On 27/01/12 02:29, Conal Elliott wrote:

I'm seeing more strictness than I'd expect for printf:

...

printf "foo %s\n" (show ([1..10] ++ undefined)) foo *** Exception: Prelude.undefined

In contrast,

*Utils.Fabprim.ToHaskell> "foo " ++ show ([1..10] ++ undefined) ++ "\n" "foo [1,2,3,4,5,6,7,8,9,10*** Exception: Prelude.undefined

It seems that the problem is in the way printf handles filling. "%s" is equivalent to "%0s". Before outputing the string, printf needs to know its length, so it can insert the appropriate amount of padding (in Text.Printf.fmt.adjust): let lstr = length str lpre = length pre fill = if lstr+lpre < width then take (width-(lstr+lpre)) (repeat (if zero then '0' else ' ')) else "" For maximal lazyness, you would need to calculate something like: fillNeeded :: Int -> [a] -> Int fillNeeded 0 _ = 0 fillNeeded i (_:xs) = fillNeeded (i-1) xs Then those lines in the printf library could be replaced by let fillAmount = fillNeeded width (str++pre) fillChar = if zero then '0' else ' ' fill = replicate fillAmount fillChar Twan