Polyvariadic Functions in Haskell

Question

After reading this article on writing polyvariadic functions in Haskell, I tried to write some of my own.

At first I thought I'd try to generalize it - so I could have a function that returned variadic functions by collapsing arguments as given.

{-# OPTIONS -fglasgow-exts #-}
module Collapse where
class Collapse a r | r -> a where
  collapse :: (a -> a -> a) -> a -> r
instance Collapse a a where
  collapse _ = id
instance (Collapse a r) => Collapse a (a -> r) where
  collapse f a a' = collapse f (f a a')

However, the compiler didn't like that:

Collapse.hs:5:9:
    Functional dependencies conflict between instance declarations:
      instance Collapse a a -- Defined at Collapse.hs:5:9-20
      instance (Collapse a r) => Collapse a (a -> r)
        -- Defined at Collapse.hs:7:9-43

If I went back and added a wrapper type for the final result, however, it worked:

module Collapse where
class Collapse a r | r -> a where
  collapse :: (a -> a -> a) -> a -> r
data C a = C a
instance Collapse a (C a) where
  collapse _ = C . id
instance (Collapse a r) => Collapse a (a -> r) where
  collapse f a a' = collapse f (f a a')
sum :: (Num a, Collapse a r) => a -> r
sum = collapse (+)

Once I made this change, it compiled fine, and I could use the collapse function in ghci.

ghci> let C s = Collapse.sum 1 2 3 in s
6

I'm not sure why the wrapper type is required for the final result. If anyone could explain that, I'd highly appreciate it. I can see that the compiler's telling me that it's some issue with the functional dependencies, but I don't really grok the proper use of fundeps yet.

Later, I tried to take a different tack, and try and define a variadic function generator for functions that took a list and returned a value. I had to do the same container trick, and also allow UndecidableInstances.

{-# OPTIONS -fglasgow-exts #-}
{-# LANGUAGE UndecidableInstances #-}
module Variadic where
class Variadic a b r | r -> a, r -> b where
  variadic :: ([a] -> b) -> r
data V a = V a
instance Variadic a b (V b) where
  variadic f = V $ f []
instance (Variadic a b r) => Variadic a b (a -> r) where
  variadic f a = variadic (f . (a:))
list :: Variadic a [a] r => r
list = variadic . id
foldl :: (Variadic b a r) => (a -> b -> a) -> a -> r
foldl f a = variadic (Prelude.foldl f a)

Without allowing UndecidableInstances the compiler complained that my instance declarations were illegal:

Variadic.hs:7:0:
    Illegal instance declaration for `Variadic a b (V b)'
        (the Coverage Condition fails for one of the functional dependencies;
         Use -XUndecidableInstances to permit this)
    In the instance declaration for `Variadic a b (V b)'

Variadic.hs:9:0:
    Illegal instance declaration for `Variadic a b (a -> r)'
        (the Coverage Condition fails for one of the functional dependencies;
         Use -XUndecidableInstances to permit this)
    In the instance declaration for `Variadic a b (a -> r)'

However, once it compiled, I could successfully use it in ghci:

ghci> let V l = Variadic.list 1 2 3 in l
[1,2,3]
ghci> let vall p = Variadic.foldl (\b a -> b && (p a)) True
ghci> :t vall
vall :: (Variadic b Bool r) => (b -> Bool) -> r
ghci> let V b = vall (>0) 1 2 3 in b
True

I guess what I'm looking for is an explanation of why the container type for the final value is necessary, as well as why all the various functional dependencies are necessary.

Also, this seemed odd:

ghci> let vsum = Variadic.foldl (+) 0

<interactive>:1:10:
    Ambiguous type variables `a', `r' in the constraint:
      `Variadic a a r'
        arising from a use of `Variadic.foldl' at <interactive>:1:10-29
    Probable fix: add a type signature that fixes these type variable(s)

<interactive>:1:10:
    Ambiguous type variable `a'in the constraint:
      `Num a' arising from the literal `0' at <interactive>:1:29
    Probable fix: add a type signature that fixes these type variable(s)
ghci> let vsum' = Variadic.foldl (+) 
ghci> :t vsum'
(Num a, Variadic a a r) => t -> a -> r
ghci> :t vsum' 0
(Num a, Variadic a a r) => a -> r
ghci> let V s = vsum' 0 1 2 3 in s
6

I'm guessing that's fallout from allowing UndecidableInstances, but I don't know, and I'd like to better understand what's going on.

Answer 1

The idea behind functional dependencies is that in a declaration like

class Collapse a r | r -> a where
  ...

the r -> a bit says that a is uniquely determined by r. So, you can't have instance Collapse (a -> r) (a -> r) and instance Collapse a (a -> r). Note that instance Collapse (a -> r) (a -> r) follows from instance Collapse a a for the complete picture.

In other words, your code is trying to establish instance Collapse t t (the type variable's name is of course unimportant) and instance Collapse a (a -> r). If you substitute (a -> r) for t in the first instance declaration, you get instance Collapse (a -> r) (a -> r). Now this is the only instance of Collapse with the second type parameter equal to (a -> r) that you can have, because the class declaration says that the first type parameter is to be deducible from the second. Yet next you try to establish instance a (a -> r), which would add another instance of Collapse with the second type parameter being (a -> r). Thus, GHC complains.

Answer 2

Michał Marczyk is absolutely correct about the fundeps and instance matching issue, and the wrapper type seems like an easy fix. On the other hand, if you're already reading Oleg's site, you might prefer to go deeper down the rabbit hole and try writing an instance for "any type that isn't a function".

As far as UndecidableInstances goes, the coverage condition is described here; it should be obvious why your instances fail it. Note that the word "undecidable" here means undecidable in roughly the same sense as in "the Halting Problem is undecidable"--that is to say, you're telling GHC to recklessly attempt to resolve code that could send it into an infinite loop based only on your assertion that it's okay. It's fun for hacking neat ideas, but consenting to be a human first-pass type-checker for GHC is a burden I personally find wearying.