Turning a Dict into a constraint

问题

I have a class Cyc c r which has functions for datas of the form c m r, where m is a phantom type. For example,

class Cyc c r where
  cyc :: (Foo m, Foo m') => c m r -> c m' r

I do have good reasons for not making m a class parameter. For the purposes of this example, the primary reason is that it reduces the number of constraints on functions. In my actual example, a more compelling need for this interface is that I work with changing and hidden phantom types, so this interface lets me get a Cyc constraint for any phantom type.

One downside to that choice is that I can't make Num (c m r) a superclass constraint of Cyc. My intention is that c m r should be a Num whenever (Cyc c r, Foo m). The current solution is very annoying: I added method to class Cyc

witNum :: (Foo m) => c m r -> Dict (Num (c m r))

which sort-of accomplishes the same thing. Now when I have a function that takes a generic Cyc and needs a Num (c m r) constraint, I can write:

foo :: (Cyc c r, Foo m) => c m r -> c m r
foo c = case witNum c of
  Dict -> c*2

Of courses I could add a Num (c m r) constraint to foo, but I'm trying to reduce the number of constraints, remember? (Cyc c r, Foo m) is supposed to imply a Num (c m r) constraint (and I need Cyc c r and Foo m for other purposes), so I don't want to have to write out the Num constraint also.

In the process of writing this question, I found a better(?) way to accomplish this, but it has its own drawbacks.

Module Foo:

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ScopedTypeVariables #-}
module Foo where

import Data.Constraint

class Foo m

class Cyc c r where
  cyc :: (Foo m, Foo m') => c m r -> c m' r  
  witNum :: (Foo m) => c m r -> Dict (Num (c m r))

instance (Foo m, Cyc c r) => Num (c m r) where
  a * b = case witNum a of
            Dict -> a * b
  fromInteger a = case witNum (undefined :: c m r) of
                    Dict -> fromInteger a

-- no Num constraint and no Dict, best of both worlds
foo :: (Foo m, Cyc c r) => c m r -> c m r
foo = (*2)

Module Bar:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, OverlappingInstances #-}
module Bar where

import Foo
import Data.Constraint

data Bar m r = Bar r deriving (Show)

instance (Num r) => Cyc Bar r where
  witNum _ = Dict

instance (Num r, Foo m) => Num (Bar m r) where
  (Bar a) * (Bar b) = Bar $ a*b
  fromInteger = Bar . fromInteger

instance Foo ()  

bar :: Bar () Int
bar = foo 3

While this approach gets me everything I'm looking for, it seems fragile. My main concerns are:

1. I'm wary of the generic instance head for Num in module Foo.
2. If any overlapping instances are imported into Foo, I suddenly need IncoherentInstances or the Num constraint on foo to defer instance selection to runtime.

Is there an alternative way to avoid using Dict in every function that needs Num (c m r) that avoids either of these downsides?

回答1:

After 6 months of thought, I finally have an answer to my dangling comment above: add a newtype wrapper!

I split the Cyc class in two:

class Foo m

class Cyc c where
  cyc :: (Foo m, Foo m') => c m r -> c m' r

class EntailCyc c where
  entailCyc :: Tagged (c m r) ((Foo m, Num r) :- (Num (c m r)))

Then I define my Cyc instance as above:

data Bar m r = ...

instance Cyc Bar where ...

instance (Num r, Foo m) => Num (Bar m r) where ...

instance EntailCyc Bar where
  witNum _ = Dict

Then I define a newtype wrapper and give a generic Cyc instance for it:

newtype W c m r = W (c m r)

instance Cyc (W c m r) where cyc (W a) = W $ cyc a

instance (EntailCyc c, Foo m, Num r) => Num (W c m r) where
  (W a) + (W b) = a + b \\ witness entailCyc a

Finally, I change all functions that used a generic c m r type to use a W c m r type:

foo :: (Cyc c, EntailCyc c, Foo m, Num r) => W c m r -> W c m r
foo = (*2)

The point here is that foo might need many constraints (e.g., Eq (W c m r), Show (W c m r), etc) that would each individually require their own constraints. However, the generic instances for W c m r for Eq, Show, etc all have exactly the constraints (EntailCyc c, Foo m, Eq/Show/... a), so the constraints on foo above are the only constraints I need to write!

来源：https://stackoverflow.com/questions/29482576/turning-a-dict-into-a-constraint