Avoid repeatedly evaluating the source if it's a complex expression

quicklens/src/main/scala-3/com/softwaremill/quicklens/QuicklensMacros.scala

@@ -40,13 +40,22 @@ object QuicklensMacros {
       case Varargs(args) => focus +: args
     }
 
-    val modF = modifyImpl(obj, focuses)
+    val modF = modifyImplCacheObj(obj, focuses)
 
     toPathModify(obj, modF)
   }
 
   def toPathModifyFromFocus[S: Type, A: Type](obj: Expr[S], focus: Expr[S => A])(using Quotes): Expr[PathModify[S, A]] =
-    toPathModify(obj, modifyImpl(obj, Seq(focus)))
+    toPathModify(obj, modifyImplCacheObj(obj, Seq(focus)))
+
+  // #114: in case obj is a complex expression (not a simple by-name reference), we cache it to avoid repeatedly
+  // evaluating it; this especially matters in chained .modify calls
+  private def modifyImplCacheObj[S: Type, A: Type](obj: Expr[S], focuses: Seq[Expr[S => A]])(using
+      Quotes
+  ): Expr[(A => A) => S] = '{
+    val v = $obj
+    ${ modifyImpl('v, focuses) }
+  }
 
   private def modifyImpl[S: Type, A: Type](obj: Expr[S], focuses: Seq[Expr[S => A]])(using
       Quotes

quicklens/src/test/scala/com/softwaremill/quicklens/RepeatedModifyTest.scala

@@ -0,0 +1,43 @@
+package com.softwaremill.quicklens
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class RepeatedModifyTest extends AnyFlatSpec with Matchers {
+  import RepeatedModifyTest._
+
+  it should "properly handle repeated modify invocations for different fields" in {
+    val c = C(B(1, 1, 1, 1, 1))
+    c
+      .modify(_.b.a1)
+      .setTo(0.0d)
+      .modify(_.b.a2)
+      .setTo(0.0d)
+      .modify(_.b.a3)
+      .setTo(0.0d)
+      .modify(_.b.a4)
+      .setTo(0.0d)
+      .modify(_.b.a5)
+      .setTo(0.0d) shouldBe C(B(0, 0, 0, 0, 0))
+  }
+
+  it should "properly handle repeated modify invocations for the same field" in {
+    val c = C(B(1, 1, 1, 1, 1))
+    c
+      .modify(_.b.a1)
+      .setTo(0.0d)
+      .modify(_.b.a1)
+      .setTo(1.0d)
+      .modify(_.b.a1)
+      .setTo(2.0d)
+      .modify(_.b.a1)
+      .setTo(3.0d)
+      .modify(_.b.a1)
+      .setTo(4.0d) shouldBe C(B(4, 1, 1, 1, 1))
+  }
+}
+
+object RepeatedModifyTest {
+  case class B(a1: Double, a2: Double, a3: Double, a4: Double, a5: Double)
+  case class C(b: B)
+}