[Haskell-cafe] working with Random.randoms

Stephen Howard stephen at enterity.com
Fri Jun 13 12:38:25 EDT 2008

Hi List,

I am a newcomer doing my obligatory struggling with Haskell's type 
system, and I've got a nut I've not been able to crack.  Given:

import Random

random_test :: Int -> String
random_test n = do
    g <- getStdGen
    take n (randoms g)::String

I'm expecting that I ought to be able to pass this action an integer and 
get back a random string that long (given, not all characters may be 

But GHCI tells me:

    Couldn't match `[]' against `IO'
      Expected type: []
      Inferred type: IO
    In a 'do' expression: g <- getStdGen
    In the definition of `random_test':
        random_test n = do
                          g <- getStdGen
                            take n (randoms g) :: String

And yet, when I run these lines in GHCI by hand, things seem to work 
(though the string is the same set of random characters each time, 
another bit that I need to solve):

Prelude> :module Random
Prelude Random> g <- getStdGen
Prelude Random> take 5 (randoms g)::String

I'm guessing that randoms is returning an IO type but I'm not sure how 
to go about extracting the String to return to the calling action.  
Changing the type signature to Int -> IO  String only gives me a 
different error.

Where am I going wrong?


More information about the Haskell-Cafe mailing list