Couldn't match expected type `IO b0' with actual type `Float'

Question

I wanted to make a simple unit converter, I wrote:

value :: String -> Float

value "mg"  = 0.001

value "g"   = 1

value "dag" = 10
value "kg"  = 1000
value "t"   = 1000000

main = do
  putStrLn "enter the number: "
  numbr <- getLine
  putStrLn "enter the unit: "
  unit <- getLine
  (read numbr*(value unit))

but it's giving me an error:

jedn.hs:16:16:
Couldn't match expected type `IO b0' with actual type `Float'
In the return type of a call of `value'
In the second argument of `(*)', namely `(value unit)'
In a stmt of a 'do' block: (read numbr * (value unit))

I believe that the problem is with changing values like "dag", "kg" to actual numbers, but how should I write it right?

I'm quite new to Haskell, so this code is probably written the wrong way.

Answer 1

You just need to print the result rather than trying to return it.

print (read numbr * value unit)

You can't return it for reasons that will become clearer as you study monads more. If you want to return from an I/O function instead, use

return (read numbr * value unit)