I want to read a string from keyboard but i don't know why isn't working, I tried a lot of methods, but nothing worked! Can someone help me or give me an idea?
type Polinom = [Int]
scriePolinom :: Polinom -> String
....
main = do
a<-getLine
--let a=[2,-5,1]
scriePolinom a
Parse the line into the corresponding data:
import Data.List
parseIntList :: String -> [Int]
parseIntList s = [read x :: Int | x <- words s]
type Polinom = [Int]
scriePolinom :: Polinom -> String
....
main = do
a <- getLine
let intlist = parseIntList a
putStrLn $ scriePolinom intlist
parseIntList :: String -> [Int] parseIntList s = [read x :: Int | x <- words s] main=do { a <- getLine scriePolinom (parseIntList a)}
GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help
Prelude> :type getLine -- you can also write :t getLine
getLine :: IO String
This tells you that getLine always gives you (after doing some kind of impure I/O operations, in this case waiting for the user to enter some stuff on the keyboard, until pressing enter) a value of type String. Which makes sense: a string, also called...
Prelude> :info String -- you can also write :i String
type String = [Char] -- Defined in ‘GHC.Base’
...list of characters. These are just the exact characters/keys the user typed in. In general, these characters may not properly describe a Polinom – what, for example, if the user enters 34958oiyq4ulbwre?
What you need to do is trying to parse the input string to a more meaningfully-typed value, like Polinom. How to do that depends on what form you actually expect the input to be. If the user should use normal Haskell syntax for the coefficients, e.g. [2, -5, 1], you can use the standard read parser:
Prelude> scriePolinom (read "[2, -5, 1]")
"[2,-5,1]"
Or if, as Daniel Sanchez assumes, you expect just a sequence of space-separated numbers in decimal notation, you can first split up these numbers as words, then read each one individually:
Prelude> scriePolinom . map read $ words "2 -5 1"
"[2,-5,1]"
To use this in an executable program, you can use the ready-build combination of getLine and read:
main :: IO ()
main = do
a <- readLn
putStrLn $ scriePolinom a
Note that read/readLn are not robust with respect to malformed input (they're not total functions) – this will just crash the program unless you wrap it in explicit exception handling. For serious applications I recommend using a full-featured parsing library, such as megaparsec.
mainis required to have typeIO (), butscriePolinom ahas typeString.