First of all, sorry for my bad english. I'm not native and try my best :)
Now to the problem: i have a list of Strings and want to convert them to a list of integers. The Problem is, it's not just numbers, basically the String is a List to.
["[1,2,3,4,5,6,7,8]","[8,7,6,5,4,3,2,1]","[1,2,3,4,5,6,7,8]"]
This is the result i get from my code i'll post further down.
Any idea how i can achieve, that the internal list of numbers are list of integers?
I tried like three hours and didn't find a solution.
Every help is appreciatet.
Kind regards
get "/authors/:author" $ do
authorName <- param "author"
directories <- liftIO(listDirectory("data/" ++ authorName))
liftIO(readFiles directories authorName)
html (T.pack (HtmlModule.h1 ("Author: " ++ authorName)))
readFiles :: [String] -> String -> IO ()
readFiles x authorName = do
let y = addPrefix x authorName
content <- mapM readFile y
putStrLn (show content)
Result: ["[1,2,3,4,5,6,7,8]","[8,7,6,5,4,3,2,1]","[1,2,3,4,5,6,7,8]"]
You can read the string into a list of ints:
let nums = map read content :: [[Int]]
You can use read :: Read a => String -> a to convert a string to a type that is a member of the Read typeclass.
Since Int is a member of the Read typeclass, and [a] is a member of the Read typeclass if a is a member of the Read typeclass, we thus can read a list of Ints:
Prelude> read "[1,2,3,4,5,6,7,8]" :: [Int]
[1,2,3,4,5,6,7,8]
We thus can convert a list of Strings with:
content <- mapM ((read :: String -> [Int]) . readFile) y
read will raise an error in case the String can not be converted. You can make use of readMaybe :: Read a => String -> Maybe a to wrap the result in a Just in case parsing was successful, and Nothing in case parsing failed.
Related
I am trying to make a function that takes a list of strings and executes the command putStrLn or print (I think they are basically equivalent, please correct me if I am wrong as I'm still new to Haskell) to every element and have it printed out on my terminal screen. I was experimenting with the map function and also with lambda/anonymous functions as I already know how to do this recursively but wanted to try a more complex non recursive version. map returned a list of the type IO() which was not what I was going for and my attempts at lambda functions did not go according to plan. The basic code was:
test :: [String] -> something
test x = map (\a->putStrLn a) x -- output for this function would have to be [IO()]
Not entirely sure what the output of the function was supposed to be either which also gave me issues.
I was thinking of making a temp :: String variable and have each String appended to temp and then putStrLn temp but was not sure how to do that entirely. I though using where would be viable but I still ran into issues. I know how to do this in languages like java and C but I am still quite new to Haskell. Any help would be appreciated.
There is a special version of map that works with monadic functions, it's called mapM:
test :: [String] -> IO [()]
test x = mapM putStrLn x
Note that this way the return type of test is a list of units - that's because each call to putStrLn returns a unit, so result of applying it to each element in a list would be a list of units. If you'd rather not deal with this silliness and have the return type be a plain unit, use the special version mapM_:
test :: [String] -> IO ()
test x = mapM_ putStrLn x
I was thinking of making a temp :: String variable and have each String appended to temp and then putStrLn temp
Good idea. A pattern of "render the message" then a separate "emit the message" is often nice to have long term.
test xs = let temp = unlines (map show xs)
in putStrLn temp
Or just
test xs = putStrLn (unlines (show <$> xs))
Or
test = putStrLn . unlines . map show
Not entirely sure what the output of the function was supposed to be either which also gave me issues.
Well you made a list of IO actions:
test :: [String] -> [IO ()]
test x = map (\a->putStrLn a) x
So with this list of IO actions when do you want to execute them? Now? Just once? The first one many times the rest never? In what order?
Presumably you want to execute them all now. Let's also eta reduce (\a -> putStrLn a) to just putStrLn since that means the same thing:
test :: [String] -> IO ()
test x = sequence_ (map (\a->putStrLn a) x)
I want to ask the user for a sentence, and then return the sentence back but with the words on separate lines.
For example, if the user inputs "hello I am tall", the computer returns:
hello,
I
am
tall
I tried to start off a bit, but don't know of a function or something I can use to try and separate the sentence. My code so far:
displayWords ::IO ()
displayWords = do putStr "Please enter a line of text"
x <- getLine
mapM print x
I get the error:
Couldn't match type ‘[()]’ with ‘()’
EDIT: One more side thing... using mapM_ print (words x) fixes what I want, but is there a way to print this without the quotation marks?
EDIT2: One more thing... Someone in the comments helped answer the previous edit, but if I change the format of this to
displayWords:: String -> IO Int()
displayWords s = do
mapM_ putStrLn s
return (length s)
I get the error
Couldn't match type 'Char' With '[Char]'
How come putStrLn doesn't work here?
You need the words function to split the string into separate words.
Someone suggested mapM_ print (words x), but since each word is a string, using print will wrap it in quote marks, which you don't want. So try
mapM_ putStrLn (words x)
Use mapM_.
mapM print returns a list, the result of calling print for each element, but since print returns just () that is [()] not very meaningful.
-- here, b = ()
mapM :: Monad m => (a -> m b) -> [a] -> m [b]
mapM_ print discards the result of each call:
mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
displayWords must be of type IO (), and the type of a do-block is the type of its last statement, this is why mapM results in a type error.
Another way would be to ignore the result of a function like mapM by adding a statement that does nothing.
do putStr "..."
x <- getLine
mapM print x
return ()
Okay, I'm new to the Haskell community having come from Python and this is driving me crazy.
I have a text file looking something like:
"1.2
1.423
2.43".
I want to read this text file and store it as a list of doubles in list_var. So list_var = [1.2,1.423,2.43]. This list_var will be used further in the program.
I just don't seem to find an answer on how to do this, most answers can print out list_var, e.g. Haskell - Read a file containing numbers into a list but I need list_var further down the line!
I have tried:
get_coefficients :: String -> [Double]
get_coefficients file_1 = do
coefficients_fromfile <- readLines "test2.txt"
let coefficients = map readDouble coefficients_fromfile
return coefficients
which doesn't work, readLines is
readLines :: FilePath -> IO [String]
readLines = fmap lines . readFile
and readDouble is
readDouble :: String -> Double
readDouble = read
Thanks in advance!
Since you use return, your output is in a monad, in this case the IO monad. As the error message tells you, you should change this line:
get_coefficients :: String -> [Double]
To this:
get_coefficients :: String -> IO [Double]
This is because of a core principle of Haskell: referential transparency.
If you want to use the [Double] produced, you still have to keep it in an IO monad, like so:
main :: IO ()
main = do
-- This can be thought of as taking out values from the monad,
-- but requires the promise that it'll be put back into a monad later.
doubles <- get_coefficients "This argument does nothing, why?"
-- This prints the list of doubles. Note: it returns an IO (),
-- thus fufills the promise!
-- print :: Show a => a -> IO ()
print doubles
I am having trouble reading in a level file in Haskell. The goal is to read in a simple txt file with two numbers seperated by a space and then commas. The problem I keep getting is this: Couldn't match type `IO' with `[]'
If I understand correctly the do statement is supposed to pull the String out of the Monad.
readLevelFile :: FilePath -> [FallingRegion]
readLevelFile f = do
fileContent <- readFile f
(map lineToFallingRegion (lines fileContent))
lineToFallingRegion :: String -> FallingRegion
lineToFallingRegion s = map textShapeToFallingShape (splitOn' (==',') s)
textShapeToFallingShape :: String -> FallingShape
textShapeToFallingShape s = FallingShape (read $ head numbers) (read $ head
$ tail numbers)
where numbers = splitOn' (==' ') s
You can't pull things out of IO. You can think of IO as a container (in fact, some interpretations of IO liken it to the box containing Schrödinger's cat). You can't see what's in the container, but if you step into the container, values become visible.
So this should work:
readLevelFile f = do
fileContent <- readFile f
return (map lineToFallingRegion (lines fileContent))
It does not, however, have the type given in the OP. Inside the do block, fileContent is a String value, but the entire block is still inside the IO container.
This means that the return type of the function isn't [FallingRegion], but IO [FallingRegion]. So if you change the type annotation for readLevelFile to
readLevelFile :: FilePath -> IO [FallingRegion]
you should be able to get past the first hurdle.
Let's look at your first function with explicit types:
readLevelFile f = do
(fileContent :: String) <-
(readFile :: String -> IO String) (f :: String) :: IO String
fileContent is indeed of type String but is only available within the execution of the IO Monad under which we are evaluating. Now what?
(map lineToFallingRegion (lines fileContent)) :: [String]
Now you are suddenly using an expression that is not an IO monad but instead is a list value - since lists are also a type of monad the type check tries to unify IO with []. What you actually wanted is to return this value:
return (map lineToFallingRegion (lines fileContent)) :: IO [String]
Now recalling that we can't ever "exit" the IO monad your readLevelFile type must be IO - an honest admission that it interacts with the outside world:
readLevelFile :: FilePath -> IO [FallingRegion]
I started with programming in Haskell about 4 month ago and now I came to the point where I have to deal with the IO system of Haskell.
I already did a lot of IO actions and haven't faced any problems I couldn't solve by myself, but this time I googled for almost two hours for no avail to get some information about the function readMaybe. So I have the following problem set to solve and I already tried a lot of different approaches to solve it but all the time I get the same failure message from my compiler:
No instance for (Read a0) arising from a use of `readMaybe'
The type variable `a0' is ambiguous
I understand what the compiler does want to tell me but I have no idea how to solve this problem. I already tried to add a class constraint, but without success.
So here is my very small and simple program that is just counting how many valid numbers the user has entered. The program is meant to terminate when the user enters an empty line.
This is just a auxiliary function I want to use for my project later on.
countNumbers :: IO Int
countNumbers = do
x <- count 0
return x where
count :: Int -> IO Int
count n = do
line <- getLine
case line of
"" -> do
return n
_ -> case readMaybe line of
Just _ -> do
x <- count (n+1)
return x
Nothing -> do
x <- count n
return x
Unfortunately I couldn't find out a lot of informations about the function readMaybe. The only thing I could find was in the Haskell library Text.Read:
readMaybe :: Read a => String -> Maybe aSource
Parse a string using the Read instance. Succeeds if there is exactly one valid result.
The very weird thing for me is that I have already written such a function that uses the readMaybe function and it worked perfectly ...
This program is just asking the user for a number and keeps asking as long as the user enters a valid number
getLineInt :: IO Int
getLineInt = do
putStrLn "Please enter your guess"
line <- getLine
case readMaybe line of
Just x -> do
return x
Nothing -> do
putStrLn "Invalid number entered"
x <- getLineInt
return x
So far as I can see there are no differences between the usage of the function readMaybe in the two programs and therefore it works in the one but not in the other :)
I would be really thankful for any hints from you!!
This has nothing to do with IO, so maybe you don't understand what the compiler is trying to tell you. There is a type variable a in readMaybe's signature; a has to have a Read instance, but other than that it can be anything. The compiler is telling you that it doesn't have any way to determine what you want a to be.
In getLineInt you don't have this problem, because you are returning the result of readMaybe and the type signature says it should be Int. In countNumbers, you're not using the result of readMaybe, so there's nothing that can be used to determine the correct type. You can fix this by adding an explicit type signature (I picked Int since you're apparently counting numbers):
_ -> case readMaybe line :: Maybe Int of
Finally a word about do notation: it's just syntactic sugar, you don't have to use it all the time. Instead of do return x you can simply write return x, and instead of
x <- getLineInt
return x
you can simply do
getLineInt
That makes things more readable:
getLineInt :: IO Int
getLineInt = do
putStrLn "Please enter your guess"
line <- getLine
case readMaybe line of
Just x -> return x
Nothing -> putStrLn "Invalid number entered" >> getLineInt
Why does this happen?
In your second function, it is clear that readMaybe line is used as String -> Maybe Int, since type inference notices that you use return x and therefore x must be an Int.
In your first function, you don't use the Maybe's value at all, you just want to check whether the read succeeded. However, since you didn't specify the type (neither explicit nor implicit with type inference), the type variable is ambiguous:
_ -> case readMaybe line of
There's an easy fix: annotate the type:
_ -> case readMaybe line :: Maybe Int of
By the way, this is exactly the same behaviour you encounter when you use read in ghci without any type context:
> read "1234"
<interactive>:10:1:
No instance for (Read a0) arising from a use of `read'
The type variable `a0' is ambiguous
As soon as you make the type clear everything is fine:
> read "1234" :: Int
1234
Making things clear
Now that we've seen why the error happens, lets make this program much simpler. First of all, we're going to use a custom readMaybe:
readMaybeInt :: String -> Maybe Int
readMaybeInt = readMaybe
Now how does one count numbers? Numbers are those words, where readMaybeInt doesn't return Nothing:
countNumbers :: String -> Int
countNumbers = length . filter isJust . map readMaybeInt . words
How does one now calculate the numbers in the standard input? We simply take input until one line is completely empty, map countNumbers on all those lines and then sum:
lineNumberCount :: IO Int
lineNumberCount =
getContents >>= return . sum . map countNumbers . takeWhile (/= "") . lines
If you're not used to the bind methods, that's basically
lineNumberCount :: IO Int
lineNumberCount = do
input <- getContents
return . sum . map countNumbers . takeWhile (/= "") . lines $ input
All in all we get the following terse solution:
import Control.Monad (liftM)
import Data.Maybe (isJust)
import Text.Read (readMaybe)
readMaybeInt :: String -> Maybe Int
readMaybeInt = readMaybe
countNumbers :: String -> Int
countNumbers = length . filter isJust . map readMaybeInt . words
lineNumberCount :: IO Int
lineNumberCount =
getContents >>= return . sum . map countNumbers . takeWhile (/= "") . lines
Now there's only one function working in the IO monad, and all functions are basically applications of standard functions. Note that getContents will close the handle to the standard input. If you want to use you're better of using something like
input :: String -> IO [String]
input delim = do
ln <- getLine
if ln == delim then return []
else input delim >>= return . (ln:)
which will extract lines until a line matching delim has been found. Note that you need to change lineNumberCount in this case:
lineNumberCount :: IO Int
lineNumberCount =
input "" >>= return . sum . map countNumbers