尝试编写 Haskell 函数以获取字符串参数并附加到文件

Question

I am trying to write a function that takes a string as input and appends it to a file. the file is the same file every time. the function compiles correctly but gives the error

"No instance for (Show (String -> IO ())) arising from a use of `print'".

The code is:

appendText :: String -> IO ()

appendText [] = return ()

appendText x = appendFile "testfile.txt" x 

I understand the problem is something to do with types and the Show type but I'm having trouble understanding the problem and how to correct it. Any help would be great.

Edit: I am calling it as a function, eg appendFile "random text"

Answer 1

Did you attempt to write something like appendText or print appendText in GHCi? If so, it's attempting to Show the function, which is a type error since function types don't have a Show instance. You need to invoke it with an argument, being the particular String that you want to append to testfile.txt :

appendText "test text"

The type of this is IO () , so it's an action that can be executed, and after running it you ought to see the text appear in the file.

Your function can also be written more simply as:

appendText x = appendFile "testfile.txt" x

Or in point-free form:

appendText = appendFile "testfile.txt"

Answer 2

So to be clear, removing the second line allowed the function to work:

appendText :: String -> IO ()

appendText = appendFile "testfile.txt"

I'm not sure why this fixed the problem, that line was there for error handling purposes but apparently wasn't needed. It works though, thanks for the help.