feat(predicates): not operator

@vates/predicates/.USAGE.md

@@ -1,7 +1,7 @@
 `undefined` predicates are ignored and `undefined` is returned if all predicates are `undefined`, this permits the most efficient composition:
 
 ```js
-const compositePredicate = every(undefined, some(predicate2, undefined))
+const compositePredicate = not(every(undefined, some(not(predicate2), undefined)))
 
 // ends up as
 
@@ -36,6 +36,21 @@ isBetween3And10(10)
 // → false
 ```
 
+### `not(predicate)`
+
+> Returns a predicate that returns the negation of the predicate.
+
+```js
+const isEven = n => n % 2 === 0
+const isOdd = not(isEven)
+
+isOdd(1)
+// true
+
+isOdd(2)
+// false
+```
+
 ### `some(predicates)`
 
 > Returns a predicate that returns `true` iff some predicate returns `true`.

@vates/predicates/README.md

@@ -19,7 +19,7 @@ Installation of the [npm package](https://npmjs.org/package/@vates/predicates):
 `undefined` predicates are ignored and `undefined` is returned if all predicates are `undefined`, this permits the most efficient composition:
 
 ```js
-const compositePredicate = every(undefined, some(predicate2, undefined))
+const compositePredicate = not(every(undefined, some(not(predicate2), undefined)))
 
 // ends up as
 
@@ -54,6 +54,21 @@ isBetween3And10(10)
 // → false
 ```
 
+### `not(predicate)`
+
+> Returns a predicate that returns the negation of the predicate.
+
+```js
+const isEven = n => n % 2 === 0
+const isOdd = not(isEven)
+
+isOdd(1)
+// true
+
+isOdd(2)
+// false
+```
+
 ### `some(predicates)`
 
 > Returns a predicate that returns `true` iff some predicate returns `true`.

@vates/predicates/index.js

@@ -51,6 +51,22 @@ exports.every = function every() {
   }
 }
 
+const notPredicateTag = {}
+exports.not = function not(predicate) {
+  if (isDefinedPredicate(predicate)) {
+    if (predicate.tag === notPredicateTag) {
+      return predicate.predicate
+    }
+
+    function notPredicate() {
+      return !predicate.apply(this, arguments)
+    }
+    notPredicate.predicate = predicate
+    notPredicate.tag = notPredicateTag
+    return notPredicate
+  }
+}
+
 exports.some = function some() {
   const predicates = handleArgs.apply(this, arguments)
   const n = predicates.length

@vates/predicates/index.spec.js

@@ -3,20 +3,14 @@
 const assert = require('assert/strict')
 const { describe, it } = require('tap').mocha
 
-const { every, some } = require('./')
+const { every, not, some } = require('./')
 
 const T = () => true
 const F = () => false
 
-const testArgsHandling = fn => {
+const testArgHandling = fn => {
-  it('returns undefined if all predicates are undefined', () => {
+  it('returns undefined if predicate is undefined', () => {
     assert.equal(fn(undefined), undefined)
-    assert.equal(fn([undefined]), undefined)
-  })
-
-  it('returns the predicate if only a single one is passed', () => {
-    assert.equal(fn(undefined, T), T)
-    assert.equal(fn([undefined, T]), T)
   })
 
   it('throws if it receives a non-predicate', () => {
@@ -24,6 +18,15 @@ const testArgsHandling = fn => {
     error.value = 3
     assert.throws(() => fn(3), error)
   })
+}
+
+const testArgsHandling = fn => {
+  testArgHandling(fn)
+
+  it('returns the predicate if only a single one is passed', () => {
+    assert.equal(fn(undefined, T), T)
+    assert.equal(fn([undefined, T]), T)
+  })
 
   it('forwards this and arguments to predicates', () => {
     const thisArg = 'qux'
@@ -36,17 +39,21 @@ const testArgsHandling = fn => {
   })
 }
 
-const runTests = (fn, truthTable) =>
+const runTests = (fn, acceptMultiple, truthTable) =>
   it('works', () => {
     truthTable.forEach(([result, ...predicates]) => {
-      assert.equal(fn(...predicates)(), result)
+      if (acceptMultiple) {
         assert.equal(fn(predicates)(), result)
+      } else {
+        assert.equal(predicates.length, 1)
+      }
+      assert.equal(fn(...predicates)(), result)
     })
   })
 
 describe('every', () => {
   testArgsHandling(every)
-  runTests(every, [
+  runTests(every, true, [
     [true, T, T],
     [false, T, F],
     [false, F, T],
@@ -54,9 +61,22 @@ describe('every', () => {
   ])
 })
 
+describe('not', () => {
+  testArgHandling(not)
+
+  it('returns the original predicate if negated twice', () => {
+    assert.equal(not(not(T)), T)
+  })
+
+  runTests(not, false, [
+    [true, F],
+    [false, T],
+  ])
+})
+
 describe('some', () => {
   testArgsHandling(some)
-  runTests(some, [
+  runTests(some, true, [
     [true, T, T],
     [true, T, F],
     [true, F, T],

CHANGELOG.unreleased.md

@@ -33,6 +33,7 @@
 
 - @vates/nbd-client major
 - @vates/otp major
+- @vates/predicates minor
 - @vates/read-chunk patch
 - @xen-orchestra/fs minor
 - @xen-orchestra/log minor