fix(transducers): add generics for compR(), fix types in mapNth()

thi-ng · Apr 18, 2018 · 3b7c9d9 · 3b7c9d9
1 parent 4c014b1
commit 3b7c9d9
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Showing 3 changed files with 29 additions and 7 deletions.
diff --git a/packages/transducers/README.md b/packages/transducers/README.md
@@ -638,7 +638,7 @@ itself. Returns nothing.
 
 #### `mapKeys(keys: IObjectOf<(x: any) => any>, copy?: boolean): Transducer<any, any>`
 
-#### `mapNth<A, B>(n: number, offset: number, fn: (x: A) => B): Transducer<A, B>`
+#### `mapNth<A, B>(n: number, offset?: number, fn: (x: A) => B): Transducer<A, A | B>`
 
 #### `mapVals<A, B>(fn: (v: A) => B, copy = true): Transducer<IObjectOf<A>, IObjectOf<B>>`
 

diff --git a/packages/transducers/src/func/compr.ts b/packages/transducers/src/func/compr.ts
@@ -1,5 +1,27 @@
 import { Reducer } from "../api";
+import { Reduced } from "../reduced";
 
-export function compR(rfn: Reducer<any, any>, fn: (acc: any, x: any) => any) {
-    return <Reducer<any, any>>[rfn[0], rfn[1], fn];
+/**
+ * Reducer composition helper. Takes existing reducer `rfn` (a 3-tuple)
+ * and a reducing function `fn`. Returns a new reducer tuple of this
+ * form:
+ *
+ * ```
+ * [rfn[0], rfn[1], fn]
+ * ```
+ *
+ * `rfn[2]` reduces values of type `B` into an accumulator of type `A`.
+ * `fn` accepts values of type `C` and produces interim results of type
+ * `B`, which are then (possibly) passed to the "inner" `rfn[2]`
+ * function. Therefore the resulting reducer takes inputs of `C` and an
+ * accumulator of type `A`.
+ *
+ * It is assumed that `fn` internally calls `rfn[2]` to pass its own
+ * results for further processing by the nested reducer `rfn`.
+ *
+ * @param rfn
+ * @param fn
+ */
+export function compR<A, B, C>(rfn: Reducer<A, B>, fn: (acc: A, x: C) => A | Reduced<A>) {
+    return <Reducer<A, C>>[rfn[0], rfn[1], fn];
 }
diff --git a/packages/transducers/src/xform/map-nth.ts b/packages/transducers/src/xform/map-nth.ts
@@ -1,9 +1,9 @@
 import { Reducer, Transducer } from "../api";
 import { compR } from "../func/compr";
 
-export function mapNth<A, B>(n: number, fn: (x: A) => B): Transducer<A, B>;
-export function mapNth<A, B>(n: number, offset: number, fn: (x: A) => B): Transducer<A, B>;
-export function mapNth<A, B>(...args: any[]): Transducer<A, B> {
+export function mapNth<A, B>(n: number, fn: (x: A) => B): Transducer<A, A | B>;
+export function mapNth<A, B>(n: number, offset: number, fn: (x: A) => B): Transducer<A, A | B>;
+export function mapNth<A, B>(...args: any[]): Transducer<A, A | B> {
     let n = args[0] - 1, offset, fn;
     if (typeof args[1] === "number") {
         offset = args[1];
@@ -12,7 +12,7 @@ export function mapNth<A, B>(...args: any[]): Transducer<A, B> {
         fn = args[1];
         offset = 0;
     }
-    return (rfn: Reducer<any, B>) => {
+    return (rfn: Reducer<any, A | B>) => {
         const r = rfn[2];
         let skip = 0, off = offset;
         return compR(rfn,