Fix Sequence tailRecM StackOverflow

Author: pt2121

kategory/src/main/kotlin/kategory/data/SequenceKW.kt

@@ -42,14 +42,19 @@ data class SequenceKW<out A> constructor(val sequence: Sequence<A>) : SequenceKW
                 buf: MutableList<B>,
                 f: (A) -> HK<SequenceKWHK, Either<A, B>>,
                 v: SequenceKW<Either<A, B>>) {
-            if (!v.isEmpty()) {
+            if (v.sequence.any()) {
                 val head: Either<A, B> = v.first()
                 when (head) {
                     is Either.Right<A, B> -> {
                         buf += head.b
                         go(buf, f, v.drop(1).k())
                     }
-                    is Either.Left<A, B> -> go(buf, f, (f(head.a).ev() + v.drop(1)).k())
+                    is Either.Left<A, B> -> {
+                        if (v.count() == 1)
+                            go(buf, f, (f(head.a).ev()).k())
+                        else
+                            go(buf, f, (f(head.a).ev() + v.drop(1)).k())
+                    }
                 }
             }
         }

kategory/src/test/kotlin/kategory/data/SequenceKWTest.kt

@@ -14,6 +14,7 @@ class SequenceKWTest : UnitSpec() {
                     a.toList() == b.toList()
         }
 
+        testLaws(MonadLaws.laws(SequenceKW.monad(), eq))
         testLaws(MonoidKLaws.laws(SequenceKW.monoidK(), applicative, eq))
         testLaws(TraverseLaws.laws(SequenceKW.traverse(), applicative, { n: Int -> SequenceKW(sequenceOf(n)) }, eq))
     }