diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 # This Travis job script has been generated by a script via
 #
-#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-installed' 'cabal.project'
+#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--ghc-head' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-unconstrained' 'cabal.project'
 #
 # For more information, see https://github.com/hvr/multi-ghc-travis
 #
@@ -36,33 +36,37 @@
 
 matrix:
   include:
-    - compiler: "ghc-7.4.2"
+    - compiler: "ghc-8.6.1"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-8.6.1], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.4.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-7.4.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.6.3"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.4.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.2.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-7.6.3], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.8.4"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.2.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.0.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-7.8.4], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.0.2], sources: [hvr-ghc]}}
     - compiler: "ghc-7.10.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.0.2"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.10.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.8.4"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.2.2"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.8.4], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.6.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.0,ghc-8.2.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.4.2"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.6.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.4.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.4.2], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.4.2], sources: [hvr-ghc]}}
     - compiler: "ghc-head"
       env: GHCHEAD=true
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
   allow_failures:
     - compiler: "ghc-head"
+    - compiler: "ghc-8.6.1"
 
 before_install:
   - HC=${CC}
@@ -80,7 +84,8 @@
   - BENCH=${BENCH---enable-benchmarks}
   - TEST=${TEST---enable-tests}
   - HADDOCK=${HADDOCK-true}
-  - INSTALLED=${INSTALLED-true}
+  - UNCONSTRAINED=${UNCONSTRAINED-true}
+  - NOINSTALLEDCONSTRAINTS=${NOINSTALLEDCONSTRAINTS-false}
   - GHCHEAD=${GHCHEAD-false}
   - travis_retry cabal update -v
   - "sed -i.bak 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config"
@@ -88,7 +93,8 @@
   # Overlay Hackage Package Index for GHC HEAD: https://github.com/hvr/head.hackage
   - |
     if $GHCHEAD; then
-      sed -i.bak 's/-- allow-newer:.*/allow-newer: *:base, *:template-haskell, *:ghc, *:Cabal/' ${HOME}/.cabal/config
+      sed -i 's/-- allow-newer: .*/allow-newer: *:base/' ${HOME}/.cabal/config
+      for pkg in $($HCPKG list --simple-output); do pkg=$(echo $pkg | sed 's/-[^-]*$//'); sed -i "s/allow-newer: /allow-newer: *:$pkg, /" ${HOME}/.cabal/config; done
 
       echo 'repository head.hackage'                                                        >> ${HOME}/.cabal/config
       echo '   url: http://head.hackage.haskell.org/'                                       >> ${HOME}/.cabal/config
@@ -98,13 +104,18 @@
       echo '              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e' >> ${HOME}/.cabal/config
       echo '   key-threshold: 3'                                                            >> ${HOME}/.cabal.config
 
+      grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
+
       cabal new-update head.hackage -v
     fi
   - grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
   - "printf 'packages: \".\" \"./examples\"\\n' > cabal.project"
   - echo 'package free-examples' >> cabal.project
   - "echo '  flags: -mandelbrot-iter' >> cabal.project"
-  - cat cabal.project
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
   - if [ -f "./configure.ac" ]; then
       (cd "." && autoreconf -i);
     fi
@@ -113,7 +124,7 @@
     fi
   - rm -f cabal.project.freeze
   - cabal new-build -w ${HC} ${TEST} ${BENCH} --project-file="cabal.project" --dep -j2 all
-  - rm -rf "."/.ghc.environment.* "./examples"/.ghc.environment.* "."/dist "./examples"/dist
+  - rm -rf .ghc.environment.* "."/dist "./examples"/dist
   - DISTDIR=$(mktemp -d /tmp/dist-test.XXXX)
 
 # Here starts the actual work to be performed for the package under test;
@@ -128,8 +139,10 @@
   - "printf 'packages: free-*/*.cabal free-examples-*/*.cabal\\n' > cabal.project"
   - echo 'package free-examples' >> cabal.project
   - "echo '  flags: -mandelbrot-iter' >> cabal.project"
-  - cat cabal.project
-
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
 
   # build & run tests, build benchmarks
   - cabal new-build -w ${HC} ${TEST} ${BENCH} all
@@ -142,5 +155,5 @@
   - rm -rf ./dist-newstyle
   - if $HADDOCK; then cabal new-haddock -w ${HC} ${TEST} ${BENCH} all; else echo "Skipping haddock generation";fi
 
-# REGENDATA ["-o",".travis.yml","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-installed","cabal.project"]
+# REGENDATA ["-o",".travis.yml","--ghc-head","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-unconstrained","cabal.project"]
 # EOF
diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,13 @@
+5.1 [2018.07.03]
+----------------
+* Generalize the type of `_Free`.
+* Allow building with `containers-0.6`.
+* Avoid incurring some dependencies when using recent GHCs.
+
+5.0.2 [2018.04.25]
+------------------
+* Add `Generic` and `Generic1` instances where possible.
+
 5.0.1 [2018.03.07]
 ------------------
 * Fix the build on old GHCs with `transformers-0.4`.
diff --git a/examples/ValidationForm.hs b/examples/ValidationForm.hs
--- a/examples/ValidationForm.hs
+++ b/examples/ValidationForm.hs
@@ -1,10 +1,13 @@
+{-# LANGUAGE CPP #-}
 module Main where
 
+#if !(MIN_VERSION_base(4,8,0))
 import Control.Applicative
+#endif
 import Control.Applicative.Free
 import Control.Monad.State
 
-import Data.Monoid
+import Data.Monoid (Sum(..))
 
 import Text.Read.Compat (readEither)
 import Text.Printf
diff --git a/examples/free-examples.cabal b/examples/free-examples.cabal
--- a/examples/free-examples.cabal
+++ b/examples/free-examples.cabal
@@ -10,7 +10,14 @@
 homepage:      http://github.com/ekmett/free/
 bug-reports:   http://github.com/ekmett/free/issues
 copyright:     Copyright (C) 2008-2015 Edward A. Kmett
-tested-with:   GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.2
+tested-with:   GHC == 7.4.2
+             , GHC == 7.6.3
+             , GHC == 7.8.4
+             , GHC == 7.10.3
+             , GHC == 8.0.2
+             , GHC == 8.2.2
+             , GHC == 8.4.3
+             , GHC == 8.6.1
 synopsis:      Monads for free
 description:   Examples projects using @free@
 build-type:    Simple
diff --git a/free.cabal b/free.cabal
--- a/free.cabal
+++ b/free.cabal
@@ -1,8 +1,8 @@
 name:          free
 category:      Control, Monads
-version:       5.0.2
+version:       5.1
 license:       BSD3
-cabal-version: >= 1.18
+cabal-version: 1.18
 license-file:  LICENSE
 author:        Edward A. Kmett
 maintainer:    Edward A. Kmett <ekmett@gmail.com>
@@ -10,7 +10,14 @@
 homepage:      http://github.com/ekmett/free/
 bug-reports:   http://github.com/ekmett/free/issues
 copyright:     Copyright (C) 2008-2015 Edward A. Kmett
-tested-with:   GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.2
+tested-with:   GHC == 7.4.2
+             , GHC == 7.6.3
+             , GHC == 7.8.4
+             , GHC == 7.10.3
+             , GHC == 8.0.2
+             , GHC == 8.2.2
+             , GHC == 8.4.3
+             , GHC == 8.6.1
 synopsis:      Monads for free
 description:
   Free monads are useful for many tree-like structures and domain specific languages.
@@ -73,19 +80,28 @@
 
   build-depends:
     base                 == 4.*,
-    bifunctors           >= 4 && < 6,
     comonad              >= 4 && < 6,
     distributive         >= 0.2.1,
     mtl                  >= 2.0.1.0 && < 2.3,
     profunctors          >= 4 && < 6,
     semigroupoids        >= 4 && < 6,
-    semigroups           >= 0.8.3.1 && < 1,
     transformers         >= 0.2.0   && < 0.6,
-    transformers-base    < 0.5,
-    transformers-compat  >= 0.3     && < 1,
+    transformers-base    >= 0.4 && < 0.5,
     template-haskell     >= 2.7.0.0 && < 3,
     exceptions           >= 0.6 && < 0.11,
-    containers           < 0.6
+    containers           < 0.7
+
+  if !impl(ghc >= 8.2)
+    build-depends: bifunctors >= 4 && < 6
+
+  if !impl(ghc >= 8.0)
+    build-depends: semigroups >= 0.8.3.1 && < 1
+
+  -- Ensure Data.Functor.Classes is always available
+  if impl(ghc >= 7.10)
+    build-depends: transformers >= 0.4.2.0
+  else
+    build-depends: transformers-compat >= 0.5.1.0 && <0.7
 
   exposed-modules:
     Control.Applicative.Free
diff --git a/include/free-common.h b/include/free-common.h
--- a/include/free-common.h
+++ b/include/free-common.h
@@ -6,6 +6,10 @@
 #define MIN_VERSION_mtl(x,y,z) 1
 #endif
 
+#ifndef MIN_VERSION_transformers_compat
+#define MIN_VERSION_transformers_compat(x,y,z) 0
+#endif
+
 #if MIN_VERSION_base(4,9,0)
 #define LIFTED_FUNCTOR_CLASSES 1
 #else
diff --git a/src/Control/Monad/Free.hs b/src/Control/Monad/Free.hs
--- a/src/Control/Monad/Free.hs
+++ b/src/Control/Monad/Free.hs
@@ -413,19 +413,19 @@
   {-# INLINE impure #-}
 {-# INLINE _Pure #-}
 
--- | This is @Prism' (Free f a) (f (Free f a))@ in disguise
+-- | This is @Prism (Free f a) (Free g a) (f (Free f a)) (g (Free g a))@ in disguise
 --
 -- >>> preview _Free (review _Free (Just (Pure 3)))
 -- Just (Just (Pure 3))
 --
 -- >>> review _Free (Just (Pure 3))
 -- Free (Just (Pure 3))
-_Free :: forall f m a p. (Choice p, Applicative m)
-      => p (f (Free f a)) (m (f (Free f a))) -> p (Free f a) (m (Free f a))
+_Free :: forall f g m a p. (Choice p, Applicative m)
+      => p (f (Free f a)) (m (g (Free g a))) -> p (Free f a) (m (Free g a))
 _Free = dimap unfree (either pure (fmap Free)) . right'
  where
   unfree (Free x) = Right x
-  unfree x        = Left x
+  unfree (Pure x) = Left (Pure x)
   {-# INLINE unfree #-}
 {-# INLINE _Free #-}
 
diff --git a/src/Control/Monad/Free/TH.hs b/src/Control/Monad/Free/TH.hs
--- a/src/Control/Monad/Free/TH.hs
+++ b/src/Control/Monad/Free/TH.hs
@@ -1,4 +1,7 @@
 {-# LANGUAGE CPP #-}
+#if __GLASGOW_HASKELL__ >= 800
+{-# OPTIONS_GHC -Wno-overlapping-patterns #-}
+#endif
 #include "free-common.h"
 
 -----------------------------------------------------------------------------