Question

Im pretty new to Haskell.
I have a datatype:

data Sentence= Prop Int
          | No Sentence
          | And [Sentence]
          | Or [Sentence]
          deriving Eq

I already wrote a Show instance for it

However, whether it makes sense or not, I would like to be able to generate a random Sentence.
How can i accomplish this in Haskell?

Answer 1

Random number generation is a typical example of an “impure” operation, since calling a random generator twice will of course yield different results – Haskell’s nature disallows this.

Therefore, you need to use a so called monad Gen a that represents a random generator, that, when run, yields a value of type a.

Fortunately, you can combine these generators in a quite nice syntax …

So, you just need some library that implements such a generator – and here we go.

randomNo   = No <$> randomSentence
randomProp = Prop <$> choose (1, 10) 
[...]

randomSentence = oneOf [randomNo, randomProp, ...]