```
spago install classless
spago install classless-arbitrary
```

For the examples in this document you'll need the following imports:

```
module Test.Classless.Arbitrary where
import Prelude
import Classless (class Init, class InitRecord, class InitSum, initRecord, initSum, noArgs, (~))
import Classless.Arbitrary as Arb
import Control.Alt ((<|>))
import Data.Either (Either)
import Data.Generic.Rep (class Generic)
import Data.Maybe (Maybe)
import Data.Tuple (Tuple)
import Record as Record
import Test.QuickCheck.Gen (Gen, chooseInt)
```

Let's assume we have the following somehow nested type:

```
type Items = Array
( Either
String
(Tuple Int Boolean)
)
```

We can define a random generator of type `Gen Items`

by reproducing the
nested structure with those primitives and combinators:

```
genItems'1 :: Gen Items
genItems'1 = Arb.array
( Arb.either
Arb.string
(Arb.tuple Arb.int Arb.boolean)
)
```

The advantage of this approach is that we have full control over how each
piece of the type is generated. E.g. the `Arb.int`

combinator will generate
Integers in some defined default range. But we can replace it with any other
generator of type `Gen Int`

, maybe with one that can be configured to produce
values in a different range.

```
customInt :: Gen Int
customInt = chooseInt 10 20
```

This library is using QuickCheck's `Gen`

type, so you can use whatever
already exists for this type.

This works well for homogenous types like Arrays or Maybes. Things get a bit more interesting when we like to write generators in this style for heterogenous structures like Records. Let's say we'd like to generate values for the following record type:

```
type User =
{ name :: String
, age :: Int
, loggedIn :: Boolean
, coordinates :: Array { x :: Int, y :: Int }
}
```

The least boilerplate we archive by using the generic `Arb.record`

function
like below:

```
genUser'1 :: Gen User
genUser'1 = Arb.record
{ name: Arb.string
, age: Arb.int
, loggedIn: Arb.boolean
, coordinates: Arb.array $ Arb.record
{ x: Arb.int
, y: Arb.int
}
}
```

There's a similar construct for ADTs which have a generic instance. This
deserves a bit more explanation. Let's assume we have a `RemoteData`

type. A
sum type with 4 constructors each having some or none positional arguments.

```
data RemoteData
= NotAsked
| Loading Int Int Int
| Error String
| Success
{ status :: Int
, body :: String
}
derive instance Generic RemoteData _
```

We can now use the `Arb.sum`

function with a specification of how each field
in the type should be correlated with a generator. If there are no arguments,
we use `noArgs`

. If we have a product of arguments we use the `~`

operator to
list generators for the fields. The syntax is inspired by the routing-duplex package

```
genRemoteData'1 :: Gen RemoteData
genRemoteData'1 = Arb.sum
{ "NotAsked": noArgs
, "Loading": Arb.int ~ Arb.int ~ Arb.int
, "Error": Arb.string
, "Success": Arb.record
{ status: Arb.int
, body: Arb.string
}
}
```

Of course, the above only compiles if all constructors are specified with the correct labels and all the other types align to the target type.

At this moment we have some powerful combinators to create Generators for most of the common data structures that we use in a program. And we have fine grained control over how each piece of a type should be generated. The downside of it is, that we have to manually write the Generators for each type. As the name suggests, this "classless" package does not provide a type class. But you can define a type class yourself. And the package is designed to make this as boilerplate free as possible. The advantage of defining the type class at your side is that you can write instances for every type without the headache of orphan instances and Newtype wrappers.

Let's see how this works:

```
class MyArbitrary a where
arbitrary :: Gen a
instance MyArbitrary Int where
arbitrary = Arb.int
instance MyArbitrary String where
arbitrary = Arb.string
instance MyArbitrary Boolean where
arbitrary = Arb.boolean
instance (MyArbitrary a) => MyArbitrary (Array a) where
arbitrary = Arb.array arbitrary
instance (MyArbitrary a) => MyArbitrary (Maybe a) where
arbitrary = Arb.maybe arbitrary
instance (MyArbitrary a, MyArbitrary b) => MyArbitrary (Either a b) where
arbitrary = Arb.either arbitrary arbitrary
instance (MyArbitrary a, MyArbitrary b) => MyArbitrary (Tuple a b) where
arbitrary = Arb.tuple arbitrary arbitrary
```

So far this, should be quite familiar and not surprising. We defined instances
for a couple of concrete types like String or Boolean. As well as a couple of
combined types like `Array a`

which refer to our instance to fill the values
of the generic type parameters. This does not work that easily for constructs
like records. Because they're completely heterogenous, meaning they can contain an
arbitrary amount of different types. Now we need a trick to "pass" our own
type class implementation to a generic function that traverses the record
fields. The concept that is used here is inspired by the heterogeneous
package and is well
documented in the librarie's README.

We define a "dummy" data type and write an instance of the `Init`

type class
for it:

```
data MyInit = MyInit
instance (MyArbitrary a) => Init MyInit (Gen a) where
init _ = arbitrary
```

Now we have everything we need to define a type class instance for records like this:

```
instance (Arb.Record r' r, InitRecord MyInit r') => MyArbitrary (Record r) where
arbitrary = Arb.record $ initRecord MyInit
```

And for sum types we can define the following helper function which may be familiar to you (see e.g. genericShow)

```
genericSum :: forall r' a. Arb.Sum r' a => InitSum MyInit r' => Gen a
genericSum = Arb.sum $ initSum MyInit
```

It does not matter if you have not fully understood the details of the previous section. It's just important to note that we have created a type class that is able to produce the three sample Generators we defined earlier completely generically:

```
genItems'2 :: Gen Items
genItems'2 = arbitrary
genUser'2 :: Gen User
genUser'2 = arbitrary
genRemoteData'2 :: Gen RemoteData
genRemoteData'2 = genericSum
```

This is very convenient but what we lose here is the ability to define
different generators for the same types that somewhere occur. Every integer
is generated in the same way and without Newtype wrappers there would be no way to
opt in for the `chooseInt`

implementation that is explained above.

Depending on the use case we can chose the one or the other approach. But wouldn't it be nice to have a combination of both somehow?

Let's try to write a Generator for the `User`

record type above where every
field except one is derived by the type class.

```
genUser'3 :: Gen User
genUser'3 = Arb.record
$ Record.union
{ age: chooseInt 0 100
}
$ initRecord MyInit
```

`initRecord`

initializes the fields for us based on our type class. However,
before we pass field specification to the `Arb.record`

function, we just
merge it with our manually defined subset of the spec and there we go.
The inference is optimized in such a way that this even works if the types
that you specify manually have no instances for your type class. As you can
see below, the integer is generated by the type class but for char there's no
instance so we have to merge it into the spec:

```
genAB :: Gen { a :: Int, b :: Char }
genAB = Arb.record
$ Record.union
{ b: Arb.char
}
$ initRecord MyInit
```

The same patten works for sum types, too! Let's try to do the same with the
`RemoteData`

sample. Let's say we only want a custom generator for the
`Error`

case.

```
genRemoteData'3 :: Gen RemoteData
genRemoteData'3 = Arb.sum
$ Record.union
{ "Error": pure "error one" <|> pure "error two"
}
$ initSum MyInit
```

We can even provide a Generator only for one field of a product:

```
genRemoteData'4 :: Gen RemoteData
genRemoteData'4 = Arb.sum
$ Record.union
{ "Loading": arbitrary ~ arbitrary ~ chooseInt 0 100
}
$ initSum MyInit
```

And finally we can combine the sum and record mechanism. E.g. below we
generically retrieve everything except the `status`

field in the `Success`

case:

```
genRemoteData'5 :: Gen RemoteData
genRemoteData'5 = Arb.sum
$ Record.union
{ "Success": Arb.record
$ Record.union
{ status: chooseInt 200 500
}
$ initRecord MyInit
}
$ initSum MyInit
```

Module documentation is published on Pursuit.