如何为TestIt类型编写SmallCheck的Serial实例？含依赖序列化需求

Let's break down how to solve your problem step by step, starting with the basic Serial instance and moving to dependent serialization.

1. Basic Serial Instance for TestIt #

First, we need to define a Serial instance for your custom D type since SmallCheck doesn't know about it out of the box:

instance Serial m D where
  series = cons0 A \/ cons1 B \/ cons1 C

• cons0 handles nullary constructors (like A), cons1 handles unary constructors (like B String and C String).

Next, we need to generate values of type m D for the y field's output. We can wrap our D series in pure to lift it into the monad:

instance Serial m D => Serial m (m D) where
  series = pure <$> series

Now, for the TestIt instance:
The y field is Int -> m D, which requires generating functions. SmallCheck uses the CoSerial class for this—CoSerial m a lets us generate functions from a to any serializable type. Since Int has a built-in CoSerial instance, we can use coseries to turn our m D series into a series of functions:

instance (Monad m, Serial m Int, Serial m String) => Serial m (TestIt m) where
  series = TestIt <$> xSeries <~> ySeries
    where
      xSeries = pure <$> series  -- Generate m Int by lifting Int samples into the monad
      ySeries = coseries series  -- Convert m D series into Int -> m D function series

• <~> is SmallCheck's applicative combinator for combining series of values.

2. Dependent Serialization for y (When x is 0) #

Yes, this is feasible—but it requires that your monad m lets you extract the underlying Int value from m Int. This works best with pure monads like Identity (since monads like Maybe or IO don't expose their values until runtime).

Here's how to implement this for Identity:

import Control.Monad.Identity (Identity(..))

instance Serial Identity (TestIt Identity) where
  series = do
    -- Generate the x value (Identity Int)
    xIdentity <- pure <$> series
    let intVal = runIdentity xIdentity  -- Extract the Int from Identity
    
    -- Generate y based on x's value
    yFunc <- if intVal == 0
             then -- When x is 0, generate y functions focused on input 0
                  dVal <- series :: Series Identity D
                  pure (\input -> if input == 0 then pure dVal else pure A)
             else -- Use normal function generation for other x values
                  coseries (pure <$> series)
    
    return $ TestIt xIdentity yFunc

• We use SmallCheck's Series monad to first generate x, then conditionally generate y based on x's value.
• When x is 0, we create functions that return a generated D value only when given 0 as input (you can adjust this logic to fit your testing needs).

Note that this approach restricts the generated TestIt instances to those where y depends on x, which is useful if you want to focus tests on the specific case where x is 0.

内容的提问来源于stack exchange，提问作者RandomB