Spagoify package; Remove MonadZero instance due to deprecation

.gitignore

@@ -4,3 +4,4 @@
 /.psc*
 /.purs*
 /.psa*
+/.spago/

packages.dhall

@@ -0,0 +1,105 @@
+{-
+Welcome to your new Dhall package-set!
+
+Below are instructions for how to edit this file for most use
+cases, so that you don't need to know Dhall to use it.
+
+## Use Cases
+
+Most will want to do one or both of these options:
+1. Override/Patch a package's dependency
+2. Add a package not already in the default package set
+
+This file will continue to work whether you use one or both options.
+Instructions for each option are explained below.
+
+### Overriding/Patching a package
+
+Purpose:
+- Change a package's dependency to a newer/older release than the
+    default package set's release
+- Use your own modified version of some dependency that may
+    include new API, changed API, removed API by
+    using your custom git repo of the library rather than
+    the package set's repo
+
+Syntax:
+where `entityName` is one of the following:
+- dependencies
+- repo
+- version
+-------------------------------
+let upstream = --
+in  upstream
+  with packageName.entityName = "new value"
+-------------------------------
+
+Example:
+-------------------------------
+let upstream = --
+in  upstream
+  with halogen.version = "master"
+  with halogen.repo = "https://example.com/path/to/git/repo.git"
+
+  with halogen-vdom.version = "v4.0.0"
+  with halogen-vdom.dependencies = [ "extra-dependency" ] # halogen-vdom.dependencies
+-------------------------------
+
+### Additions
+
+Purpose:
+- Add packages that aren't already included in the default package set
+
+Syntax:
+where `<version>` is:
+- a tag (i.e. "v4.0.0")
+- a branch (i.e. "master")
+- commit hash (i.e. "701f3e44aafb1a6459281714858fadf2c4c2a977")
+-------------------------------
+let upstream = --
+in  upstream
+  with new-package-name =
+    { dependencies =
+       [ "dependency1"
+       , "dependency2"
+       ]
+    , repo =
+       "https://example.com/path/to/git/repo.git"
+    , version =
+        "<version>"
+    }
+-------------------------------
+
+Example:
+-------------------------------
+let upstream = --
+in  upstream
+  with benchotron =
+      { dependencies =
+          [ "arrays"
+          , "exists"
+          , "profunctor"
+          , "strings"
+          , "quickcheck"
+          , "lcg"
+          , "transformers"
+          , "foldable-traversable"
+          , "exceptions"
+          , "node-fs"
+          , "node-buffer"
+          , "node-readline"
+          , "datetime"
+          , "now"
+          ]
+      , repo =
+          "https://github.com/hdgarrood/purescript-benchotron.git"
+      , version =
+          "v7.0.0"
+      }
+-------------------------------
+-}
+let upstream =
+      https://github.com/purescript/package-sets/releases/download/psc-0.15.4/packages.dhall
+        sha256:a6d66723b6109f1e3eaf6575910f1c51aa545965ce313024ba329360e2f009ac
+
+in  upstream

spago.dhall

@@ -0,0 +1,17 @@
+{-
+Welcome to a Spago project!
+You can edit this file as you like.
+
+Need help? See the following resources:
+- Spago documentation: https://github.com/purescript/spago
+- Dhall language tour: https://docs.dhall-lang.org/tutorials/Language-Tour.html
+
+When creating a new Spago project, you can use
+`spago init --no-comments` or `spago init -C`
+to generate this file without the comments in this block.
+-}
+{ name = "backtrack"
+, dependencies = [ "control", "effect", "prelude", "tailrec", "transformers" ]
+, packages = ./packages.dhall
+, sources = [ "src/**/*.purs" ]
+}

src/Control/Transformers/Back/Trans.purs

@@ -8,7 +8,6 @@ import Control.Monad.Rec.Class (class MonadRec, Step(..), tailRecM)
 import Control.Monad.State.Class (class MonadState, state)
 import Control.Monad.Trans.Class (class MonadTrans, lift)
 import Control.MonadPlus (class Plus)
-import Control.MonadZero (class MonadZero)
 
 data FailBack a = BackPoint a | NoBack a | GoBack
 newtype BackT m a = BackT (m(FailBack a))
@@ -94,4 +93,3 @@ instance plusBackT :: Monad m ⇒ Plus (BackT m) where
 
 instance alternativeBackT :: Monad m ⇒ Alternative (BackT m)
 
-instance monadZeroBackT :: Monad m ⇒ MonadZero (BackT m)