Type-variable-dependent default method implementation in Haskell

Question

I am trying to define default method implementations, but only if the class's type variables derive certain other classes.

I have tried creating type-dependent instances using => (am I even using it correctly?), but I get a "duplicate instance declaration error": (https://repl.it/@solly_ucko/Distributions)

{-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, InstanceSigs #-}

import Data.Int
import Data.Ratio
import Data.Set
import System.Random

duplicate :: a -> (a, a)
duplicate a = (a, a)

listRange :: Enum a => a -> a -> [a]
listRange a b = [a..b]

class Fractional w => Distribution d v w where
    probability :: d v w -> v -> w

    probabilityOfRange :: Ord v => d v w -> v -> v -> w

    ranges :: (Ord v) => d v w -> Set (v, v)
    ranges = (Data.Set.map duplicate) . values

    sample :: RandomGen g => d v w -> g -> (v, g)
    --sample d g = (scanl1 (+) $ flip Prelude.map $ probability d, g) -- Will need to implement some sort of binary tree, most likely.

    sampleIO :: d v w -> IO v
    sampleIO = getStdRandom . sample

    values :: d v w -> Set v

instance (Ord v, Fractional w) => Distribution d v w where
    probability d v = probabilityOfRange d v v

instance Enum v => Distribution d v w where
    probabilityOfRange d v1 v2 = sum $ Prelude.map (probability d) [v1..v2]

instance (Enum v, Ord v) => Distribution d v w where
    values = fromList . (concatMap $ uncurry listRange) . toList . ranges

When I then try to add real instances (and comment out some of the "instances" I created earlier so that the compiler can reach that point), it gives me an error about conflicting instances.

data Empty v w = Empty

instance Distribution Empty v (Ratio Int8) where
    sample _ g = (undefined, g)
    sampleIO _ = return undefined
    probabilityOfRange _ _ _ = 0
    values _ = empty

data Singleton v w = Singleton v

instance Distribution Singleton v Integer where
    sample (Singleton v) g = (v, g)
    sampleIO (Singleton v) = return v
    probabilityOfRange (Singleton v1) v2 v3
        | v2 <= v1 && v1 <= v3 = 1
        | otherwise        = 0

data Uniform v w = Uniform (Set v)

To clarify, my goal is for probability and values to be defined for all Distributions, and for probabilityOfRange to be defined for all Distributions with values deriving Ord. I also wish to provide defaults when additional constraints are met, because without them, a reasonable default (based on other methods) is impossible.

You definitely can't do this in GHC Haskell (or any other existing Haskell variant I know of). Those three default instances are "duplicates" because they have the same instance heads. The compiler ignores instance constraints until it has actually chosen an instance. Those other instances are overlapping, which is a bit less fatal, but requires special annotations and will likely make life difficult. — dfeuer
– dfeuer, Commented Apr 5, 2019 at 3:54

K. A. Buhr · Accepted Answer · 2019-04-05 04:09:55Z

Trying to specify a default for a single method using something like:

instance (Ord v, Fractional w) => Distribution d v w where
    probability d v = probabilityOfRange d v v

won't work. Haskell instances don't "accumulate". For a given triple of types d v w, at most one instance Distribution d v w clause will apply. (If multiple clauses could apply because of "overlapping" instances, there are mechanisms to choose the "best" match, but there are no direct mechanisms to combine methods from multiple instance clauses.)

In general, if you have a class method:

class Distribution d v w where
    probability :: d v w -> v w

and you'd like to define a default method with a more restrictive type signature (i.e., with constraints on some of the types):

probability :: (Ord v) => d v w -> v -> w
probability d v = probabilityOfRange d v v

there are two approaches.

The first is to make use of the DefaultSignatures extension. This allows you to separate the type signature for the method from the (possibly more restrictive) type signature of the default method. The syntax is:

class Fractional w => Distribution d v w where
    probability :: d v w -> v -> w
    default probability :: (Ord v) => d v w -> v -> w
    probability d v = probabilityOfRange d v v
    ...

The caveat here is that, if you define an instance that doesn't override the default method, then it must satisfy the constraint Ord v, or it won't typecheck.

If you want more control over when the default method is used, then the usual approach is to separate the default definition out into a separate function that must be explicitly included in instances that wish to use it. So, you'd have:

class Fractional w => Distribution d v w where
    probability :: d v w -> v -> w

probabilityDefault :: (Distribution d v w, Ord v) => d v w -> v -> w
probabilityDefault d v = probabilityOfRange d v v

and an instance that wishes to use the default must do so explicitly:

instance Distribution Whatever Int w where
    probability = probabilityDefault

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