I'm writing a simple program in Haskell that lets user input exponent of the power of 2
power a = 2^a
main = do
number <- readLn
let x = (read number :: Int)
power x
I figured out that I need to convert String to Int but i still get following error: No instance for (Num (IO t0)) arising from a use of 'power'
How do I make it work?
It helps when you write out the types.
power :: (Num a, Integral b) => b -> a
power a = 2^a
main :: IO ()
main = do
number <- getLine
let x = read number :: Int
power x
But let's look what these things really return. power looks right, but main is trying to return a Num a, not an IO (). main is not meant to return values -- you should do something with them. Maybe you want to print the result? print :: Show a => a -> IO ()
main :: IO ()
main = do
number <- getLine
let x = read number :: Int
print $ power x
.exe file I get this message: user error (Prelude.readIO: no parse)readLn to getLine as @HTNW suggested
readMaybe from Text.Read instead of read because read is not total.
number <- readLn as in the original. But there is no need to read the number -- readLn already does that for you. Just print $ power numberGot it running that way - number is not an Int:
power :: (Num a, Integral b) => b -> a
power a = 2^a
main :: IO ()
main = do
number <- getLine
let x = read number
print $ power x
reading afterreadLnis a bad idea.readLn = readIO =<< getLineso you'll read a line, parse that into aString(requiring proper quoting and escaping), and then parse that again into a number. Just usereadLn