CODEWITHSUNDEEP
haskellinstance
Luxzero
Luxzero

Reputation: 43

Instancing arbitrary

I'm having trouble writing an instance of Arbitrary for my data type. Which is the following:

    data FavoriteList a = FL [(a, Bool)] deriving Eq

This is what I have:

    instance Arbitrary a => Arbitrary(FavoriteList a) where
       arbitrary = oneof [liftM FavoriteList arbitrary]

But I'm getting the following error:

    Not in scope: data constructor 'FavoriteList'

I'm probably not understanding something about types and constructors I think...Can someone help me out?

Upvotes: 0

Views: 117

Answers (1)

Rein Henrichs
Rein Henrichs

Reputation: 15605

You're attempting to use the type-level term FavoriteList at the value-level. FL is the value-level term that constructs a value of type FavoriteList a for whatever a you choose. FL <$> arbitrary should be sufficient, or equivalently fmap FL arbitrary or liftM FL arbitrary. For a more thorough explanation of Haskell's type- and value-level languages, see this excellent answer by Conor McBride.

Upvotes: 3

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