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?
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]"