monads - Haskell error: Couldn't match expected type `Bool' against inferred type `IO b' -

i not understand problem is. 'a' not bool , should not bool. why bool expected?

code:

probablyprime n 0 = false probablyprime n t =       <- randomrio(3, n-1 :: integer)                let comp = defcomp(a,n)               let ret  = (not comp) && (probablyprime n t-1)          return ret   defcomp n = xcon1 && xcon2 (s,m) = findsm n       x = a^m `mod` n       xcon1 = x /= 1 || x /= n-1       xcon2 = comploop x n s   comploop x n 0 = false comploop x n s = x1 || (comploop x n (s-1))     x1 = (x^2 `mod` n) == 1   findsm n = (s,m) m = findm n       s = n/m   findm n = m   f = (logbase 2 n) - (truncate (logbase 2 n))         m' = 2^f         m = m_ify m'   m_ify m | m mod 1 == 0 = m      | otherwise = m_ify (m*2) 

error:

couldn't match expected type `bool' against inferred type `io b' in stmt of 'do' expression:     <- randomrio (3, n - 1 :: integer) in expression:     { <- randomrio (3, n - 1 :: integer);          let comp = defcomp ...;          let ret = (not comp) && (probablyprime n t - 1);          return ret } in definition of `probablyprime':     probablyprime n t                     = { <- randomrio (3, n - 1 :: integer);                            let comp = ...;                            let ret = ...;                            .... } 

probablyprime n 0 = false 

this tells haskell return type of probablyprime bool. in second case, you're dealing monads , returning io bool, types don't match.

change false return false , work.

you have change

let ret  = (not comp) && (probablyprime n t-1) 

to

prob <- probablyprime n (t-1) let ret = (not comp) && prob 

or like

ret <- liftm ((not comp) &&) (probablyprime n (t-1)) 

as andrew jaffe pointed out.