Fix empty error branches in complex either expressions

Fixes #83

CHANGELOG.md

@@ -1,3 +1,11 @@
+v1.8.2
+------
+**Improved error reporting:**
+
+- Fix bug where empty error branches could be shown in complex either
+  expressions (fixes #83)
+
+
 v1.8.1
 ------
 - Fix: revert accidentally emitting $ReadOnlyArray types in array decoders

src/__tests__/either.test.js

@@ -7,29 +7,56 @@ import { constant, undefined_ } from '../constants';
 import { either, either4, either9 } from '../either';
 import { guard } from '../guard';
 import { number } from '../number';
-import { string } from '../string';
+import { object } from '../object';
+import { regex, string } from '../string';
 import { INPUTS } from './fixtures';
 
 describe('either', () => {
-    const decoder = guard(either(string, boolean));
+    const stringOrBooleanDecoder = guard(either(string, boolean));
     const [okay, not_okay] = partition(INPUTS, x => typeof x === 'string' || typeof x === 'boolean');
 
     it('valid', () => {
         expect(okay.length).not.toBe(0);
         for (const value of okay) {
-            expect(decoder(value)).toBe(value);
+            expect(stringOrBooleanDecoder(value)).toBe(value);
         }
     });
 
     it('invalid', () => {
         expect(not_okay.length).not.toBe(0);
         for (const value of not_okay) {
-            expect(() => decoder(value)).toThrow();
+            expect(() => stringOrBooleanDecoder(value)).toThrow();
         }
     });
 
-    it('errors nicely', () => {
-        expect(() => decoder(42)).toThrow('Either:');
+    it('errors nicely in trivial eithers', () => {
+        expect(() => stringOrBooleanDecoder(42)).toThrow('Either:');
+    });
+
+    it('errors nicely in common, simple eithers (ie optional)', () => {
+        // Either undefined or string
+        const g1 = guard(either(undefined_, string));
+        expect(() => g1(42)).toThrow('Either:');
+        expect(() => g1({})).toThrow('Either:');
+
+        // Either undefined or object
+        const g2 = guard(either(undefined_, object({ name: string })));
+        expect(() => g2(42)).toThrow('Either:');
+        expect(() => g2({ name: 42 })).toThrow('Either');
+
+        const g3 = guard(either(regex(/1/, 'Must contain 1'), regex(/2/, 'Must contain 2')));
+        expect(() => g3(42)).toThrow('Either');
+        expect(() => g3('foobar')).toThrow('Either');
+    });
+
+    it.skip('errors nicely in complex eithers (with two wildly different branches)', () => {
+        const g = guard(either(object({ foo: string }), object({ bar: number })));
+        expect(() =>
+            g({
+                foo: 123,
+                bar: 'not a number',
+            })
+        ).toThrow('XXX FIXME - this Either: error looks horrendous');
     });
 });
 

src/either.js

@@ -1,6 +1,7 @@
 // @flow strict
 
 import { annotate, indent } from 'debrief';
+import { summarize } from 'debrief';
 import { Err, Ok } from 'lemons';
 
 import type { Decoder, anything } from './types';
@@ -21,7 +22,16 @@ export function either<T1, T2>(d1: Decoder<T1>, d2: Decoder<T2>): Decoder<T1 | T
                 d2(blob).dispatch(
                     value2 => Ok(value2),
                     err2 =>
-                        Err(annotate(blob, ['Either:', itemize(err1.annotation), itemize(err2.annotation)].join('\n')))
+                        Err(
+                            annotate(
+                                blob,
+                                [
+                                    'Either:',
+                                    itemize(summarize(err1).join('\n')),
+                                    itemize(summarize(err2).join('\n')),
+                                ].join('\n')
+                            )
+                        )
                 )
         );
 }