diff --git a/.hlint.yaml b/.hlint.yaml
new file mode 100644
--- /dev/null
+++ b/.hlint.yaml
@@ -0,0 +1,12 @@
+- arguments: [--cpp-define=HLINT, --cpp-ansi]
+
+# not viable
+- ignore: {name: Reduce duplication}
+
+# don't want to!
+- ignore: {name: Use infix}
+
+# these don't consider the corner cases when using doubles
+- ignore: {name: "Use >"}
+- ignore: {name: "Use <="}
+- ignore: {name: "Use >="}
diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,7 +4,7 @@
 #
 # For more information, see https://github.com/haskell-CI/haskell-ci
 #
-# version: 0.3.20190425
+# version: 0.5.20180830
 #
 language: c
 dist: xenial
@@ -35,44 +35,43 @@
   include:
     - compiler: ghc-8.8.1
       addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.8.1","cabal-install-3.0"]}}
-      env: GHCHEAD=true
     - compiler: ghc-8.6.5
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.6.5","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.6.5","cabal-install-3.0"]}}
     - compiler: ghc-8.4.4
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.4.4","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.4.4","cabal-install-3.0"]}}
     - compiler: ghc-8.2.2
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.2.2","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.2.2","cabal-install-3.0"]}}
     - compiler: ghc-8.0.2
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.0.2","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-8.0.2","cabal-install-3.0"]}}
     - compiler: ghc-7.10.3
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.10.3","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.10.3","cabal-install-3.0"]}}
     - compiler: ghc-7.8.4
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.8.4","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.8.4","cabal-install-3.0"]}}
     - compiler: ghc-7.6.3
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.6.3","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.6.3","cabal-install-3.0"]}}
     - compiler: ghc-7.4.2
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.4.2","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.4.2","cabal-install-3.0"]}}
     - compiler: ghc-7.2.2
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.2.2","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.2.2","cabal-install-3.0"]}}
     - compiler: ghc-7.0.4
-      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.0.4","cabal-install-2.4"]}}
+      addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-7.0.4","cabal-install-3.0"]}}
     - compiler: ghc-head
       addons: {"apt":{"sources":["hvr-ghc"],"packages":["ghc-head","cabal-install-head"]}}
-      env: GHCHEAD=true
   allow_failures:
     - compiler: ghc-head
     - compiler: ghc-7.0.4
     - compiler: ghc-7.2.2
-    - compiler: ghc-8.8.1
 before_install:
   - HC=$(echo "/opt/$CC/bin/ghc" | sed 's/-/\//')
+  - WITHCOMPILER="-w $HC"
+  - HADDOCK=$(echo "/opt/$CC/bin/haddock" | sed 's/-/\//')
   - HCPKG="$HC-pkg"
   - unset CC
   - CABAL=/opt/ghc/bin/cabal
   - CABALHOME=$HOME/.cabal
   - export PATH="$CABALHOME/bin:$PATH"
   - TOP=$(pwd)
-  - HCNUMVER=$(( $(${HC} --numeric-version|sed -E 's/([0-9]+)\.([0-9]+)\.([0-9]+).*/\1 * 10000 + \2 * 100 + \3/') ))
+  - "HCNUMVER=$(${HC} --numeric-version|perl -ne '/^(\\d+)\\.(\\d+)\\.(\\d+)(\\.(\\d+))?$/; print(10000 * $1 + 100 * $2 + ($3 == 0 ? $5 != 1 : $3))')"
   - echo $HCNUMVER
   - CABAL="$CABAL -vnormal+nowrap+markoutput"
   - set -o pipefail
@@ -100,11 +99,13 @@
   - TEST=--enable-tests
   - if [ $HCNUMVER -lt 70600 ] ; then TEST=--disable-tests ; fi
   - BENCH=--enable-benchmarks
-  - GHCHEAD=${GHCHEAD-false}
+  - HEADHACKAGE=false
+  - if [ $HCNUMVER -gt 80801 ] ; then HEADHACKAGE=true ; fi
   - rm -f $CABALHOME/config
   - |
     echo "verbose: normal +nowrap +markoutput"          >> $CABALHOME/config
     echo "remote-build-reporting: anonymous"            >> $CABALHOME/config
+    echo "write-ghc-environment-files: always"          >> $CABALHOME/config
     echo "remote-repo-cache: $CABALHOME/packages"       >> $CABALHOME/config
     echo "logs-dir:          $CABALHOME/logs"           >> $CABALHOME/config
     echo "world-file:        $CABALHOME/world"          >> $CABALHOME/config
@@ -118,15 +119,14 @@
     echo "repository hackage.haskell.org"               >> $CABALHOME/config
     echo "  url: http://hackage.haskell.org/"           >> $CABALHOME/config
   - |
-    if $GHCHEAD; then
+    if $HEADHACKAGE; then
     echo "allow-newer: $($HCPKG list --simple-output | sed -E 's/([a-zA-Z-]+)-[0-9.]+/*:\1/g')" >> $CABALHOME/config
-    
-    echo "repository head.hackage"                                                        >> $CABALHOME/config
-    echo "   url: http://head.hackage.haskell.org/"                                       >> $CABALHOME/config
+    echo "repository head.hackage.ghc.haskell.org"                                        >> $CABALHOME/config
+    echo "   url: https://ghc.gitlab.haskell.org/head.hackage/"                           >> $CABALHOME/config
     echo "   secure: True"                                                                >> $CABALHOME/config
-    echo "   root-keys: 07c59cb65787dedfaef5bd5f987ceb5f7e5ebf88b904bbd4c5cbdeb2ff71b740" >> $CABALHOME/config
-    echo "              2e8555dde16ebd8df076f1a8ef13b8f14c66bad8eafefd7d9e37d0ed711821fb" >> $CABALHOME/config
-    echo "              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e" >> $CABALHOME/config
+    echo "   root-keys: 7541f32a4ccca4f97aea3b22f5e593ba2c0267546016b992dfadcd2fe944e55d" >> $CABALHOME/config
+    echo "              26021a13b401500c8eb2761ca95c61f2d625bfef951b939a8124ed12ecf07329" >> $CABALHOME/config
+    echo "              f76d08be13e9a61a377a85e2fb63f4c5435d40f8feb3e12eb05905edb8cdea89" >> $CABALHOME/config
     echo "   key-threshold: 3"                                                            >> $CABALHOME/config
     fi
   - cat $CABALHOME/config
@@ -136,17 +136,16 @@
   - rm -rf cabal.project cabal.project.local cabal.project.freeze
   - touch cabal.project
   - |
-    echo 'packages: "."' >> cabal.project
+    echo "packages: ." >> cabal.project
   - |
-    echo "write-ghc-environment-files: always" >> cabal.project
-  - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | grep -vE -- '^(intervals)$' | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"
+  - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(intervals)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"
   - cat cabal.project || true
   - cat cabal.project.local || true
   - if [ -f "./configure.ac" ]; then (cd "." && autoreconf -i); fi
-  - ${CABAL} v2-freeze -w ${HC} ${TEST} ${BENCH} | color_cabal_output
+  - ${CABAL} v2-freeze $WITHCOMPILER ${TEST} ${BENCH} | color_cabal_output
   - "cat cabal.project.freeze | sed -E 's/^(constraints: *| *)//' | sed 's/any.//'"
   - rm  cabal.project.freeze
-  - ${CABAL} v2-build -w ${HC} ${TEST} ${BENCH} --dep -j2 all | color_cabal_output
+  - ${CABAL} v2-build $WITHCOMPILER ${TEST} ${BENCH} --dep -j2 all | color_cabal_output
 script:
   - DISTDIR=$(mktemp -d /tmp/dist-test.XXXX)
   # Packaging...
@@ -154,26 +153,27 @@
   # Unpacking...
   - mv dist-newstyle/sdist/*.tar.gz ${DISTDIR}/
   - cd ${DISTDIR} || false
-  - find . -maxdepth 1 -name '*.tar.gz' -exec tar -xvf '{}' \;
+  - find . -maxdepth 1 -type f -name '*.tar.gz' -exec tar -xvf '{}' \;
+  - find . -maxdepth 1 -type f -name '*.tar.gz' -exec rm       '{}' \;
+  - PKGDIR_intervals="$(find . -maxdepth 1 -type d -regex '.*/intervals-[0-9.]*')"
   # Generate cabal.project
   - rm -rf cabal.project cabal.project.local cabal.project.freeze
   - touch cabal.project
   - |
-    echo 'packages: "intervals-*/*.cabal"' >> cabal.project
+    echo "packages: ${PKGDIR_intervals}" >> cabal.project
   - |
-    echo "write-ghc-environment-files: always" >> cabal.project
-  - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | grep -vE -- '^(intervals)$' | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"
+  - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(intervals)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"
   - cat cabal.project || true
   - cat cabal.project.local || true
   # Building with tests and benchmarks...
   # build & run tests, build benchmarks
-  - ${CABAL} v2-build -w ${HC} ${TEST} ${BENCH} all | color_cabal_output
+  - ${CABAL} v2-build $WITHCOMPILER ${TEST} ${BENCH} all | color_cabal_output
   # Testing...
-  - if [ $HCNUMVER -ge 70600 ] ; then ${CABAL} v2-test -w ${HC} ${TEST} ${BENCH} all | color_cabal_output ; fi
+  - if [ $HCNUMVER -ge 70600 ] ; then ${CABAL} v2-test $WITHCOMPILER ${TEST} ${BENCH} all | color_cabal_output ; fi
   # cabal check...
-  - (cd intervals-* && ${CABAL} -vnormal check)
+  - (cd ${PKGDIR_intervals} && ${CABAL} -vnormal check)
   # haddock...
-  - ${CABAL} v2-haddock -w ${HC} ${TEST} ${BENCH} all | color_cabal_output
+  - ${CABAL} v2-haddock $WITHCOMPILER --with-haddock $HADDOCK ${TEST} ${BENCH} all | color_cabal_output
 
 # REGENDATA ["--output=.travis.yml","--config=cabal.haskell-ci","cabal.project"]
 # EOF
diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,9 @@
+0.9.1 [2020.01.29]
+------------------
+* Add `Semigroup` instances for the `Interval` types in `Numeric.Interval`,
+  `Numeric.Interval.Kaucher`, and `Numeric.Interval.NonEmpty`.
+  Add a `Monoid` instance for the `Interval` type in `Numeric.Interval`.
+
 0.9 [2019.05.10]
 ----------------
 * Remove the `Foldable` instances for the `Interval` types from
diff --git a/HLint.hs b/HLint.hs
deleted file mode 100644
--- a/HLint.hs
+++ /dev/null
@@ -1,12 +0,0 @@
-import "hint" HLint.HLint
-
--- not viable
-ignore "Reduce duplication"
-
--- don't want to!
-ignore "Use infix"
-
--- these don't consider the corner cases when using doubles
-ignore "Use >"
-ignore "Use <="
-ignore "Use >="
diff --git a/intervals.cabal b/intervals.cabal
--- a/intervals.cabal
+++ b/intervals.cabal
@@ -1,5 +1,5 @@
 name:              intervals
-version:           0.9
+version:           0.9.1
 synopsis:          Interval Arithmetic
 description:
   A 'Numeric.Interval.Interval' is a closed, convex set of floating point values.
@@ -29,10 +29,10 @@
                   , GHC == 8.6.5
                   , GHC == 8.8.1
 extra-source-files:
+  .hlint.yaml
   .travis.yml
   CHANGELOG.markdown
   README.markdown
-  HLint.hs
   Warning.hs
 
 source-repository head
@@ -74,6 +74,9 @@
 
   if impl(ghc >=7.4)
     build-depends: ghc-prim
+
+  if !impl(ghc >=8.0)
+    build-depends: semigroups >=0.11 && <1
 
   ghc-options:     -Wall -O2
 
diff --git a/src/Numeric/Interval/Internal.hs b/src/Numeric/Interval/Internal.hs
--- a/src/Numeric/Interval/Internal.hs
+++ b/src/Numeric/Interval/Internal.hs
@@ -65,6 +65,9 @@
 import Numeric.Interval.Exception
 import Prelude hiding (null, elem, notElem)
 
+import qualified Data.Semigroup
+import qualified Data.Monoid
+
 -- $setup
 
 data Interval a = I !a !a | Empty deriving
@@ -78,6 +81,14 @@
 #endif
 #endif
   )
+
+-- | 'Data.Semigroup.<>' is 'hull'
+instance Ord a => Data.Semigroup.Semigroup (Interval a) where
+  (<>) = hull
+
+instance Ord a => Data.Monoid.Monoid (Interval a) where
+  mempty = empty
+  mappend = (Data.Semigroup.<>)
 
 infix 3 ...
 infixl 6 +/-
diff --git a/src/Numeric/Interval/Kaucher.hs b/src/Numeric/Interval/Kaucher.hs
--- a/src/Numeric/Interval/Kaucher.hs
+++ b/src/Numeric/Interval/Kaucher.hs
@@ -65,7 +65,6 @@
 #endif
   )
 import Data.Function (on)
-import Data.Monoid
 import Data.Traversable
 #if __GLASGOW_HASKELL__ >= 704
 import GHC.Generics
@@ -73,6 +72,9 @@
 import Numeric.Interval.Exception
 import Prelude hiding (null, elem, notElem)
 
+import qualified Data.Semigroup
+import qualified Data.Monoid
+
 -- $setup
 
 data Interval a = I !a !a deriving
@@ -87,12 +89,16 @@
 #endif
   )
 
+-- | 'Data.Semigroup.<>' is 'hull'
+instance Ord a => Data.Semigroup.Semigroup (Interval a) where
+  (<>) = hull
+
 instance Functor Interval where
   fmap f (I a b) = I (f a) (f b)
   {-# INLINE fmap #-}
 
 instance Foldable Interval where
-  foldMap f (I a b) = f a `mappend` f b
+  foldMap f (I a b) = f a `Data.Monoid.mappend` f b
   {-# INLINE foldMap #-}
 
 instance Traversable Interval where
@@ -662,6 +668,10 @@
 --
 -- >>> hull (15 ... 85 :: Interval Double) (0 ... 10 :: Interval Double)
 -- 0.0 ... 85.0
+--
+-- >>> hull (10 ... 20 :: Interval Double) (15 ... 0 :: Interval Double)
+-- 10.0 ... 20.0
+--
 hull :: Ord a => Interval a -> Interval a -> Interval a
 hull x@(I a b) y@(I a' b')
   | null x = y
diff --git a/src/Numeric/Interval/NonEmpty/Internal.hs b/src/Numeric/Interval/NonEmpty/Internal.hs
--- a/src/Numeric/Interval/NonEmpty/Internal.hs
+++ b/src/Numeric/Interval/NonEmpty/Internal.hs
@@ -58,9 +58,10 @@
 #if __GLASGOW_HASKELL__ >= 704
 import GHC.Generics
 #endif
-import Numeric.Interval.Exception
 import Prelude hiding (null, elem, notElem)
 
+import qualified Data.Semigroup
+
 -- $setup
 -- >>> import Test.QuickCheck.Arbitrary
 -- >>> import Test.QuickCheck.Gen hiding (scale)
@@ -88,6 +89,10 @@
 #endif
 #endif
   )
+
+-- | 'Data.Semigroup.<>' is 'hull'
+instance Ord a => Data.Semigroup.Semigroup (Interval a) where
+  (<>) = hull
 
 infix 3 ...