The standard Haskell\'s Double uses the standard double-precision arithmetic:
data DoubleDouble-precision floating point numbers. It is de
As chuff has pointed out, you might want to take a look a the numbers package on hackage. You can install it with cabal install numbers. Here is an example:
import Data.Number.CReal -- from numbers
main :: IO ()
main = putStrLn (showCReal 100 (sqrt 2))
-- output: 1.4142135623730950488016887242096980785696718753769480731766797379907324784621070388503875343276415727
As the documentation states, showCReal returns a string showing a given number of type CReal with the given number of decimals.