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more efficient genBoundedEnum #33

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more efficient genBoundedEnum #33

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25 changes: 16 additions & 9 deletions src/Data/Enum/Gen.purs
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Expand Up @@ -2,17 +2,24 @@ module Data.Enum.Gen where

import Prelude

import Control.Monad.Gen (class MonadGen, elements)
import Data.Enum (class BoundedEnum, succ, enumFromTo)
import Data.Maybe (Maybe(..))
import Control.Monad.Gen (class MonadGen, chooseInt, elements)
import Data.Enum (class BoundedEnum, Cardinality, cardinality, enumFromTo, fromEnum, succ, toEnum)
import Data.Maybe (Maybe(..), fromJust)
import Data.Newtype (unwrap)
import Data.NonEmpty ((:|))
import Partial.Unsafe (unsafePartial)

-- | Create a random generator for a finite enumeration.
genBoundedEnum :: forall m a. MonadGen m => BoundedEnum a => m a
genBoundedEnum =
case succ bottom of
Just a →
let possibilities = enumFromTo a top :: Array a
in elements (bottom :| possibilities)
Nothing →
pure bottom
let topInt = fromEnum (top :: a)
bottomInt = fromEnum (bottom :: a)
enumRange = topInt - bottomInt
in if enumRange == unwrap (cardinality :: Cardinality a)
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Shouldn't this always be the case?

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My idea here was that if the Enum instance is bad, this test will fail. If it's good, then the test passes and the following line is safe. I'll rethink this... I'm probably missing something.

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Ah, I see what you mean... A bad Enum instance could just be lying about its Cardinality. Argh.

then unsafePartial $ fromJust <<< toEnum <$> chooseInt bottomInt topInt
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This actually is unsafe though, if the Enum instance is bad, right?

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If you excluded bottom, and added it back in fromMaybe, this could be safe though.

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But then it might be harder to ensure this is uniformly distributed :(

else case succ bottom of
Just a →
let possibilities = enumFromTo a top :: Array a
in elements (bottom :| possibilities)
Nothing →
pure bottom