Allow mapper function in map() calls to throw an error to reject the …

…decoder

CHANGELOG.md

@@ -1,3 +1,13 @@
+v1.16.0
+-------
+
+**New feature:**
+
+- Allow `map()` calls to throw an exception in the mapper function to reject
+  the decoder.  Previously, these mapper functions were not expected to ever
+  throw.
+
+
 v1.15.0
 -------
 

src/__tests__/compose.test.js

@@ -4,6 +4,7 @@ import { annotate } from 'debrief';
 import { Err, Ok } from 'lemons/Result';
 
 import { guard } from '../guard';
+import { number } from '../number';
 import { string } from '../string';
 import { compose, map } from '../utils';
 
@@ -44,4 +45,23 @@ describe('map', () => {
         expect(upcase('123').unwrap()).toEqual('123');
         expect(upcase('I am Hulk').unwrap()).toEqual('I AM HULK');
     });
+
+    it('a failing mapper will fail the decoder', () => {
+        const odd = map(number, n => {
+            if (n % 2 !== 0) return n;
+            throw new Error('Must be odd');
+        });
+        expect(odd(13).unwrap()).toEqual(13);
+        expect(() => guard(odd)(4)).toThrow('^ Must be odd');
+        expect(odd(3).isErr()).toBe(false);
+        expect(odd(4).isErr()).toBe(true);
+
+        const weirdEven = map(number, n => {
+            if (n % 2 === 0) return n;
+            throw 'Must be even'; // Throwing a string, not an Error is non-conventional, but won't break anything
+        });
+        expect(weirdEven(3).isErr()).toBe(true);
+        expect(() => guard(weirdEven)(3)).toThrow('^ Must be even');
+        expect(weirdEven(4).unwrap()).toEqual(4);
+    });
 });

src/utils.js

@@ -22,7 +22,13 @@ export const isDate = (value: mixed): boolean %checks =>
 export function map<T, V>(decoder: Decoder<T>, mapper: T => V): Decoder<V> {
     return compose(
         decoder,
-        x => Ok(mapper(x))
+        x => {
+            try {
+                return Ok(mapper(x));
+            } catch (e) {
+                return Err(annotate(x, e instanceof Error ? e.message : String(e)));
+            }
+        }
     );
 }