foldLeft, foldRight, other Foldable specializations (#11592)

* FoldableContravariant, a mapping for Foldable instances

* use FoldableContravariant to specialize several ImmArraySeq, NonEmpty methods

* folding specializations for ImmArray

* a few docs for FoldableContravariant

* specializations for FrontStack

* no changelog

CHANGELOG_BEGIN
CHANGELOG_END

daml-lf/data/src/main/scala/com/digitalasset/daml/lf/data/FrontStack.scala

@@ -4,8 +4,6 @@
 package com.daml.lf.data
 
 import ScalazEqual.{orderBy, toIterableForScalazInstances}
-import scalaz.syntax.applicative._
-import scalaz.syntax.traverse._
 import scalaz.{Applicative, Order, Traverse}
 
 import scala.annotation.tailrec
@@ -154,11 +152,26 @@ object FrontStack extends FrontStackInstances {
   implicit val `FrontStack covariant`: Traverse[FrontStack] = new Traverse[FrontStack] {
     override def traverseImpl[G[_]: Applicative, A, B](
         fa: FrontStack[A]
-    )(f: A => G[B]): G[FrontStack[B]] =
+    )(f: A => G[B]): G[FrontStack[B]] = {
+      import scalaz.syntax.applicative._, scalaz.syntax.traverse._
       fa.toBackStack.bqFoldRight(FrontStack.empty[B].pure[G])(
         (a, z) => ^(f(a), z)(_ +: _),
         (iaa, z) => ^(iaa traverse f, z)(_ ++: _),
       )
+    }
+
+    override def map[A, B](fa: FrontStack[A])(f: A => B) = fa map f
+
+    override def foldLeft[A, B](fa: FrontStack[A], z: B)(f: (B, A) => B) =
+      fa.iterator.foldLeft(z)(f)
+
+    override def foldRight[A, B](fa: FrontStack[A], z: => B)(f: (A, => B) => B) =
+      fa.toBackStack.bqFoldRight(z)(
+        (a, z) => f(a, z),
+        (iaa, z) => iaa.foldRight(z)(f(_, _)),
+      )
+
+    override def length[A](fa: FrontStack[A]) = fa.length
   }
 
   implicit def `FrontStack Order`[A: Order]: Order[FrontStack[A]] = {

daml-lf/data/src/main/scala/com/digitalasset/daml/lf/data/ImmArray.scala

@@ -3,6 +3,7 @@
 
 package com.daml.lf.data
 
+import com.daml.scalautil.FoldableContravariant
 import com.daml.scalautil.Statement.discard
 
 import ScalazEqual.{equalBy, orderBy, toIterableForScalazInstances}
@@ -393,6 +394,12 @@ object ImmArray extends ImmArrayInstances {
         }
         .map(_.toImmArray)
     }
+
+    override def foldLeft[A, B](fa: ImmArray[A], z: B)(f: (B, A) => B) =
+      fa.foldLeft(z)(f)
+
+    override def foldRight[A, B](fa: ImmArray[A], z: => B)(f: (A, => B) => B) =
+      fa.foldRight(z)(f(_, _))
   }
 
   implicit def immArrayOrderInstance[A: Order]: Order[ImmArray[A]] = {
@@ -429,12 +436,13 @@ object ImmArray extends ImmArrayInstances {
   object ImmArraySeq extends ImmArraySeqCompanion {
     val Empty: ImmArraySeq[Nothing] = ImmArray.Empty.toSeq
     implicit val `immArraySeq Traverse instance`: Traverse[ImmArraySeq] = new Traverse[ImmArraySeq]
-      with Foldable.FromFoldr[ImmArraySeq] {
+      with Foldable.FromFoldr[ImmArraySeq]
+      with FoldableContravariant[ImmArraySeq, ImmArray] {
       override def map[A, B](fa: ImmArraySeq[A])(f: A => B) = fa.toImmArray.map(f).toSeq
-      override def foldLeft[A, B](fa: ImmArraySeq[A], z: B)(f: (B, A) => B) =
-        fa.foldLeft(z)(f)
-      override def foldRight[A, B](fa: ImmArraySeq[A], z: => B)(f: (A, => B) => B) =
-        fa.foldRight(z)(f(_, _))
+
+      protected[this] override def Y = Foldable[ImmArray]
+      protected[this] override def ctmap[A](xa: ImmArraySeq[A]) = xa.toImmArray
+
       override def traverseImpl[F[_], A, B](
           immArr: ImmArraySeq[A]
       )(f: A => F[B])(implicit F: Applicative[F]): F[ImmArraySeq[B]] = {

daml-lf/data/src/test/scala/com/digitalasset/daml/lf/data/FrontStackSpec.scala

@@ -75,6 +75,10 @@ class FrontStackSpec
 
   "Traverse instance" should {
     checkLaws(ScalazProperties.traverse.laws[FrontStack])
+
+    "reconstruct itself with foldRight" in forAll { fs: FrontStack[Int] =>
+      scalaz.Foldable[FrontStack].foldRight(fs, FrontStack.empty[Int])(_ +: _) should ===(fs)
+    }
   }
 
   "Equal instance" should {

libs-scala/scala-utils/src/main/scala/com/daml/scalautil/FoldableContravariant.scala

@@ -0,0 +1,60 @@
+// Copyright (c) 2021 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package com.daml.scalautil
+
+import scalaz.{Monoid, Foldable, Semigroup}
+import FoldableContravariant._
+
+// Specialized overrides for when a Foldable wraps another Foldable.
+// If you need to hand-write some of these, just mix in the traits you
+// don't want to hand-write.
+private[daml] trait FoldableContravariant[X[_], Y[_]]
+    extends CoreOps[X, Y]
+    with Semigroupoids[X, Y]
+    with Conversions[X, Y]
+    with Lookups[X, Y]
+
+private[daml] object FoldableContravariant {
+  trait CtMap[X[_], Y[_]] {
+    protected[this] def Y: Foldable[Y]
+    protected[this] def ctmap[A](ax: X[A]): Y[A]
+  }
+
+  // non-derived combinators
+  trait CoreOps[X[_], Y[_]] extends Foldable[X] with CtMap[X, Y] {
+    override final def foldLeft[A, B](xa: X[A], z: B)(f: (B, A) => B) = Y.foldLeft(ctmap(xa), z)(f)
+    override final def foldRight[A, B](xa: X[A], z: => B)(f: (A, => B) => B) =
+      Y.foldRight(ctmap(xa), z)(f)
+    override final def foldMap[A, Z: Monoid](xa: X[A])(f: A => Z) = Y.foldMap(ctmap(xa))(f)
+  }
+
+  // plays on functions from the semigroupoids library
+  trait Semigroupoids[X[_], Y[_]] extends Foldable[X] with CtMap[X, Y] {
+    override final def foldMapRight1Opt[A, B](xa: X[A])(z: A => B)(f: (A, => B) => B) =
+      Y.foldMapRight1Opt(ctmap(xa))(z)(f)
+    override final def foldMapLeft1Opt[A, B](xa: X[A])(z: A => B)(f: (B, A) => B) =
+      Y.foldMapLeft1Opt(ctmap(xa))(z)(f)
+    override final def foldMap1Opt[A, B: Semigroup](xa: X[A])(f: A => B) =
+      Y.foldMap1Opt(ctmap(xa))(f)
+  }
+
+  // to (collection type) converters
+  trait Conversions[X[_], Y[_]] extends Foldable[X] with CtMap[X, Y] {
+    override final def toStream[A](xa: X[A]) = Y.toStream(ctmap(xa))
+    override final def toSet[A](xa: X[A]) = Y.toSet(ctmap(xa))
+    override final def toList[A](xa: X[A]) = Y.toList(ctmap(xa))
+    override final def toVector[A](xa: X[A]) = Y.toVector(ctmap(xa))
+    override final def toIList[A](xa: X[A]) = Y.toIList(ctmap(xa))
+    override final def toEphemeralStream[A](xa: X[A]) =
+      Y.toEphemeralStream(ctmap(xa))
+  }
+
+  // list-like operations
+  trait Lookups[X[_], Y[_]] extends Foldable[X] with CtMap[X, Y] {
+    override final def index[A](xa: X[A], i: Int) = Y.index(ctmap(xa), i)
+    override final def length[A](xa: X[A]) = Y.length(ctmap(xa))
+    override final def all[A](xa: X[A])(p: A => Boolean): Boolean = Y.all(ctmap(xa))(p)
+    override final def any[A](xa: X[A])(p: A => Boolean): Boolean = Y.any(ctmap(xa))(p)
+  }
+}

libs-scala/scala-utils/src/main/scala/com/daml/scalautil/nonempty/NonEmpty.scala

@@ -6,10 +6,12 @@ package com.daml.scalautil.nonempty
 import scala.collection.compat._
 import scala.collection.{immutable => imm}, imm.Map, imm.Set
 import scalaz.Id.Id
-import scalaz.{Foldable, Foldable1, Monoid, OneAnd, Semigroup, Traverse}
+import scalaz.{Foldable, Foldable1, OneAnd, Semigroup, Traverse}
 import scalaz.Leibniz, Leibniz.===
 import scalaz.Liskov, Liskov.<~<
 import scalaz.syntax.std.option._
+
+import com.daml.scalautil.FoldableContravariant
 import NonEmptyCollCompat._
 
 /** The visible interface of [[NonEmpty]]; use that value to access
@@ -169,7 +171,7 @@ sealed abstract class NonEmptyCollInstances extends NonEmptyCollInstances0 {
 
 sealed abstract class NonEmptyCollInstances0 {
   implicit def foldable1[F[_]](implicit F: Foldable[F]): Foldable1[NonEmptyF[F, *]] =
-    NonEmpty.substF(new Foldable1[F] {
+    NonEmpty.substF(new Foldable1[F] with FoldableContravariant[F, F] {
       private[this] def errEmpty(fa: F[_]) =
         throw new IllegalArgumentException(
           s"empty structure coerced to non-empty: $fa: ${fa.getClass.getSimpleName}"
@@ -187,12 +189,9 @@ sealed abstract class NonEmptyCollInstances0 {
       override def foldMapLeft1[A, B](fa: F[A])(z: A => B)(f: (B, A) => B) =
         assertNE(fa, F.foldMapLeft1Opt(fa)(z)(f))
 
-      override def foldMap[A, B: Monoid](fa: F[A])(f: A => B) = F.foldMap(fa)(f)
-
-      override def foldRight[A, B](fa: F[A], z: => B)(f: (A, => B) => B) = F.foldRight(fa, z)(f)
+      protected[this] override def Y = F
 
-      override def foldLeft[A, B](fa: F[A], z: B)(f: (B, A) => B) =
-        F.foldLeft(fa, z)(f)
+      protected[this] override def ctmap[A](xa: F[A]) = xa
     })
 
   import scala.language.implicitConversions