Reproducing a bug in unbound-generics https://github.com/lambdageek/unbound-generics/issues/50

Readme.md

Steps to reproduce:

1. Download files into a new directory
2. Create a sub-directory app
3. Move Main.hs there
4. run nix-build . "" in the shell
5. run ./result/bin/test
6. Get the following error:

LVar left
test: LocallyNameless.open: inconsistent sorts
CallStack (from HasCallStack):
error, called at src/Unbound/Generics/LocallyNameless/Alpha.hs:717:20 in unbound-generics-0.4.2-B2j2SNQtM51IWCI7gQ83CW:Unbound.Generics.LocallyNameless.Alpha

default.nix

let
  defpkgs = builtins.fetchTarball {
    # Descriptive name to make the store path easier to identify
    name = "nixos-22.05.1271.babb041b716";
    url = "https://releases.nixos.org/nixos/22.05-small/nixos-22.05.1271.babb041b716/nixexprs.tar.xz";
    # Hash obtained using `nix-prefetch-url --unpack <url>`
    sha256 = "0g8vwni83zn6kgkczrm5vwmyhl473rrs9d4k4hn5gfbgfsyv7ls8";
  };
  config = compiler: {
    packageOverrides = pkgs: rec {
      haskell = pkgs.haskell // {
        packages = pkgs.haskell.packages // {
          "${compiler}" = pkgs.haskell.packages."${compiler}".override {
            overrides = haskellPackagesNew: haskellPackagesOld: rec {
              unbound-generics =
                let unbound-src =
                      pkgs.fetchzip {
                        url = "https://github.com/lambdageek/unbound-generics/archive/a2a558058012b09bb313b3eb03cddb734fcf4a98.zip";
                        sha256 = "af4592a93d0d280591b3bcff3ebe244956cc3637bf20ed2315fb6b2e070caef4";
  };
                in haskellPackagesNew.callCabal2nix "unbound-generics" unbound-src {};
            };
          };
        };
      };
    };
  };
in
{ compiler ? "ghc902",
  nixpkgs ? import defpkgs {config = (config compiler);} }:
nixpkgs.pkgs.haskell.packages."${compiler}".callCabal2nix "test" ./. {}

Main.hs

{-# LANGUAGE DeriveAnyClass, DeriveGeneric #-}

import qualified Unbound.Generics.LocallyNameless as Unbound
import           GHC.Generics (Generic)
import           Control.DeepSeq (NFData, force)

type LName = Unbound.Name LTerm

data LTerm
  = -- | variables  `x`
    LVar LName
  | -- | abstraction  `\x. a`
    LLam (Unbound.Bind LName LTerm)
  | -- | application `a b`
    LApp LTerm LTerm
  deriving (Show, Generic, Unbound.Alpha, NFData)

type RName = Unbound.Name RTerm

data RTerm
  = -- | variables  `x`
    RVar RName
  | -- | abstraction  `\x. a`
    RLam (Unbound.Bind RName RTerm)
  | -- | application `a b`
    RApp RTerm RTerm
  deriving (Show, Generic, Unbound.Alpha, NFData)

lterm :: LTerm
lterm =
  let name = (Unbound.s2n "left")
      var = LVar name
  in LLam (Unbound.bind name var)

rterm :: RTerm
rterm =
  let name = (Unbound.s2n "right")
      var = RVar name
  in RLam (Unbound.bind name var)

test :: IO (LTerm, RTerm)
test = Unbound.runFreshMT $ do
  let (LLam lbind) = lterm
  let (RLam rbind) = rterm
  (lv, lb, rv, rb) <- Unbound.unbind2Plus lbind rbind
  return (lb, rb)

main :: IO ()
main = do
  (l,r) <- test
-- this doesn't cause errors
  let dl = force l
  let dr = force r
-- this does
  putStrLn $ show $ dl
  putStrLn $ show $ dr
  putStrLn "done"

test.cabal

cabal-version:      2.4
name:               test
version:            0.1.0.0

-- A short (one-line) description of the package.
-- synopsis:

-- A longer description of the package.
-- description:

-- A URL where users can report bugs.
-- bug-reports:

-- The license under which the package is released.
-- license:
author:             Name
maintainer:         email@example.com

-- A copyright notice.
-- copyright:
-- category:
extra-source-files: CHANGELOG.md

executable test
    main-is:          Main.hs

    -- Modules included in this executable, other than Main.
    -- other-modules:

    -- LANGUAGE extensions used by modules in this package.
    -- other-extensions:
    hs-source-dirs:   app
    default-language: Haskell2010
    build-depends:
                  base >= 4 && < 5,
                  unbound-generics >= 0.2,
                  deepseq