fix(at): at decoder handles undefined values gracefully

src/Decoder.ts

@@ -215,9 +215,9 @@ export const array = <A>(decoder: Decoder<A>): Decoder<A[]> =>
     }
 
     return value.reduce((memo: Result<string, A[]>, element, idx) => {
-      const result = decoder.decodeAny(element);
       return memo.andThen(results => {
-        return result
+        return decoder
+          .decodeAny(element)
           .mapError(s => `I found an error in the array at [${idx}]: ${s}`)
           .map(v => results.concat([v]));
       });
@@ -260,6 +260,11 @@ export const at = <A>(
   decoder: Decoder<A>
 ): Decoder<A> =>
   new Decoder<A>(value => {
+    if (value == null) {
+      return err(
+        `I found an error. Could not apply 'at' path to an undefined or null value.`
+      );
+    }
     let val = value;
     let idx = 0;
     while (idx < path.length) {

tests/Decoder.test.ts

@@ -1,19 +1,5 @@
 import * as test from 'tape';
-import {
-  array,
-  at,
-  boolean,
-  date,
-  dict,
-  field,
-  keyValuePairs,
-  maybe,
-  nullable,
-  number,
-  oneOf,
-  string,
-  succeed,
-} from '../src/index';
+import { array, at, boolean, date, dict, field, keyValuePairs, maybe, nullable, number, oneOf, string, succeed } from '../src/index';
 
 test('string decoder', t => {
   string.decodeJson('"foo"').cata({
@@ -148,6 +134,13 @@ test('at decoder', t => {
       Ok: v => t.pass(`at is compatible with nullable`),
     });
 
+  at(['foo', 0, 'bar'], number)
+    .decodeAny(undefined)
+    .cata({
+      Err: () => t.pass(`at-ing an undefined to should be an error, but not crash`),
+      Ok: () => t.fail(`at-ing an undefined should not pass`),
+    });
+
   t.end();
 });