[Haskell-cafe] Unexpected result from Data.Compact.inCompact?

Viktor Dukhovni ietf-dane at dukhovni.org
Wed Jun 26 06:24:54 UTC 2019

I am seeing surprising results from 'inCompact' after 'compact'.
It seems there are additional limitations on what can be compacted,
with 'compact' not throwing a 'CompactionFailed' exception in some
cases, and yet not compacting the value.  Is this expected?  When
the below is compiled and run:

    {-# LANGUAGE OverloadedStrings #-}
    module Main (main) where
    import           Data.Compact
    import qualified Data.ByteString.Short as S
    import qualified Data.Map.Strict as M
    fib :: [Int]
    fib = 0:1:zipWith (+) fib (tail fib)
    u = if M.lookup 3 m == Just 2 then () else undefined
    f3 = head $ drop 3 fib
    m = M.fromList $ zip [0..9] fib
    main :: IO ()
    main = do
        test "explicit ()" ()
        test "computed ()" u
        test "explicit Int" (42 :: Int)
        test "computed Int" f3
        test "empty string" ("" :: String)
        test "non-empty string" ("some string" :: String)
        test "empty short bytestring" S.empty
        test "non-empty short bytestring" (S.toShort "some bytestring")
        test "explict empty map" (M.empty :: M.Map Int Int)
        test "computed fib table" m
        test :: String -> a -> IO ()
        test msg val = do
            putStr (msg ++ ": ")
            c <- compact val
            inCompact c (getCompact c) >>= print

it outputs:

    explicit (): False
    computed (): False
    explicit Int: False
    computed Int: True
    empty string: False
    non-empty string: True
    empty short bytestring: True
    non-empty short bytestring: True
    explict empty map: False
    computed fib table: True

but the documentation promises:

    getCompact :: Compact a -> a #

	Retrieve a direct pointer to the value pointed at by a Compact
	reference. If you have used compactAdd, there may be multiple
	Compact references into the same compact region. Upholds the

	    inCompact c (getCompact c) == True

so when 'compact' does not bottom, I'd expect 'True'.


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