fspec
=====

Usage: ```fspec([options])```

Takes :args :ret and (optional) :fn kwargs whose values are preds
and returns a spec whose conform/explain take a fn and validates it
using generative testing. The conformed value is always the fn itself.

See 'fdef' for a single operation that creates an fspec and
registers it, as well as a full description of :args, :ret and :fn

fspecs can generate functions that validate the arguments and
fabricate a return value compliant with the :ret spec, ignoring
the :fn spec if present.

Optionally takes :gen generator-fn, which must be a fn of no args
that returns a test.check generator.

[Source](https://github.com/mrijk/speculaas/blob/master/lib/fspec.js)

Example:

```js
const s = require('speculaas');
const {isInteger} = s.utils;

const adder = x => y => x + y;

s.fdef(adder, {
    args: s.cat(':x', isInteger),
    ret: s.fspec({
        args: s.cat(':y', isInteger),
        ret: isInteger
    }),
    fn: f => f.args[':x'] === f.ret(0)
});

s.test.check(adder);
// {sym: 'adder', result: true, ...}
```