[Haskell-beginners] Stack overflow, but hard to understand

[Michael Mossey]

Okay, beginner encountering first major bug.

I'm getting a stack overflow on a program that uses a lot of laziness to 
construct and modify lists---but the overflow happens even if I take just 
one element from the final list! The overflow seems to have started when I 
added the functions 'gauss' and 'gaussList' below, but I am at a loss to 
know why---these functions work perfectly in test cases and can even 
generate a long list.

The following program is distilled (with a lot of work!) down to the 
essence of what triggers the problem.

import Control.Monad
import Control.Monad.Random
import Data.List

-- Here I put a moderate-sized list of filenames which just generic
-- text or code. The longer the list,
-- the more likely to overflow stack. Which is curious, because if IO were
-- lazy here, it shouldn't be reading more than the first character of the
-- first file. Methinks a hint to the problem.
fileList =  ...

data TextGroup = Single Char Float
                deriving (Show)

textGroup_addDelay :: TextGroup -> Float -> TextGroup
textGroup_addDelay (Single c del) del2 = Single c (del+del2)

mkTextGroup :: Char -> Rand StdGen TextGroup
mkTextGroup c = liftM (Single c) gauss

-- Here's the kinda weird thing I'm doing with Rand. I want to
-- use fromList as the first computation to randomly choose
-- a second Rand computation. It turns out if you replace
-- this funkiness with
--   gauss = getRandomR (0,1)
-- the whole thing works.

gaussList :: Rand StdGen (Rand StdGen Float)
gaussList =
     fromList  [ (getRandomR( -1.0       ,-0.8 ),  2)
               , (getRandomR( -0.8       ,-0.6 ),  2)
               , (getRandomR( -0.6       ,-0.4 ),  4)
               , (getRandomR( -0.4       ,-0.2 ),  8)
               , (getRandomR( -0.2       , 0.0 ), 12)
               , (getRandomR(  0.0       , 0.2 ), 12)
               , (getRandomR(  0.2       , 0.4 ),  8)
               , (getRandomR(  0.4       , 0.6 ),  4)
               , (getRandomR(  0.6       , 0.8 ),  2)
               , (getRandomR(  0.8       , 1.0 ),  2)
               ]

gauss :: Rand StdGen Float
gauss = do
   m <- gaussList
   m

main = do
   gen0 <- newStdGen
   bufs <- mapM readFile fileList
   let buf = concat bufs
   let tgs = evalRand (do orig <- mapM mkTextGroup buf
                          addBreaks orig) gen0
   writeFile "output.ank" (show $ take 1 tgs)