类型声明错误：[Char]和[[Char]]

Question

With the below snippet I want to append/prepend a string to a list that is part of a value stored in a map.

From the snippet below I get the error

Couldn't match expected type `Char' with actual type `[Char]'
    Expected type: Map.Map ([Char], Integer) [Char]
      Actual type: Map.Map ([Char], Integer) [[Char]]

and I am not quite sure what that should tell me. Can that be solved with a change in the code or must there be something like

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances #-}

Together with an instance declaration?

import Data.Time
import Data.Time.Clock.POSIX
import qualified Data.PSQueue as PSQ
import qualified Data.Map as Map
import Data.Maybe
import Control.Category
import Control.Concurrent
import Control.Concurrent.MVar
import Control.Monad

key = ("192.168.1.1", 4711)
messages = ["aaa", "bbbb", "ccccc"]

newRq = do
      time <- getPOSIXTime
      let q = PSQ.singleton key time
      let m = Map.singleton key messages
      return (q, m)

appendMsg :: String -> (String, Integer) -> Map.Map ([Char], Integer) [Char] -> Map.Map ([Char], Integer) [Char]
appendMsg  a (b, c) m = m2
      where 
          f x = x ++ a
          m2 = Map.adjust f (b, c) m

main :: IO()
main = do
     (q, m) <- newRq
     let m2 = appendMsg "first" key m
     print (m2)

Answer 1

You need a change in the code. You start with a Map (String, Integer) [String] , by creating a singleton with value messages . Your update function however is written for Map (String, Integer) String . Either change it to the appropriate type by making it fx = x ++ [a] (and change the type signature to

appendMsg :: String -> (String, Integer) -> Map (String, Integer) [String]
                                         -> Map (String, Integer) [String]

too) or change your initial map by using eg unwords messages . (I think you'll rather want the first.)

Answer 2

[Char] and String are the same type. The Haskell prelude has the following type synonym definition:

type String = [Char]

... which basically means those two types mean the exact same thing and are completely interchangeable. This means we can rewrite the compiler error as:

Expected type: Map (String, Integer) String
  Actual type: Map (String, Integer) [String]

The two type parameters of the Map type are its Key type and its Value type. So if you have a Map Int String type, that means it's a map with Int s as keys and Strings as values. This means you can now interpret the compiler message a little more clearly:

Expected type: Map from keys of type "(String, Integer)" to values of type "String"
  Actual type: Map from keys of type "(String, Integer)" to values of type "[String]"

This tells us that somewhere in your code you gave it a map where every value is a [String] (ie a list of strings ), but it was expecting a map where every value was just a single String . In fact, you can trace the error to this line of code:

let m = Map.singleton key messages

messages is, you guessed it, an list of String s (ie [String] ), and you told it to store the entire array messages as a single element in the map m when you used the singleton function to initialize it. So Haskell correctly inferred the value type of m had to be [String] (which was obviously not what you intended).

The compiler then noticed something was amiss when you tried to append a string to every element in your map in the following line:

let m2 = appendMsg "first" key m

You told the compiler "Please concatenate this string onto every element of my map" and the compiler then tells you "But you don't have multiple strings stored in your map; You have just one list of strings stored as a single value in your map, so I can't append 'first' onto the array itself".

The fix is simple where instead of using singleton to initialize m , you just fromList , which takes a list of elements (ie messages ) and converts each element in the list to an element of the map, which is what you intended.

Answer 3

The map you're creating with Map.singleton key messages is a Map.Map (String, Integer) [String] . However your declaration of appendMsg indicates that you really want a Map.Map (String, Integer) String . You need to revisit your newRq and figure out what you're really trying to do with that Map.singleton line.