diff --git a/CHANGELOG.txt b/CHANGELOG.txt
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,3 +1,8 @@
+Changelog ideas-math-types-1.0 => ideas-math-types-1.1
+
+* adding missing semigroup instances 
+* ghc-8.7.6 compatibility (including containers library)
+
 ideas-math-types-1.0
 
 Initial version of the library. Some modules have been released in 2015 as part 
diff --git a/ideas-math-types.cabal b/ideas-math-types.cabal
--- a/ideas-math-types.cabal
+++ b/ideas-math-types.cabal
@@ -1,5 +1,5 @@
 name:                   ideas-math-types
-version:                1.0
+version:                1.1
 synopsis:               Common types for mathematical domain reasoners
 homepage:               http://ideas.cs.uu.nl/
 description:
@@ -22,7 +22,7 @@
 extra-source-files:     NOTICE.txt, CHANGELOG.txt
 build-type:             Simple
 cabal-version:          >= 1.10
-tested-with:            GHC == 7.10.3
+tested-with:            GHC == 7.10.3, GHC == 8.6.5
 
 source-repository head
   type:     svn
diff --git a/src/Domain/Math/Data/SquareRoot.hs b/src/Domain/Math/Data/SquareRoot.hs
--- a/src/Domain/Math/Data/SquareRoot.hs
+++ b/src/Domain/Math/Data/SquareRoot.hs
@@ -52,7 +52,7 @@
 
 -- re-establish invariants
 makeMap :: (Eq a,Num a) => SqMap a -> SqMap a
-makeMap = M.filter (/=0) . M.foldWithKey f M.empty
+makeMap = M.filter (/=0) . M.foldrWithKey f M.empty
  where
    f k a m
       | a == 0    = m
@@ -77,7 +77,7 @@
       _ ->
          let op n a = M.unionWith (+) (f n (fmap (a *) m1))
              f i    = M.mapKeys (*i)
-         in makeMap (M.foldWithKey op M.empty m2)
+         in makeMap (M.foldrWithKey op M.empty m2)
 
 recipSqMap :: (Eq a,Fractional a) => SqMap a -> SqMap a
 recipSqMap m =
@@ -196,5 +196,5 @@
    (a, b) = (numerator r, denominator r)
 
 eval :: Floating a => SquareRoot a -> a
-eval (S _ m) = M.foldWithKey f 0 m
+eval (S _ m) = M.foldrWithKey f 0 m
  where f n a b = a * Prelude.sqrt (fromIntegral n) + b