#2583: printf %s is not lazy enough
------------------------+---------------------------------------------------
Reporter: olsner | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 6.8.3 | Severity: minor
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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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-}
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2583>
GHC <http://www.haskell.org/ghc/>
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