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?
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?
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,
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?
Extremely sorry,
The definition of fromFile is String -> [Int] where input parameter is the
name of the file.
---------- Forwarded message ---------
From: Yugesh Kothari
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?
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@gmail.com> Date: Wed, 27 Mar, 2019, 9:18 AM Subject: Re: [Haskell-beginners] IO String and String using readFile To: Cc: <[2]beginners@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@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@gmail.com 2. mailto:beginners@haskell.org 3. mailto:ian@zenhack.net
Sounds like you want concatMap: https://hackage.haskell.org/package/base-4.12.0.0/docs/Prelude.html#v:concat... Quoting Yugesh Kothari (2019-03-26 23:48:35)
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, <[1]ian@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:ian@zenhack.net
I'm guessing in the .hs file, you have this in a do block? Something like:
foo = do
contents <- readFile
words contents
If so, the problem is the return type of `words`. Each line of the do block
has to have an IO value, and words is a pure function. To lift its result
into the monad, use the aptly-named `pure` function:
foo = do
contents <- readFile
pure $ words contents
On Tue, Mar 26, 2019, 8:35 PM Yugesh Kothari
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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
participants (3)
-
Ian Denhardt -
Theodore Lief Gannon -
Yugesh Kothari