2: "bar"

(you would be able to parse the above into the MyType type using Aeson's decodeFileEither).

Answer 1

After some digging, I found three workarounds. Skip to the bottom if you want the best workaround until toMap lands.

As of 2019-05-05 there is no way to represent maps in Dhall like how it is possible with Aeson/YAML (although support for a native toMap function is coming soon). So for now we basically have to use a list of homogeneous records. It's a bit clunky, but at least you get the native unmarshaling.

If we want to use a list of tuples instead of a map, we can do this:

{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE GeneralizedNewtypeDeriving   #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE RecordWildCards     #-}

module Tuple where

import Dhall
import qualified Data.Text as T

data MyType = MyType { env :: [MyTuple] }
    deriving (Generic, Show)

instance Interpret MyType

newtype MyTuple = MyTuple (T.Text, T.Text)
    deriving (Interpret, Show)

-- input auto "{env = [{_1= \"HOME\", _2 = \"foo\"}] }" :: IO MyType

The above was adapted from this answer, which showed a way to parse IP addresses as 4-element tuples.

For parsing into a Map, we can do:

{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE FlexibleInstances     #-}

module MapA where

import Data.Map (Map)
import Data.Text (Text)
import Dhall

import qualified Data.Map

data MyType = MyType { env :: Map Text Text }
    deriving (Generic, Show)

data KeyValue a = KeyValue { mapKey :: Text, mapValue :: a }
    deriving (Generic, Show)

toMap :: [KeyValue a] -> Map Text a
toMap keyValues = Data.Map.fromList (map adapt keyValues)
  where
    adapt (KeyValue {..}) = (mapKey, mapValue)

instance Interpret MyType
instance Interpret a => Interpret (KeyValue a)

-- Wrap `Map` in a newtype if you want to avoid an orphan instance
instance Interpret a => Interpret (Map Text a) where
    autoWith options = fmap toMap (autoWith options)

-- input auto "{env = [{mapKey = \"HOME\", mapValue = \"foo\"}] }" :: IO MapA.MyType

The above was adapted from this comment. The idea is to make records that look like { mapKey = X, mapValue = Y} parseable, and then to convert any lists of such records into a Map. Notice how we support any value type, not just text (so we can have env in MyType be Map Text Int or something else, if we wanted to). This solution has just 1 type variable a for the values in the map, but I suppose it is possible to make the keys more generic as well.

OK so after some tweaking, I got the following to compile, which supports both keys and values to be generic as well:

{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE FlexibleInstances     #-}

module MapKV where

import Data.Map (Map)
import Data.Text (Text)
import Dhall

import qualified Data.Map

data MyType = MyType { env :: Map Text Text }
    deriving (Generic, Show)

data MyTypeInts = MyTypeInts { envInts :: Map Integer Integer }
    deriving (Generic, Show)

data KeyValue k v = KeyValue { mapKey :: k, mapValue :: v }
    deriving (Generic, Show)

toMap :: Ord k => [KeyValue k v] -> Map k v
toMap keyValues = Data.Map.fromList (map adapt keyValues)
  where
    adapt (KeyValue {..}) = (mapKey, mapValue)

instance Interpret MyType
instance Interpret MyTypeInts
instance (Interpret k, Interpret v) => Interpret (KeyValue k v)

-- Wrap `Map` in a newtype if you want to avoid an orphan instance
instance (Ord k, Interpret k, Interpret v) => Interpret (Map k v) where
    autoWith options = fmap toMap (autoWith options)

-- input auto "{env = [{mapKey = +1, mapValue = \"foo\"}] }" :: IO MapKV.MyType
-- input auto "{envInts = [{mapKey = +1, mapValue = -22 }] }" :: IO MapKV.MyTypeInts

Lastly here is a version that avoids the orphan instance that uses an Env newtype wrapper:

{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE FlexibleInstances     #-}

module MapKV where

import Data.Map (Map)
import Dhall

import qualified Data.Map

data MyType = MyType { env :: Env }
    deriving (Generic, Show)

newtype Env = Env (Map Text Text)
  deriving (Eq, Generic, Show)

data KeyValue k v = KeyValue { mapKey :: k, mapValue :: v }
    deriving (Generic, Show)

toMap :: Ord k => [KeyValue k v] -> Map k v
toMap keyValues = Data.Map.fromList (map adapt keyValues)
  where
    adapt (KeyValue {..}) = (mapKey, mapValue)

instance Interpret MyType
instance (Interpret k, Interpret v) => Interpret (KeyValue k v)

instance Interpret Env where
    autoWith options = fmap (Env . toMap) (autoWith options)

-- input auto "{env = [{mapKey = \"HOME\", mapValue = \"foo\"}] }" :: IO MapKV.MyType