How to use Reader?

20 Jun 2018

      Hi all,

I think I'm pretty close but not quite there yet. Let's say I have the
following:

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Prelude (IO)
import Data.Text (Text, append)
import Data.Text.IO (putStrLn)

main :: IO ()
main =
    putStrLn (showText "Hello World!" config)
    where
        config :: Config
        config = Config (A "Boo")

data A = A Text

data Config = Config { a :: A }

showText :: Text -> Config -> Text
showText text config =
    "[" `append` text `append` (showA config) `append` "]"

showA :: Config -> Text
showA config =
    "'" `append` text `append` "'"
    where
        A text = a config

Now I want to use Reader. This is as close as I can get:

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Prelude (IO, return, ($))
import Data.Text (Text, append)
import Data.Text.IO (putStrLn)
import Control.Monad.Reader (Reader, asks, runReader)

main :: IO ()
main =
    putStrLn (runReader (showText "Hello World!") config)
    where
        config :: Config
        config = Config (A "Boo")

data A = A Text

data Config = Config { a :: A }

showText :: Text -> Reader Config Text
showText text =
    return $
        "[" `append` text `append` showA `append` "]"

showA :: Reader Config Text
showA = do
    A text <- asks a
    return $
        "'" `append` text `append` "'"

This is almost correct, except for the showA invocation in showText.
It wants a Text but it's getting a Reader Config Text. What is the
magic to make this work?

Cheers,
Hilco

Hilco Wijbenga

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