export laws sans `Arbitrary`

[matthewleon]

It should be possible to support quickchecking laws without making use of Arbitrary. Here is an example for Semigroup:

checkSemigroupGen
  ∷ ∀ eff s
  . Semigroup s
  ⇒ Eq s
  ⇒ Gen s
  → QC eff Unit
checkSemigroupGen gen = do

  log "Checking 'Associativity' law for Semigroup"
  quickCheck' 1000 $ associativity <$> gen <*> gen <*> gen

  where

  associativity ∷ s → s → s → Boolean
  associativity x y z = ((x <> y) <> z) == (x <> (y <> z))

[garyb]

👍

I've also been meaning to export some of the actual properties as standalone functions (like associativity)

[matthewleon]

I've also been meaning to export some of the actual properties as standalone functions also (like associativity)

I believe there are some PRs around, at least on the Prelude, to do precisely this.

[garyb]

That's true! They've been there for so long I forgot 😬

[hdgarrood]

Yeah so I started on this with purescript/purescript-prelude#111, and I was starting to think that returning a { lhs :: a, rhs :: a } might be better; see purescript/purescript-prelude#111 (comment)

[hdgarrood]

Ah, but that's separate really, isn't it? ok never mind then.