Data.JSString.pack breaks when using irrefutable patterns

[matthewbauer]

Using Data.JSString.pack gives incorrect results. For example:

import Data.JSString (pack)

main :: IO ()
main = print $ pack $ fst $ foldr (select (== 'a')) ([], []) "ababababa" -- "a"
  where
    -- taken from Data.OldList
    select p x ~(ts, fs) | p x       = (x:ts, fs  )
                         | otherwise = (ts  , x:fs)

Outputs "a", and it should output "aaaaa". But if we remove the ~ for irrefutable patterns, we get correct results:

import Data.JSString (pack)

main :: IO ()
main = print $ pack $ fst $ foldr (select (== 'a')) ([], []) "ababababa" -- "aaaaa"
  where
    -- taken from Data.OldList
    select p x (ts, fs) | p x       = (x:ts, fs  )
                        | otherwise = (ts  , x:fs)

I believe this issue has something to do with GHCJS packing the lazy String too early, before the fold is complete.

/cc @eskimor @luite

[luite]

I think this may be a bug related to the handling of selector thunks. It looks like they're not always correctly updated