fn: add FindIdx function

fn/slice.go

@@ -53,7 +53,7 @@ func Map[A, B any](f func(A) B, s []A) []B {
 
 // Filter creates a new slice of values where all the members of the returned
 // slice pass the predicate that is supplied in the argument.
-func Filter[A any](pred func(A) bool, s []A) []A {
+func Filter[A any](pred Pred[A], s []A) []A {
 	res := make([]A, 0)
 
 	for _, val := range s {
@@ -92,7 +92,7 @@ func Foldr[A, B any](f func(A, B) B, seed B, s []A) B {
 
 // Find returns the first value that passes the supplied predicate, or None if
 // the value wasn't found.
-func Find[A any](pred func(A) bool, s []A) Option[A] {
+func Find[A any](pred Pred[A], s []A) Option[A] {
 	for _, val := range s {
 		if pred(val) {
 			return Some(val)
@@ -102,6 +102,18 @@ func Find[A any](pred func(A) bool, s []A) Option[A] {
 	return None[A]()
 }
 
+// FindIdx returns the first value that passes the supplied predicate along with
+// its index in the slice. If no satisfactory value is found, None is returned.
+func FindIdx[A any](pred Pred[A], s []A) Option[T2[int, A]] {
+	for i, val := range s {
+		if pred(val) {
+			return Some(NewT2[int, A](i, val))
+		}
+	}
+
+	return None[T2[int, A]]()
+}
+
 // Flatten takes a slice of slices and returns a concatenation of those slices.
 func Flatten[A any](s [][]A) []A {
 	sz := Foldr(

fn/slice_test.go

@@ -340,3 +340,37 @@ func TestPropForEachConcOutperformsMapWhenExpensive(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestPropFindIdxFindIdentity(t *testing.T) {
+	f := func(div, mod uint8, s []uint8) bool {
+		if div == 0 || div == 1 {
+			return true
+		}
+
+		pred := func(i uint8) bool {
+			return i%div == mod
+		}
+
+		foundIdx := FindIdx(pred, s)
+
+		// onlyVal :: Option[T2[A, B]] -> Option[B]
+		onlyVal := MapOption(func(t2 T2[int, uint8]) uint8 {
+			return t2.Second()
+		})
+
+		valuesEqual := Find(pred, s) == onlyVal(foundIdx)
+
+		idxGetsVal := ElimOption(
+			foundIdx,
+			func() bool { return true },
+			func(t2 T2[int, uint8]) bool {
+				return s[t2.First()] == t2.Second()
+			})
+
+		return valuesEqual && idxGetsVal
+	}
+
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}