Data.TypeRepMap.toList causes segfaulting

[meditans]

Hi,
I am experiencing a bug when writing some code using Data.TypeRepMap (version 0.3.0 on hackage, ghc version 8.6.3); I've distilled the problem to its essence below (load this file in ghci and read the comments):

{-# language ConstraintKinds, GADTs #-}

module CrashTypeRepMap where

import Data.TypeRepMap
import GHC.Exts (toList, fromList)

data C a = C a deriving Show

-- Being unsatisfied with the Show instance for TypeRepMap, I want to bundle
-- show constraints in the map
data Constrained c f a = c a => Constrained (f a)
type Structure = TypeRepMap (Constrained Show C)

-- I intend to use this function to do the printing:
helper :: WrapTypeable (Constrained Show C) -> String
helper (WrapTypeable (Constrained c)) = show c

-- An example of structure I'll use in the repl below
ex_structure :: Structure
ex_structure = fromList
  [ WrapTypeable (Constrained (C 3   :: C Int))
  , WrapTypeable (Constrained (C 'a' :: C Char))]

Now let's test some expressions in the repl:

-- >>> :t ex_structure
-- ex_structure :: Structure

-- >>> toList ex_structure
-- [ e8fa2f3ef0863b7b3e43faa8f94caea7
-- , e8fa2f3ef0863b7b3e43faa8f94caea7
-- ]

Aside question: why are these two fingerprint equal? Is this normal?

You can calculate the type of this expression, but evaluating it crashes ghci:

-- >>> :t map helper (toList ex_structure)
-- map helper (toList ex_structure) :: [String]

-- >>> map helper (toList ex_structure)
-- CRASHES
-- /tmp/nix-shell-24560-0/rc: line 1: 25210 Segmentation fault      ghci

Do you know what causes this problem? Did I not understand how toList/fromList were supposed to be used?

[int-index]

Aside question: why are these two fingerprint equal? Is this normal?

No, it's not. I would expect the fingerprints to match those of Int and Char:

ghci> typeRepFingerprint (typeRep @Int)
b1460030427ac0fa458cbf347f168b53
ghci> typeRepFingerprint (typeRep @Char)
913d973ca4cebc0b4829aec8318f79e1

I don't know where e8fa2f3ef0863b7b3e43faa8f94caea7 is coming from.

[int-index]

Smaller example:

module CrashTypeRepMap where

import Data.TypeRepMap
import Data.Functor.Identity
import GHC.Exts (toList, fromList)

ex_list :: [WrapTypeable Identity]
ex_list = [ WrapTypeable (Identity (3 :: Int)), WrapTypeable (Identity 'a')]

ex_structure :: TypeRepMap Identity
ex_structure = fromList ex_list

Evaluating toList ex_structure segfaults.

[meditans]

Right, don't you find interesting that in your example you can't even say toList ex_structure, while in mine toList ex_structure prints out the wrong hashes? What's the difference? I'm playing around with typeRepFingerprint (typeRep @Stuff), but I had no luck in understanding where that hash comes from.

[int-index]

@meditans Check if #65 fixes it for you, the implementation of toList unsafe coerced to WrapTypeable instead of the element type.

[meditans]

@int-index I tried that pull request both in the example problems and my original code, and it fixed the crashes I was having. Thanks for the speed in reviewing this issue, you may close it!

[chshersh]

@int-index Thanks for fixing the issue so fast!
@vrom911 Thank you very much for reviewing and preparing new release!
@meditans Thank you for reporting this bug! I've just uploaded typerep-map-0.3.1 with bugfix to Hackage:

• https://hackage.haskell.org/package/typerep-map-0.3.1