Modify mappend to avoid discarding

Closes #2

CHANGELOG.md

@@ -1,3 +1,15 @@
+## 0.5.0
+
+The behavior of `(<>)` for the `Ini` type has changed.
+
+- `<>` previously discarded all `iniGlobals`. Now it concatenates
+  the globals from the two `Ini` values.
+
+- When two `Ini` values contained `iniSections` with the same name,
+  `<>` previously returned the section from the left value and
+  discarded the section of the same name from the right value.
+  Now it concatenates the sections of the same name.
+
 ## 0.4.2
 
 _2022-07-26, Andreas Abel_

ini.cabal

@@ -1,6 +1,6 @@
 cabal-version:       >= 1.10
 name:                ini
-version:             0.4.2
+version:             0.5.0
 synopsis:            Configuration files in the INI format.
 description:         Quick and easy configuration files in the INI format.
 license:             BSD3

src/Data/Ini.hs

@@ -89,8 +89,13 @@ data Ini =
     }
   deriving (Show, Eq)
 
+-- | '<>' concatenates the lists of entries within each section (since @ini-0.5.0@)
 instance Semigroup Ini where
-  x <> y = Ini {iniGlobals = mempty, iniSections = iniSections x <> iniSections y}
+  x <> y =
+    Ini
+      { iniGlobals = iniGlobals x ++ iniGlobals y
+      , iniSections = M.unionWith (++) (iniSections x) (iniSections y)
+      }
 
 instance Monoid Ini where
   mempty = Ini {iniGlobals = mempty, iniSections = mempty}