Haskell：无法将类型“Char”与“[Char]”匹配

Question

I'm new to Haskell and I'm trying to create a simple function that manipulates a string similar to toLower , toUpper , and switchCase in other languages. However, whenever I try to pass isUpper or isLower into the template function, it throws at me this error.

* Couldn't match type `Char' with `[Char]'
      Expected: [Char] -> Bool
        Actual: Char -> Bool
    * In the second argument of `lambdaTextConv', namely `isLower'
      In the expression: lambdaTextConv str isLower
      In an equation for `toLower':
          toLower str = lambdaTextConv str isLower
   |
53 | toLower str = lambdaTextConv str isLower
   |                                  ^^^^^^^
Failed, no modules loaded.

The code itself is a simple function that takes a string, and switches cases whenever appropriate.

import System.IO
import Data.List
import Data.Maybe  

letters = take 26 (zip ['A'..] ['a'..])
upperLetters = ['A'..'Z']
lowerLetters = ['a'..'z']

lambdaTextConv str ltrFnc =
    [
        lambdaNewLetter x ltrFnc
    | x <- str ]
lambdaNewLetter x ltrFnc =
    if (ltrFnc x)
    then x
    else ((switchCase x:[]) !! 0)

isUpper char = char `elemIndex` upperLetters /= Nothing
isLower char = char `elemIndex` lowerLetters /= Nothing

toUpper str = lambdaTextConv str isUpper
toLower str = lambdaTextConv str isLower

switchCase str =
    [
        if (isUpper x)
        then snd(letters !! fromJust(x `elemIndex` upperLetters))
        else fst(letters !! fromJust(x `elemIndex` lowerLetters))
    | x <- str ]

Answer 1

The solution turned out to be really simple. The GHC compiler inferred that lambdaTextConv was of type [Char] -> ([Char] -> Bool) -> [Char] because of the line

    else ((switchCase x:[]) !! 0)

when it should have been

    else ((switchCase (x:[])) !! 0)

The source now looks more like this (imports omitted)

letters = take 26 (zip ['A'..] ['a'..])
upperLetters = ['A'..'Z']
lowerLetters = ['a'..'z']

lambdaTextConv :: [Char] -> (Char -> Bool) -> String
lambdaTextConv str ltrFnc =
    [
        lambdaNewLetter x ltrFnc
    | x <- str ]

lambdaNewLetter :: Char -> (Char -> Bool) -> Char
lambdaNewLetter x ltrFnc =
    if (ltrFnc x)
    then x
    else ((switchCase (x:[])) !! 0)

isUpper char = char `elemIndex` upperLetters /= Nothing
isLower char = char `elemIndex` lowerLetters /= Nothing

toUpper str = lambdaTextConv str isUpper
toLower str = lambdaTextConv str isLower

switchCase str =
    [
        if (isUpper x)
        then snd(letters !! fromJust(x `elemIndex` upperLetters))
        else fst(letters !! fromJust(x `elemIndex` lowerLetters))
    | x <- str ]