Question

I want to store the value returned from hashUnique into a list, but I can’t do that:

import Data.Unique
import Data.List as L

cnter = do
   u <- newUnique
   return (hashUnique u)

main = cnter:[]

It will give out the error message : No instance for (Show (IO Int)), arising from a use of 'print' at <interative>

Answer 1

cnter is an IO action that returns an Int. That is, cnter has type IO Int. You are trying to use it as an Int. What you really want is to execute the action, obtaining the Int, and then use that result:

import Data.Unique
import Data.List as L

cnter = do
   u <- newUnique
   return (hashUnique u)

main = cnter >>= \c -> print [c]

Or with do notation:

main = do c <- cnter
          print [c]

But I’m not sure why you want to construct a list just to print it, I’d just print c, personally:

main = cnter >>= print