From ef7a798d35c172d50b2f4453f4522260d0fe81e4 Mon Sep 17 00:00:00 2001
From: Gavin Cannizzaro <gcannizzaro@mindgrub.com>
Date: Mon, 1 Jun 2020 03:48:20 -0400
Subject: [PATCH] fix(transducers): #186, Fix crash when using empty string as
 source for several transducers.

---
 packages/transducers/src/xform/benchmark.ts   |  3 +-
 packages/transducers/src/xform/drop-nth.ts    |  3 +-
 packages/transducers/src/xform/drop.ts        |  3 +-
 packages/transducers/src/xform/duplicate.ts   |  3 +-
 packages/transducers/src/xform/filter.ts      |  3 +-
 .../transducers/src/xform/flatten-with.ts     |  5 +--
 packages/transducers/src/xform/interleave.ts  |  3 +-
 packages/transducers/src/xform/interpolate.ts |  3 +-
 packages/transducers/src/xform/interpose.ts   |  3 +-
 packages/transducers/src/xform/labeled.ts     |  4 +--
 packages/transducers/src/xform/map-deep.ts    |  5 ++-
 packages/transducers/src/xform/map.ts         |  3 +-
 packages/transducers/src/xform/mapcat.ts      |  3 +-
 packages/transducers/src/xform/match-first.ts |  3 +-
 packages/transducers/src/xform/match-last.ts  |  3 +-
 .../transducers/src/xform/moving-average.ts   |  3 +-
 packages/transducers/src/xform/pad-last.ts    |  3 +-
 .../transducers/src/xform/partition-of.ts     |  3 +-
 .../transducers/src/xform/partition-time.ts   |  3 +-
 packages/transducers/src/xform/pluck.ts       |  5 ++-
 packages/transducers/src/xform/select-keys.ts |  5 ++-
 packages/transducers/src/xform/struct.ts      |  3 +-
 packages/transducers/src/xform/swizzle.ts     |  5 ++-
 packages/transducers/src/xform/take-last.ts   |  3 +-
 packages/transducers/src/xform/take-nth.ts    |  3 +-
 packages/transducers/src/xform/take.ts        |  3 +-
 .../transducers/src/xform/throttle-time.ts    |  3 +-
 packages/transducers/src/xform/throttle.ts    |  3 +-
 packages/transducers/src/xform/toggle.ts      |  3 +-
 packages/transducers/test/drop.ts             | 21 +++++++++++++
 packages/transducers/test/filter.ts           | 31 +++++++++++++++++++
 packages/transducers/test/flatten.ts          |  5 +++
 packages/transducers/test/map.ts              | 18 +++++++++++
 packages/transducers/test/take.ts             | 21 +++++++++++++
 34 files changed, 163 insertions(+), 31 deletions(-)
 create mode 100644 packages/transducers/test/drop.ts
 create mode 100644 packages/transducers/test/filter.ts
 create mode 100644 packages/transducers/test/map.ts
 create mode 100644 packages/transducers/test/take.ts

diff --git a/packages/transducers/src/xform/benchmark.ts b/packages/transducers/src/xform/benchmark.ts
index 11a9f78020..6386d1d8bd 100644
--- a/packages/transducers/src/xform/benchmark.ts
+++ b/packages/transducers/src/xform/benchmark.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
 import type { Reducer, Transducer } from "../api";
@@ -21,7 +22,7 @@ import type { Reducer, Transducer } from "../api";
 export function benchmark(): Transducer<any, number>;
 export function benchmark(src: Iterable<any>): IterableIterator<number>;
 export function benchmark(src?: Iterable<any>): any {
-    return src
+    return isIterable(src)
         ? iterator1(benchmark(), src)
         : (rfn: Reducer<any, number>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/drop-nth.ts b/packages/transducers/src/xform/drop-nth.ts
index 3d7fdc22ad..7b53fa8e9d 100644
--- a/packages/transducers/src/xform/drop-nth.ts
+++ b/packages/transducers/src/xform/drop-nth.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { throttle } from "./throttle";
 import type { Transducer } from "../api";
@@ -5,7 +6,7 @@ import type { Transducer } from "../api";
 export function dropNth<T>(n: number): Transducer<T, T>;
 export function dropNth<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function dropNth<T>(n: number, src?: Iterable<T>): any {
-    if (src) {
+    if (isIterable(src)) {
         return iterator1(dropNth(n), src);
     }
     n = Math.max(0, n - 1);
diff --git a/packages/transducers/src/xform/drop.ts b/packages/transducers/src/xform/drop.ts
index 33d41c151c..c8beb49de3 100644
--- a/packages/transducers/src/xform/drop.ts
+++ b/packages/transducers/src/xform/drop.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
 import type { Reducer, Transducer } from "../api";
@@ -5,7 +6,7 @@ import type { Reducer, Transducer } from "../api";
 export function drop<T>(n: number): Transducer<T, T>;
 export function drop<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function drop<T>(n: number, src?: Iterable<T>): any {
-    return src
+  return isIterable(src)
         ? iterator1(drop(n), src)
         : (rfn: Reducer<any, T>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/duplicate.ts b/packages/transducers/src/xform/duplicate.ts
index ad812ac590..192ef79712 100644
--- a/packages/transducers/src/xform/duplicate.ts
+++ b/packages/transducers/src/xform/duplicate.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator } from "../iterator";
 import { isReduced } from "../reduced";
@@ -6,7 +7,7 @@ import type { Reducer, Transducer } from "../api";
 export function duplicate<T>(n?: number): Transducer<T, T>;
 export function duplicate<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function duplicate<T>(n = 1, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(duplicate(n), src)
         : (rfn: Reducer<any, T>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/filter.ts b/packages/transducers/src/xform/filter.ts
index da8ddfb9c8..985c788b23 100644
--- a/packages/transducers/src/xform/filter.ts
+++ b/packages/transducers/src/xform/filter.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
 import type { Predicate } from "@thi.ng/api";
@@ -9,7 +10,7 @@ export function filter<T>(
     src: Iterable<T>
 ): IterableIterator<T>;
 export function filter<T>(pred: Predicate<T>, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator1(filter(pred), src)
         : (rfn: Reducer<any, T>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/flatten-with.ts b/packages/transducers/src/xform/flatten-with.ts
index 990831c6a9..3a2d2db90d 100644
--- a/packages/transducers/src/xform/flatten-with.ts
+++ b/packages/transducers/src/xform/flatten-with.ts
@@ -1,3 +1,4 @@
+import { isIterable, isString } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator } from "../iterator";
 import { isReduced } from "../reduced";
@@ -15,8 +16,8 @@ export function flattenWith<T>(
     fn: Fn<T, Iterable<T>>,
     src?: Iterable<T | Iterable<T>>
 ): any {
-    return src
-        ? iterator(flattenWith(fn), src)
+    return isIterable(src)
+        ? iterator(flattenWith(fn), isString(src) ? <any>[src] : src)
         : (rfn: Reducer<any, T>) => {
               const reduce = rfn[2];
               const flatten = (acc: any, x: any) => {
diff --git a/packages/transducers/src/xform/interleave.ts b/packages/transducers/src/xform/interleave.ts
index 33259e5d14..6e8deb2002 100644
--- a/packages/transducers/src/xform/interleave.ts
+++ b/packages/transducers/src/xform/interleave.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator } from "../iterator";
 import { isReduced } from "../reduced";
@@ -10,7 +11,7 @@ export function interleave<A, B>(
     src: Iterable<A>
 ): IterableIterator<A | B>;
 export function interleave<A, B>(sep: any, src?: Iterable<A>): any {
-    return src
+    return isIterable(src)
         ? iterator(interleave(sep), src)
         : (rfn: Reducer<any, A | B>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/interpolate.ts b/packages/transducers/src/xform/interpolate.ts
index 8c65299c5d..17cdad71cb 100644
--- a/packages/transducers/src/xform/interpolate.ts
+++ b/packages/transducers/src/xform/interpolate.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { comp } from "../func/comp";
 import { normRange } from "../iter/norm-range";
 import { iterator } from "../iterator";
@@ -51,7 +52,7 @@ export function interpolate<T>(fn: Fn2<T[], number, T>, window: number, n: numbe
 export function interpolate<T>(fn: Fn2<T[], number, T>, window: number, n: number, src: Iterable<number>): IterableIterator<number>;
 // prettier-ignore
 export function interpolate<T>(fn: Fn2<T[], number, T>, window: number, n: number, src?: Iterable<number>) {
-    return src
+  return isIterable(src)
         ? iterator(interpolate(fn, window, n), src)
         : comp(
               partition<T>(window, 1),
diff --git a/packages/transducers/src/xform/interpose.ts b/packages/transducers/src/xform/interpose.ts
index 9a6421c3b4..5f1855e642 100644
--- a/packages/transducers/src/xform/interpose.ts
+++ b/packages/transducers/src/xform/interpose.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator } from "../iterator";
 import { isReduced } from "../reduced";
@@ -10,7 +11,7 @@ export function interpose<A, B>(
     src: Iterable<A>
 ): IterableIterator<A | B>;
 export function interpose<A, B>(sep: any, src?: Iterable<A>): any {
-    return src
+    return isIterable(src)
         ? iterator(interpose(sep), src)
         : (rfn: Reducer<any, A | B>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/labeled.ts b/packages/transducers/src/xform/labeled.ts
index 5d158ff830..4222d291de 100644
--- a/packages/transducers/src/xform/labeled.ts
+++ b/packages/transducers/src/xform/labeled.ts
@@ -1,4 +1,4 @@
-import { isFunction } from "@thi.ng/checks";
+import { isFunction, isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { map } from "./map";
 import type { Transducer } from "../api";
@@ -11,7 +11,7 @@ export function labeled<L, T>(
     src: Iterable<T>
 ): IterableIterator<[L, T]>;
 export function labeled<L, T>(id: LabelFn<L, T>, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator1(labeled(id), src)
         : map(isFunction(id) ? (x: T) => [id(x), x] : (x: T) => [id, x]);
 }
diff --git a/packages/transducers/src/xform/map-deep.ts b/packages/transducers/src/xform/map-deep.ts
index a682262b00..f0ecdc53fc 100644
--- a/packages/transducers/src/xform/map-deep.ts
+++ b/packages/transducers/src/xform/map-deep.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { deepTransform } from "../func/deep-transform";
 import { iterator1 } from "../iterator";
 import { map } from "./map";
@@ -17,5 +18,7 @@ export function mapDeep(
     src: Iterable<any>
 ): IterableIterator<any>;
 export function mapDeep(spec: TransformSpec, src?: Iterable<any>): any {
-    return src ? iterator1(mapDeep(spec), src) : map(deepTransform(spec));
+    return isIterable(src)
+        ? iterator1(mapDeep(spec), src)
+        : map(deepTransform(spec));
 }
diff --git a/packages/transducers/src/xform/map.ts b/packages/transducers/src/xform/map.ts
index a3896d7dd7..82b4279bf9 100644
--- a/packages/transducers/src/xform/map.ts
+++ b/packages/transducers/src/xform/map.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
 import type { Fn } from "@thi.ng/api";
@@ -18,7 +19,7 @@ import type { Reducer, Transducer } from "../api";
 export function map<A, B>(fn: Fn<A, B>): Transducer<A, B>;
 export function map<A, B>(fn: Fn<A, B>, src: Iterable<A>): IterableIterator<B>;
 export function map<A, B>(fn: Fn<A, B>, src?: Iterable<A>): any {
-    return src
+    return isIterable(src)
         ? iterator1(map(fn), src)
         : (rfn: Reducer<any, B>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/mapcat.ts b/packages/transducers/src/xform/mapcat.ts
index 3295191acd..ae08e9c056 100644
--- a/packages/transducers/src/xform/mapcat.ts
+++ b/packages/transducers/src/xform/mapcat.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { comp } from "../func/comp";
 import { iterator } from "../iterator";
 import { cat } from "./cat";
@@ -33,5 +34,5 @@ export function mapcat<A, B>(
     fn: Fn<A, Iterable<B> | null | undefined>,
     src?: Iterable<A>
 ): any {
-    return src ? iterator(mapcat(fn), src) : comp(map(fn), cat());
+    return isIterable(src) ? iterator(mapcat(fn), src) : comp(map(fn), cat());
 }
diff --git a/packages/transducers/src/xform/match-first.ts b/packages/transducers/src/xform/match-first.ts
index 3bfdd91433..23c2778cfc 100644
--- a/packages/transducers/src/xform/match-first.ts
+++ b/packages/transducers/src/xform/match-first.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { comp } from "../func/comp";
 import { iterator1 } from "../iterator";
 import { filter } from "./filter";
@@ -41,7 +42,7 @@ export function matchFirst<T>(
     src: Iterable<T>
 ): T | undefined;
 export function matchFirst<T>(pred: Predicate<T>, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? [...iterator1(matchFirst(pred), src)][0]
         : comp(filter(pred), take(1));
 }
diff --git a/packages/transducers/src/xform/match-last.ts b/packages/transducers/src/xform/match-last.ts
index 8d25dc0fa4..1b6bdaa6cf 100644
--- a/packages/transducers/src/xform/match-last.ts
+++ b/packages/transducers/src/xform/match-last.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { comp } from "../func/comp";
 import { iterator } from "../iterator";
 import { filter } from "./filter";
@@ -41,7 +42,7 @@ export function matchLast<T>(
     src: Iterable<T>
 ): T | undefined;
 export function matchLast<T>(pred: Predicate<T>, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? [...iterator(matchLast(pred), src)][0]
         : comp(filter(pred), takeLast(1));
 }
diff --git a/packages/transducers/src/xform/moving-average.ts b/packages/transducers/src/xform/moving-average.ts
index 9ce949f470..7734c9044f 100644
--- a/packages/transducers/src/xform/moving-average.ts
+++ b/packages/transducers/src/xform/moving-average.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { illegalArgs } from "@thi.ng/errors";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
@@ -24,7 +25,7 @@ export function movingAverage(period: number): Transducer<number, number>;
 // prettier-ignore
 export function movingAverage(period: number, src: Iterable<number>): IterableIterator<number>;
 export function movingAverage(period: number, src?: Iterable<number>): any {
-    return src
+    return isIterable(src)
         ? iterator1(movingAverage(period), src)
         : (rfn: Reducer<any, number>) => {
               period |= 0;
diff --git a/packages/transducers/src/xform/pad-last.ts b/packages/transducers/src/xform/pad-last.ts
index 28768f4983..a19a5c1955 100644
--- a/packages/transducers/src/xform/pad-last.ts
+++ b/packages/transducers/src/xform/pad-last.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator } from "../iterator";
 import { isReduced } from "../reduced";
 import type { Reducer, Transducer } from "../api";
@@ -41,7 +42,7 @@ export function padLast<T>(
     src: Iterable<T>
 ): IterableIterator<T>;
 export function padLast<T>(n: number, fill: T, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(padLast(n, fill), src)
         : ([init, complete, reduce]: Reducer<any, T>) => {
               let m = 0;
diff --git a/packages/transducers/src/xform/partition-of.ts b/packages/transducers/src/xform/partition-of.ts
index e35f2ad6ee..bc8cddbdbe 100644
--- a/packages/transducers/src/xform/partition-of.ts
+++ b/packages/transducers/src/xform/partition-of.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator } from "../iterator";
 import { partitionBy } from "./partition-by";
 import type { Transducer } from "../api";
@@ -26,7 +27,7 @@ export function partitionOf<T>(
     src: Iterable<T>
 ): IterableIterator<T[]>;
 export function partitionOf<T>(sizes: number[], src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(partitionOf(sizes), src)
         : partitionBy(() => {
               let i = 0,
diff --git a/packages/transducers/src/xform/partition-time.ts b/packages/transducers/src/xform/partition-time.ts
index b4e33f814a..b54c291b79 100644
--- a/packages/transducers/src/xform/partition-time.ts
+++ b/packages/transducers/src/xform/partition-time.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator } from "../iterator";
 import { partitionBy } from "./partition-by";
 import type { Transducer } from "../api";
@@ -38,7 +39,7 @@ export function partitionTime<T>(
     src: Iterable<T>
 ): IterableIterator<T[]>;
 export function partitionTime<T>(period: number, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(partitionTime(period), src)
         : partitionBy(() => {
               let last = 0;
diff --git a/packages/transducers/src/xform/pluck.ts b/packages/transducers/src/xform/pluck.ts
index 0d9804d21e..dbfe37dfdc 100644
--- a/packages/transducers/src/xform/pluck.ts
+++ b/packages/transducers/src/xform/pluck.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { map } from "./map";
 import type { Transducer } from "../api";
@@ -20,5 +21,7 @@ export function pluck<A, B>(
     src: Iterable<A>
 ): IterableIterator<B>;
 export function pluck<A>(key: PropertyKey, src?: Iterable<A>): any {
-    return src ? iterator1(pluck(key), src) : map((x: any) => x[key]);
+    return isIterable(src)
+        ? iterator1(pluck(key), src)
+        : map((x: any) => x[key]);
 }
diff --git a/packages/transducers/src/xform/select-keys.ts b/packages/transducers/src/xform/select-keys.ts
index 2e5f03e995..975df3b1e3 100644
--- a/packages/transducers/src/xform/select-keys.ts
+++ b/packages/transducers/src/xform/select-keys.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { keySelector } from "../func/key-selector";
 import { iterator1 } from "../iterator";
 import { map } from "./map";
@@ -33,5 +34,7 @@ export function selectKeys<T>(
     src: Iterable<T>
 ): IterableIterator<any>;
 export function selectKeys<T>(keys: PropertyKey[], src?: Iterable<T>): any {
-    return src ? iterator1(selectKeys(keys), src) : map(keySelector(keys));
+    return isIterable(src)
+        ? iterator1(selectKeys(keys), src)
+        : map(keySelector(keys));
 }
diff --git a/packages/transducers/src/xform/struct.ts b/packages/transducers/src/xform/struct.ts
index c981fb8123..5326fb7134 100644
--- a/packages/transducers/src/xform/struct.ts
+++ b/packages/transducers/src/xform/struct.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { comp } from "../func/comp";
 import { iterator } from "../iterator";
 import { mapKeys } from "./map-keys";
@@ -52,7 +53,7 @@ export function struct<T>(
     src: Iterable<any>
 ): IterableIterator<T>;
 export function struct(fields: StructField[], src?: Iterable<any>): any {
-    return src
+    return isIterable(src)
         ? iterator(struct(fields), src)
         : comp(
               partitionOf(fields.map((f) => f[1])),
diff --git a/packages/transducers/src/xform/swizzle.ts b/packages/transducers/src/xform/swizzle.ts
index 32a042ec1c..e216dd7e76 100644
--- a/packages/transducers/src/xform/swizzle.ts
+++ b/packages/transducers/src/xform/swizzle.ts
@@ -1,4 +1,5 @@
 import { swizzle as _swizzle } from "@thi.ng/arrays";
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { map } from "./map";
 import type { Transducer } from "../api";
@@ -30,5 +31,7 @@ export function swizzle<T>(
     src: Iterable<any>
 ): IterableIterator<any[]>;
 export function swizzle(order: PropertyKey[], src?: Iterable<any>): any {
-    return src ? iterator1(swizzle(order), src) : map(_swizzle(order));
+    return isIterable(src)
+        ? iterator1(swizzle(order), src)
+        : map(_swizzle(order));
 }
diff --git a/packages/transducers/src/xform/take-last.ts b/packages/transducers/src/xform/take-last.ts
index 84237b6ba4..b82312c7c8 100644
--- a/packages/transducers/src/xform/take-last.ts
+++ b/packages/transducers/src/xform/take-last.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { __drain } from "../internal/drain";
 import { iterator } from "../iterator";
 import type { Reducer, Transducer } from "../api";
@@ -18,7 +19,7 @@ import type { Reducer, Transducer } from "../api";
 export function takeLast<T>(n: number): Transducer<T, T>;
 export function takeLast<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function takeLast<T>(n: number, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(takeLast(n), src)
         : ([init, complete, reduce]: Reducer<any, T>) => {
               const buf: T[] = [];
diff --git a/packages/transducers/src/xform/take-nth.ts b/packages/transducers/src/xform/take-nth.ts
index ff048203b0..6b5ab5da0b 100644
--- a/packages/transducers/src/xform/take-nth.ts
+++ b/packages/transducers/src/xform/take-nth.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { throttle } from "./throttle";
 import type { Transducer } from "../api";
@@ -17,7 +18,7 @@ import type { Transducer } from "../api";
 export function takeNth<T>(n: number): Transducer<T, T>;
 export function takeNth<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function takeNth<T>(n: number, src?: Iterable<T>): any {
-    if (src) {
+    if (isIterable(src)) {
         return iterator1(takeNth(n), src);
     }
     n = Math.max(0, n - 1);
diff --git a/packages/transducers/src/xform/take.ts b/packages/transducers/src/xform/take.ts
index 2ab7d646b5..b706b388f0 100644
--- a/packages/transducers/src/xform/take.ts
+++ b/packages/transducers/src/xform/take.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator } from "../iterator";
 import { ensureReduced, reduced } from "../reduced";
@@ -18,7 +19,7 @@ import type { Reducer, Transducer } from "../api";
 export function take<T>(n: number): Transducer<T, T>;
 export function take<T>(n: number, src: Iterable<T>): IterableIterator<T>;
 export function take<T>(n: number, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator(take(n), src)
         : (rfn: Reducer<any, T>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/throttle-time.ts b/packages/transducers/src/xform/throttle-time.ts
index d2efa2ca5c..712ebf76b1 100644
--- a/packages/transducers/src/xform/throttle-time.ts
+++ b/packages/transducers/src/xform/throttle-time.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import { throttle } from "./throttle";
 import type { Transducer } from "../api";
@@ -22,7 +23,7 @@ export function throttleTime<T>(
     src: Iterable<T>
 ): IterableIterator<T>;
 export function throttleTime<T>(delay: number, src?: Iterable<T>): any {
-    return src
+    return isIterable(src)
         ? iterator1(throttleTime(delay), src)
         : throttle<T>(() => {
               let last = 0;
diff --git a/packages/transducers/src/xform/throttle.ts b/packages/transducers/src/xform/throttle.ts
index b52d29e07d..e981b1b696 100644
--- a/packages/transducers/src/xform/throttle.ts
+++ b/packages/transducers/src/xform/throttle.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { compR } from "../func/compr";
 import { iterator1 } from "../iterator";
 import type { StatefulPredicate } from "@thi.ng/api";
@@ -29,7 +30,7 @@ export function throttle<T>(
     pred: StatefulPredicate<T>,
     src?: Iterable<T>
 ): any {
-    return src
+    return isIterable(src)
         ? iterator1(throttle(pred), src)
         : (rfn: Reducer<any, T>) => {
               const r = rfn[2];
diff --git a/packages/transducers/src/xform/toggle.ts b/packages/transducers/src/xform/toggle.ts
index 8068471946..dabc5c4625 100644
--- a/packages/transducers/src/xform/toggle.ts
+++ b/packages/transducers/src/xform/toggle.ts
@@ -1,3 +1,4 @@
+import { isIterable } from "@thi.ng/checks";
 import { iterator1 } from "../iterator";
 import type { Reducer, Transducer } from "../api";
 
@@ -33,7 +34,7 @@ export function toggle<T>(
     initial = false,
     src?: Iterable<any>
 ): any {
-    return src
+    return isIterable(src)
         ? iterator1(toggle(on, off, initial), src)
         : ([init, complete, reduce]: Reducer<any, T>) => {
               let state = initial;
diff --git a/packages/transducers/test/drop.ts b/packages/transducers/test/drop.ts
new file mode 100644
index 0000000000..f40d680a7d
--- /dev/null
+++ b/packages/transducers/test/drop.ts
@@ -0,0 +1,21 @@
+import { drop, range } from "../src";
+
+import * as assert from "assert";
+
+describe("drop", () => {
+    it("starts iterating after N items", () => {
+        assert.deepEqual([...drop(0, [true, false])], [true, false]);
+        assert.deepEqual([...drop(1, [true, false])], [false]);
+        assert.deepEqual([...drop(2, [true, false])], []);
+        assert.deepEqual([...drop(3, [true, false])], []);
+        assert.deepEqual([...drop(2, range(0, 4))], [2, 3]);
+        assert.deepEqual([...drop(0, ["", "ab", "c"])], ["", "ab", "c"]);
+        assert.deepEqual([...drop(1, ["", "ab", "c"])], ["ab", "c"]);
+        assert.deepEqual([...drop(2, ["", "ab", "c"])], ["c"]);
+        assert.deepEqual([...drop(0, "")], []);
+        assert.deepEqual([...drop(1, "")], []);
+        assert.deepEqual([...drop(0, "abc")], ["a", "b", "c"]);
+        assert.deepEqual([...drop(1, "abc")], ["b", "c"]);
+        assert.deepEqual([...drop(2, "abc")], ["c"]);
+    });
+});
diff --git a/packages/transducers/test/filter.ts b/packages/transducers/test/filter.ts
new file mode 100644
index 0000000000..0d5dad12e4
--- /dev/null
+++ b/packages/transducers/test/filter.ts
@@ -0,0 +1,31 @@
+import { filter, range } from "../src";
+
+import * as assert from "assert";
+
+describe("filter", () => {
+    const identity = (x: any) => x;
+    const always = () => true;
+    const never = () => false;
+    const vowel = (s: string) => /[aeiou]/.test(s);
+    const even = (n: number) => n % 2 === 0;
+
+    it("applies predicate over iterable and forwards values testing truthy", () => {
+        assert.deepEqual(
+            [...filter(identity, [true, false, "a", "", 0, 1, []])],
+            [true, "a", 1, []]
+        );
+        assert.deepEqual(
+            [...filter(always, [true, false, "a", "", 0, 1, []])],
+            [true, false, "a", "", 0, 1, []]
+        );
+        assert.deepEqual(
+            [...filter(never, [true, false, "a", "", 0, 1, []])],
+            []
+        );
+        assert.deepEqual([...filter(vowel, ["", "a", "bc"])], ["a"]);
+        assert.deepEqual([...filter(even, range(1, 5))], [2, 4]);
+        assert.deepEqual([...filter(always, "")], []);
+        assert.deepEqual([...filter(always, "abc")], ["a", "b", "c"]);
+        assert.deepEqual([...filter(vowel, "abc")], ["a"]);
+    });
+});
diff --git a/packages/transducers/test/flatten.ts b/packages/transducers/test/flatten.ts
index 2ae2f686f7..f2f2c1661a 100644
--- a/packages/transducers/test/flatten.ts
+++ b/packages/transducers/test/flatten.ts
@@ -14,6 +14,11 @@ describe("flatten", () => {
     it("strings", () => {
         assert.deepEqual([...flatten(["", "a"])], ["", "a"]);
         assert.deepEqual([...flatten([[], ["a"], ""])], ["a", ""]);
+        assert.deepEqual([...flatten([["abc"]])], ["abc"]);
+        assert.deepEqual([...flatten(["abc"])], ["abc"]);
+        assert.deepEqual([...flatten("abc")], ["abc"]);
+        assert.deepEqual([...flatten([""])], [""]);
+        assert.deepEqual([...flatten("")], [""]);
     });
     it("iterators", () => {
         assert.deepEqual([...flatten(range(0))], []);
diff --git a/packages/transducers/test/map.ts b/packages/transducers/test/map.ts
new file mode 100644
index 0000000000..56f0fdc262
--- /dev/null
+++ b/packages/transducers/test/map.ts
@@ -0,0 +1,18 @@
+import { map, range } from "../src";
+
+import * as assert from "assert";
+
+describe("map", () => {
+    const identity = <T>(t: T): T => t;
+    const upper = (s: string) => s.toUpperCase();
+    const square = (n: number) => n * n;
+
+    it("applies function over iterable", () => {
+        assert.deepEqual([...map(identity, [])], []);
+        assert.deepEqual([...map(identity, ["", "ab", "c"])], ["", "ab", "c"]);
+        assert.deepEqual([...map(upper, ["", "ab", "c"])], ["", "AB", "C"]);
+        assert.deepEqual([...map(square, range(1, 4))], [1, 4, 9]);
+        assert.deepEqual([...map(upper, "")], []);
+        assert.deepEqual([...map(upper, "abc")], ["A", "B", "C"]);
+    });
+});
diff --git a/packages/transducers/test/take.ts b/packages/transducers/test/take.ts
new file mode 100644
index 0000000000..ac5bfbd53b
--- /dev/null
+++ b/packages/transducers/test/take.ts
@@ -0,0 +1,21 @@
+import { take, range } from "../src";
+
+import * as assert from "assert";
+
+describe("take", () => {
+    it("iterates up to N items", () => {
+        assert.deepEqual([...take(0, [true, false])], []);
+        assert.deepEqual([...take(1, [true, false])], [true]);
+        assert.deepEqual([...take(2, [true, false])], [true, false]);
+        assert.deepEqual([...take(3, [true, false])], [true, false]);
+        assert.deepEqual([...take(2, range(0, 4))], [0, 1]);
+        assert.deepEqual([...take(0, ["", "ab", "c"])], []);
+        assert.deepEqual([...take(1, ["", "ab", "c"])], [""]);
+        assert.deepEqual([...take(2, ["", "ab", "c"])], ["", "ab"]);
+        assert.deepEqual([...take(0, "")], []);
+        assert.deepEqual([...take(1, "")], []);
+        assert.deepEqual([...take(0, "abc")], []);
+        assert.deepEqual([...take(1, "abc")], ["a"]);
+        assert.deepEqual([...take(2, "abc")], ["a", "b"]);
+    });
+});