feat: when: allow one or more x to be given to f and g

BREAKING CHANGE: when(pred)(...x): predicate can return logical true. see #29

src/when.js

@@ -4,6 +4,7 @@
  */
 
 const curry = require('./curry');
+const T = require('./T');
 
 /**
  * @summary
@@ -22,4 +23,7 @@ const curry = require('./curry');
  * @param {?} x
  * @returns {?}
  */
-module.exports = curry((f, g, x) => f(x) === true ? g(x) : undefined);
+module.exports = curry(function (f, g, x /* <- x is needed to maintain the function arity */) {
+  const [/*f*/,/*g*/,...xs] = Array.from(arguments);
+  return T(f(...xs)) ? g(...xs) : undefined;
+});

test/when.test.js

@@ -2,24 +2,49 @@ const td = require('testdouble');
 const test = require('tape');
 const {when: sut} =  require('..');
 
-test('when returns `g(x)` if `f(x)` returns `true`', t => {
+test('when(f)(g)(...x)', t => {
+  // parameters symbols
+  const a = Symbol();
+  const b = Symbol();
+  const c = Symbol();
+  const d = Symbol();
+
+  // return symbol
+  const z = Symbol();
+
   const f = td.func();
   const g = td.func();
 
-  td.when(f(1)).thenReturn(true);
-  td.when(f(2)).thenReturn(false);
-  td.when(f(3)).thenReturn('truthy');
+  // logical true
+  td.when(f(a, a)).thenReturn(true);
+  td.when(f(a, b)).thenReturn(NaN);
+  td.when(f(a, c)).thenReturn(0);
+  td.when(f(a, d)).thenReturn('');
+
+  // logical false
+  td.when(f(b, b)).thenReturn(false);
+  td.when(f(b, c)).thenReturn(null);
+  td.when(f(b, d)).thenReturn(undefined);
+
+  td.when(g(a, a)).thenReturn(z);
+  td.when(g(a, b)).thenReturn(z);
+  td.when(g(a, c)).thenReturn(z);
+  td.when(g(a, d)).thenReturn(z);
 
-  td.when(g(1)).thenReturn('OK');
+  t.same(sut(f)(g)(a, a), z, 'Returns `g(a, a)` as `f(a, a)` is logical true (`true`).');
+  t.same(sut(f)(g)(a, b), z, 'Returns `g(a, b)` as `f(a, b)` is logical true (`NaN`).');
+  t.same(sut(f)(g)(a, c), z, 'Returns `g(a, c)` as `f(a, c)` is logical true (`0`).');
+  t.same(sut(f)(g)(a, d), z, 'Returns `g(a, d)` as `f(a, d)` is logical true (``).');
 
-  t.same(sut(f)(g)(1), 'OK',
-    'expected `g(x)` because f(x) returned `true`.');
+  t.same(sut(f)(g)(b, b), undefined, 'Returns `undefined` as `f(b, b)` is logical false (`false`).');
+  t.same(sut(f)(g)(b, c), undefined, 'Returns `undefined` as `f(b, c)` is logical false (`null`).');
+  t.same(sut(f)(g)(b, d), undefined, 'Returns `undefined` as `f(b, d)` is logical false (`undefined`).');
 
-  t.same(sut(f)(g)(2), undefined,
-    'expected `undefined` because `f(x)` returned `false`.');
+  td.verify(g(b, b), {times: 0, ignoreExtraArgs: true});
+  td.verify(g(b, c), {times: 0, ignoreExtraArgs: true});
+  td.verify(g(b, d), {times: 0, ignoreExtraArgs: true});
 
-  t.same(sut(f)(g)(3), undefined,
-    'expected `undefined` because `f(x)` returned truthy not `true`.');
+  t.same(sut(f, g, a, a), z, 'Can supply one or more `x` immediately.');
 
   t.end();
 });