On Sun, May 4, 2014 at 3:39 PM, Mathijs Kwik
An expression that demands itself isn't necessarily an infinite loop either. So this still boils down to the halting problem.
a :: [Int] a = [length a * 2, length a * 3, length a * 4]
To be more precise, I believe we're looking to catch an expression where evaluating it to head-normal form demands itself already evaluated in head-normal form. In your example, there is no loop because `length a` can be evaluated without evaluating any of the items in `a`. But you get an infinite loop if you make cons strict in the item: {-# LANGUAGE BangPatterns #-} import Prelude hiding (length) -- | List whose cons is strict in the value. data SList a = Nil | Cons !a (SList a) deriving Show infixr 5 `Cons` length :: SList a -> Int length xs0 = go 0 xs0 where go !n Nil = n go !n (Cons _ xs) = go (n+1) xs a :: SList Int a = (length a * 2) `Cons` (length a * 3) `Cons` (length a * 4) `Cons` Nil main :: IO () main = print a