Kinds of bugs that are more likely/prone in Haskell programs than in other languages?

Question

One of the highly-touted features is that if a program compiles, it highly likely to be mostly correct, more so than a program written in a language with a less sophisticated or

Answer 1

Laziness, especially lazy I/O where pure functions can force IO actions or closing a file Handle prior to reading. The Haskell course by Stanford University has good information on this in the Iteratee lecture. Imho, this lecture series is very well written and covers a lot of ground.

Answer 2

Consider the following dataype:

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE DeriveTraversable #-}

import Data.Foldable
import qualified Data.Traversable as T
import Control.Monad.Random

data U a = U [a] a deriving (Show,Functor,Foldable,T.Traversable)

I want to create a U Int with random values. It's easy using the Traversable instance:

ri :: Rand StdGen Int
ri = getRandomR (0,3)

randomU :: U Int
randomU =  flip evalRand (mkStdGen 7) 
         . T.sequence
         $ U (replicate 3 ri) ri  

putStrLn . show $ randomU -- works

Now I create a random infinite U Int and print the first three values of the list:

randomInfiniteU :: U Int
randomInfiniteU =  flip evalRand (mkStdGen 7) 
                 . T.sequence
                 $ U (repeat ri) ri  

putStrLn . show $ take 3 $ (\(U l _) -> l) $ randomInfiniteU

Works fine as well. As a last test, let's print the single value to the right:

putStrLn . show $ (\(U _ i) -> i) $ randomInfiniteU

Ugh. This hangs.

Answer 3

It doesn’t have to be asymptotic, but space leaks due to lazyness are a problem in real-world applications of Haskell. I know of Haskell-using companies that completely switched to strict datatypes (while still using the laziness of function parameters).

For sources on that view, see:

• E. Hesselink. Silk: making the sematic web functional. Functional Programming Exchange 2012, London, March 2012.
• C. J. Sampson. Experience report: Haskell in the ’real world’: writing a commercial application in a lazy functional lanuage. In ICFP ’09, pages 185–190, 2009.
• S. Wehr. Kommerzielle Softwareentwicklung mit Haskell. Hal6, Leipzig, Oct 2011. Slides (in German).