Minify: Collapse adjacent var/const declarations

Variable and constant declarations like:

        var x=1;var y=2;

now becomes:

        var x=1,y=2;

and the same for const declarations. Interspersed var/const declarations
are not handled yet.

Tests/Minify.hs

@@ -218,6 +218,11 @@ testMinifyProg = describe "Minify programs:" $ do
         minifyProg "{a;;x;};y;z;;" `shouldBe` "a;x;y;z"
         minifyProg "{b;;{x;y;};};z;;" `shouldBe` "b;x;y;z"
 
+    it "variable declaration" $ do
+        minifyProg " var a = 1 ; var b = 2 ;" `shouldBe` "var a=1,b=2"
+        minifyProg " var c=1;var d=2;var e=3;" `shouldBe` "var c=1,d=2,e=3"
+        minifyProg " const f = 1 ; const g = 2 ;" `shouldBe` "const f=1,g=2"
+        minifyProg " var h = 1 ; const i = 2 ;" `shouldBe` "var h=1;const i=2"
 
 -- -----------------------------------------------------------------------------
 -- Minify test helpers.

src/Language/JavaScript/Process/Minify.hs

@@ -101,8 +101,25 @@ fixStatementList =
     fixList s [JSStatementBlock _ blk _ _] = fixList s blk
     fixList s [x] = [fixStmtE s x]
     fixList s (JSStatementBlock _ blk _ _:xs) = fixList semi (filter (not . isRedundant) blk) ++ fixList s xs
+    fixList s (JSConstant _ vs1 _:JSConstant _ vs2 _: xs) = fixList s (JSConstant spaceAnnot (concatCommaList vs1 vs2) s : xs)
+    fixList s (JSVariable _ vs1 _:JSVariable _ vs2 _: xs) = fixList s (JSVariable spaceAnnot (concatCommaList vs1 vs2) s : xs)
     fixList s (x:xs) = fixStmtE semi x : fixList s xs
 
+concatCommaList :: JSCommaList a -> JSCommaList a -> JSCommaList a
+concatCommaList xs JSLNil = xs
+concatCommaList JSLNil ys = ys
+concatCommaList xs (JSLOne y) = JSLCons xs emptyAnnot y
+concatCommaList xs ys =
+    let recurse (z, zs) = concatCommaList (JSLCons xs emptyAnnot z) zs
+    in  maybe xs recurse $ headCommaList ys
+
+headCommaList :: JSCommaList a -> Maybe (a, JSCommaList a)
+headCommaList JSLNil = Nothing
+headCommaList (JSLOne x) = Just (x, JSLNil)
+headCommaList (JSLCons (JSLOne x) _ y) = Just (x, JSLOne y)
+headCommaList (JSLCons xs _ y) =
+    let rebuild (x, ys) = (x, JSLCons ys emptyAnnot y)
+    in  rebuild <$> headCommaList xs
 
 -- -----------------------------------------------------------------------------
 -- JSExpression and the rest can use the MinifyJS typeclass.