printf %s is not lazy enough

Taking a fixpoint of printf fix (printf "a%s") :: String, or printf'ing an infinite string printf "%s" (repeat ' ') :: String either fails with stack overflow or diverges.

The culprit is in this code:

fmt cs us =
	let (width, prec, ladj, zero, plus, cs', us') = getSpecs False False False cs us
	    adjust (pre, str) = 
		let lstr = length str
		    lpre = length pre
		    fill = if lstr+lpre < width then take (width-(lstr+lpre)) (repeat (if zero then '0' else ' ')) else ""
		in  if ladj then pre ++ str ++ fill else if zero then pre ++ fill ++ str else fill ++ pre ++ str

If width == 0, then no filling should be done, but the code is still forcing the evaluation of lstr and lpre (which in the case of an infinite str does not ever complete). A simple fix is to check that width is larger than zero before trying to test width against lstr and lpre, like this:

fill = if width > 0 && lstr+lpre < width then {-etc-}

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown