Simplify API

[gunar]

The current API is expressive enough, but I believe it could be made more appealing to people new to PBT.

I suggest

1. Merging fc.assert and fc.property into fc.test.
2. Allowing primitive constructors as identifiers to built-in arbitraries.
3. Presenting pre() earlier on in the documentation.

@dubzzz what do you think?

---

-const fc = require('fast-check');
+const { test } = require('fast-check');
 
 // Code under test
 const contains = (text, pattern) => text.indexOf(pattern) >= 0;
@@ -7,10 +7,10 @@
 describe('properties', () => {
   // string text always contains itself
   it('should always contain itself', () => {
-    fc.assert(fc.property(fc.string(), text => contains(text, text)));
+    test(String, text => contains(text, text));
   });
   // string a + b + c always contains b, whatever the values of a, b and c
   it('should always contain its substrings', () => {
-    fc.assert(fc.property(fc.string(), fc.string(), fc.string(), (a,b,c) => contains(a+b+c, b)));
+    test(String, String, String, (a,b,c) => contains(a+b+c, b));
   });
 });

[dubzzz]

Hi @gunar,

First of all thanks for the suggestions. Here are my feedbacks concerning those ones:

Merging fc.assert and fc.property into fc.test

I believe the split between assert and property is the common way to go with property based testing.

One of the advantages is that you can easily change fc.assert into:

• fc.check -> not to throw
• fc.sample -> to see the generated values
• fc.statistics -> to analyze the look and feel of the generated values

I think the best thing would be to provide wrapper specific for each framework as I already did for https://github.com/dubzzz/ava-fast-check/ or am trying to do with jestjs/jest#8035

Here are some examples taken from different projects in different programming languages:

• scalacheck in Scala

  @Test
  def propertyDoNotHoldOnInfiniteStreams() {
    val inputGen = /* ... */
    check(Prop.forAll(inputGen) {gens: (List[Int], Int) => gens match {
      case (l, num) => LineRound.ofInt.merge_tiles(Stream.continually(l.toStream).flatten).take(num).size == num
    }})
}

check would be the equivalent of fc.assert and Prop.forAll the equivalent of fc.property.

• jsverify in JavaScript

it(name, function () {
  jsc.assert(jsc.forall(...));
}

• testcheck in JavaScript

check(
  property(
    gen.int, gen.int,
    (a, b) => a + b >= a && a + b >= b
  )
)

Allowing primitive constructors as identifiers to built-in arbitraries

I am not really fond of using primitive constructors as shorthands for arbitraries:

• String is not shorter than string
• String: is it supposed to be a fc.string(), fc.unicodeString()...?
• Number: is it supposed to be an integer, a float, infinity?

Moreover it will highly complexify all the typings and require lots of changes to have it everywhere. Additionally there is no certainty that the users of fast-check building additional wrappers (wrappers similar to array, tuple...) will handle them as arbitraries.

Presenting pre() earlier on in the documentation

Feel free to suggest the associated change in the documentation with a PR. So that we could discuus your suggestion together. The easier the documentation the better.

[dubzzz]

@gunar Any updates? Any feedbacks?

[gunar]

Not really, your answer pretty much nailed it. Thanks @dubzzz.