What's wrong with "func :: String -> [Int]; func = read "[3,5,7]""

Question

In a very simple module test where I have the following function

func :: String -> [Int]
func = read "[3,5,7]"

Since I have explicit type annotations, I expect to get [3,5,7] when I load the module test and call func in ghci. However, I got

    • No instance for (Read (String -> [Int]))
        arising from a use of ‘read’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: read "[3,5,7]"
      In an equation for ‘func’: func = read "[3,5,7]"
   |
11 | func = read "[3,5,7]"
   |        ^^^^^^^^^^^^^^

But when I do read "[3,5,7]" :: [Int], [3,5,7] is returned as expected. Why an error was raised when I loaded the module instead?

Answer 1

Your function type is String -> [Int] but you didn't specify its argument so the compiler "thinks" that you want to return a function String -> [Int] instead of [Int].

You probably want:

func :: String -> [Int]
func s = read s

and then use it as:

func "[3,5,7]"

or just:

func :: String -> [Int]
func _ = read "[3,5,7]"