[Haskell-beginners] Fwd: IO String and String using readFile

[Ian Denhardt]

Ah, in that case you'll need the type to be

    String -> IO [Int]

In Haskell, functions *only* do computation -- no side effects, like
reading a file. So a String -> [Int] can't do what you want. We have a
separate type, IO, for doing things that affect the outside world. For
example:

ghci> :type readFile
readFile :: FilePath -> IO String

Also, just a style tip: there's a type alias for String defined called
FilePath, which would make the type signature more clear in this case.

Quoting Yugesh Kothari (2019-03-26 23:54:40)
>    Extremely sorry,
>    The definition of fromFile is String -> [Int] where input parameter is
>    the name of the file.
>    ---------- Forwarded message ---------
>    From: Yugesh Kothari <[1]kothariyugesh at gmail.com>
>    Date: Wed, 27 Mar, 2019, 9:18 AM
>    Subject: Re: [Haskell-beginners] IO String and String using readFile
>    To:
>    Cc: <[2]beginners at haskell.org>
>    I see. anyway, my use case is something like this-
>    I have two functions
>    fromFile :: [String] -> [Int]
>    func :: String -> [Int]
>    I want to use the "words contents" output and pass each element in it
>    to func and send back [Int] from "fromFile" function (where I
>    originally read the file.)
>    Could you suggest a better way to do this?
>    Quoting� Ian Denhardt
>    On Wed, 27 Mar, 2019, 9:13 AM Ian Denhardt, <[3]ian at zenhack.net> wrote:
>
>      It would help to see the complete source file, but I'll hazard a
>      guess
>      you're doing something like:
>      main = do
>      �  �  contents <- readFile "file.txt"
>      �  �  words contents
>      At the GHCi prompt, each line can be any expression, and GHCi will
>      evaluate it and then display it. The two lines in your session
>      aren't
>      really related.
>      In contrast, the do block in the source file expects the expression
>      at
>      the end to be an IO action.
>      What do you want your program to do when you get to "words
>      contents"?
>      display it? (If you want to display it, pass it to `print`).
>      Quoting Yugesh Kothari (2019-03-26 23:35:14)
>      >�  �  Hi,�
>      >�  �  I am trying to read data from a file.
>      >�  �  When I do-
>      >�  �  ghci> contents <- readFile "file.txt"
>      >�  �  ghci> words contents
>      >�  �  Then everything works.
>      >�  �  The same thing when done in a .hs file gives an IO String vs
>      [String]
>      >�  �  error.
>      >�  �  Why is that so?
>
> Verweise
>
>    1. mailto:kothariyugesh at gmail.com
>    2. mailto:beginners at haskell.org
>    3. mailto:ian at zenhack.net