Data.JSString.pack throws RangeError on lists with certain thunk structures

[Dridus]

Might be related to #128

I encountered it in the wild by way of showFFloat (Just 6) (2.2e-2), here's a reproduction case

import Numeric (showFFloat)
import Data.JSString (pack, unpack)

main :: IO ()
main = do
  putStrLn . unpack . pack $ showFFloat (Just 2) 2.2e-2 ""

ghcjs ./test.hs && node test.jsexe/all.js results in:

Haskell main thread: RangeError: Invalid code point: NaN
RangeError: Invalid code point: NaN
    at /home/ross/3rd/reflex-platform/test.jsexe/all.js:20550:29
    at h$jsstringPack (/home/ross/3rd/reflex-platform/test.jsexe/all.js:20711:36)
    at h$$Kt (/home/ross/3rd/reflex-platform/test.jsexe/all.js:57018:11)
    at h$runThreadSlice (/home/ross/3rd/reflex-platform/test.jsexe/all.js:6845:11)
    at h$runThreadSliceCatch (/home/ross/3rd/reflex-platform/test.jsexe/all.js:6814:12)
    at Immediate.h$mainLoop [as _onImmediate] (/home/ross/3rd/reflex-platform/test.jsexe/all.js:6809:9)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)

via node and something similar via Safari.

this is using GHCJS 8.6.0.1 (GHC 8.6.5), as packaged with reflex-platform 58873162d542c14cf198e9711986c046b06a18da

when compiled via GHC with Data.Text (pack, unpack), instead the output is the expected

0.022000

[Dridus]

using Safari devtools, the exception stack is:

• https://github.com/ghcjs/ghcjs-base/blob/master/jsbits/jsstring.js#L64

  • arguments: Arguments (8)
      0 {d1: null, d2: null, f: function, m: 3}
      1 46
      2 48
      3 50
      4 50
      5 48
      6 48
      7 48
    codePoint: NaN
    index: 0
    length: 8
    

• https://github.com/ghcjs/ghcjs-base/blob/master/jsbits/jsstring.js#L234

pushing on the test case a bit, I found that reproduction is strangely fragile. this doesn't blow up:

{-# OPTIONS_GHC -Wall #-}
import Data.Text (pack, unpack)
import GHC.Float
import GHC.Show (intToDigit)
import System.IO (hFlush, stdout)

test :: String
test = 
  let
    (is, e) = floatToDigits 10 (2.2e-2 :: Double)
    (ei,is') = roundTo 10 6 (replicate (-e) 0 ++ is)
    d:ds' = map intToDigit (if ei > 0 then is' else 0:is')
  in
    d : (if null ds' then "" else '.':ds')

main :: IO ()
main = do
  putStrLn . unpack . pack $ test
  --- hFlush stdout

and I see the correct output (0.022000) but it blows up if I uncomment the flush