dalaing

% select(X,HasXs,OneLessXs) deletes a single X from HasXs
select(X, [X|Xs], Xs).
select(X, [Y|Ys], [Y|Zs]) <- select(X, Ys, Zs).

% insert(X,OneLessXs, HasXs) inserts X into OneLessXs at a non-deterministic position
insert(X, Ys, Zs) <- select(X, Zs, Ys).

% permutation1(Input, Output)
% non-deterministically select the head of the output, permute the remaining input and use as the tail
permutation1(Xs, [Z|Zs]) <- select(Z,Xs,Ys), permutation(Ys,Zs).

dalaing

{ nixpkgs ? import <nixpkgs> {}, compiler ? "default" }:

let

  inherit (nixpkgs) pkgs;

  haskellPackages = if compiler == "default"
                       then pkgs.haskellPackages
                       else pkgs.haskell.packages.${compiler};

dalaing

module Queue (
    empty
  , isEmpty
  , peek
  , remove
  , add
  ) where

data Queue a = Queue [a] [a] [a]
                 deriving (Eq, Show)

dalaing

import Control.Monad.Free

data ExprF a k = Lit a | Add k k

instance Functor (ExprF a) where
  fmap f (Lit x) = Lit x
  fmap f (Add x y) = Add (f x) (f y)

type Expr a = Free (ExprF a) ()

dalaing

import Data.Profunctor
import qualified Data.Set as S
import Control.Monad.Trans (liftIO)
import Control.Monad.Trans.State

type MyMonad r = StateT (S.Set Int) IO r

runMyMonad :: MyMonad r -> IO r
runMyMonad m = evalStateT m S.empty

dalaing

diff --git a/compiler/typecheck/TcGenDeriv.lhs b/compiler/typecheck/TcGenDeriv.lhs
index 77bda82..1347f14 100644
--- a/compiler/typecheck/TcGenDeriv.lhs
+++ b/compiler/typecheck/TcGenDeriv.lhs
@@ -171,7 +171,7 @@ gen_Eq_binds loc tycon
     fall_through_eqn
       | no_tag_match_cons   -- All constructors have arguments
       = case pat_match_cons of
-          []  -> []   -- No constructors; no fall-though case
+          []  -> [([], undefined_Expr)]   -- No constructors; no fall-though case