Add a draft of the catmorphisms post
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code/catamorphisms/Cata.hs
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code/catamorphisms/Cata.hs
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{-# LANGUAGE LambdaCase, DeriveFunctor, DeriveFoldable, MultiParamTypeClasses #-}
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import Prelude hiding (length, sum, fix)
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length :: [a] -> Int
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length [] = 0
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length (_:xs) = 1 + length xs
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lengthF :: ([a] -> Int) -> [a] -> Int
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lengthF rec [] = 0
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lengthF rec (_:xs) = 1 + rec xs
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lengthF' = \rec -> \case
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[] -> 0
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_:xs -> 1 + rec xs
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fix f = let x = f x in x
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length' = fix lengthF
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data MyList = MyNil | MyCons Int MyList
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data MyListF a = MyNilF | MyConsF Int a
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newtype Fix f = Fix { unFix :: f (Fix f) }
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testList :: Fix MyListF
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testList = Fix (MyConsF 1 (Fix (MyConsF 2 (Fix (MyConsF 3 (Fix MyNilF))))))
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myOut :: MyList -> MyListF MyList
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myOut MyNil = MyNilF
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myOut (MyCons i xs) = MyConsF i xs
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myIn :: MyListF MyList -> MyList
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myIn MyNilF = MyNil
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myIn (MyConsF i xs) = MyCons i xs
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instance Functor MyListF where
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fmap f MyNilF = MyNilF
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fmap f (MyConsF i a) = MyConsF i (f a)
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mySumF :: MyListF Int -> Int
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mySumF MyNilF = 0
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mySumF (MyConsF i rest) = i + rest
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mySum :: MyList -> Int
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mySum = mySumF . fmap mySum . myOut
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myCata :: (MyListF a -> a) -> MyList -> a
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myCata f = f . fmap (myCata f) . myOut
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myLength = myCata $ \case
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MyNilF -> 0
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MyConsF _ l -> 1 + l
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myMax = myCata $ \case
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MyNilF -> 0
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MyConsF x y -> max x y
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myMin = myCata $ \case
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MyNilF -> 0
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MyConsF x y -> min x y
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myTestList = MyCons 2 (MyCons 1 (MyCons 3 MyNil))
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pack :: a -> (Int -> a -> a) -> MyListF a -> a
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pack b f MyNilF = b
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pack b f (MyConsF x y) = f x y
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unpack :: (MyListF a -> a) -> (a, Int -> a -> a)
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unpack f = (f MyNilF, \i a -> f (MyConsF i a))
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class Functor f => Cata a f where
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out :: a -> f a
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cata :: Cata a f => (f b -> b) -> a -> b
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cata f = f . fmap (cata f) . out
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instance Cata MyList MyListF where
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out = myOut
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data ListF a b = Nil | Cons a b deriving Functor
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instance Cata [a] (ListF a) where
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out [] = Nil
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out (x:xs) = Cons x xs
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sum :: Num a => [a] -> a
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sum = cata $ \case
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Nil -> 0
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Cons x xs -> x + xs
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data BinaryTree a = Node a (BinaryTree a) (BinaryTree a) | Leaf deriving (Show, Foldable)
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data BinaryTreeF a b = NodeF a b b | LeafF deriving Functor
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instance Cata (BinaryTree a) (BinaryTreeF a) where
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out (Node a l r) = NodeF a l r
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out Leaf = LeafF
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invert :: BinaryTree a -> BinaryTree a
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invert = cata $ \case
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LeafF -> Leaf
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NodeF a l r -> Node a r l
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381
content/blog/haskell_catamorphisms.md
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content/blog/haskell_catamorphisms.md
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---
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title: "Generalizing Folds in Haskell"
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date: 2022-04-22T12:19:22-07:00
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draft: true
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tags: ["Haskell"]
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---
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### Recursive Functions
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Let's start off with a little bit of a warmup, and take a look at a simple recursive function:
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`length`. Here's a straightforward definition:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 4 6 >}}
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Haskell is nice because it allows for clean definitions of recursive functions; `length` can
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just reference itself in its definition, and everything works out in the end. In the underlying
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lambda calculus, though, a function definition doesn't come with a name -- you only get anonymous
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functions via the lambda abstraction. There's no way for such functions to just refer to themselves by
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their name in their body. But the lambda calculus is Turing complete, so something is making recursive
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definitions possible.
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The trick is to rewrite your recursive function in such a way that instead of calling itself by its name
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(which, with anonymous functions, is hard to come by), it receives a reference to itself as an argument.
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As a concrete example:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 8 10 >}}
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This new function can easily me anonymous; if we enable the `LambdaCase` extension,
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we can write it using only lambda functions as:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 12 14 >}}
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This function is not equivalent to `length`, however. It expects "itself", or a function
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which has type `[a] -> Int`, to be passed in as the first argument. Once fed this `rec`
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argument, though, `lengthF` returns a length function. Let's try feed it something, then!
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```Haskell
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lengthF _something
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```
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But if `lengthF` produces a length function when given this _something_, why can't we feed
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this newly-produced length function back to it?
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```Haskell
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lengthF (lengthF _something)
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```
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And again:
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```Haskell
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lengthF (lengthF (lengthF _something))
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```
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If we kept going with this process infinitely, we'd eventually have what we need:
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{{< latex >}}
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\text{length} = \text{lengthF}(\text{lengthF}(\text{lengthF}(...)))
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{{< /latex >}}
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But hey, the stuff inside the first set of parentheses is still an infinite sequence of applications
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of the function $\\text{lengthF}$, and we have just defined this to be $\\text{length}$. Thus,
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we can rewrite the above equation as:
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{{< latex >}}
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\text{length} = \text{lengthF}(\text{length})
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{{< /latex >}}
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What we have just discovered is that the actual function that we want, `length`, is a [fixed point](https://mathworld.wolfram.com/FixedPoint.html)
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of the non-recursive function `lengthF`. Fortunately, Haskell comes with a function that can find
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such a fixed point. It's defined like this:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 16 16 >}}
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This definition is as declarative as can be; `fix` returns the \\(x\\) such that \\(x = f(x)\\). With
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this, we finally write:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 18 18 >}}
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Loading up the file in GHCi, and running the above function, we get exactly the expected results.
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```
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ghci> Main.length' [1,2,3]
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3
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```
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You may be dissatisfied with the way we handled `fix` here; we went through and pretended that we didn't
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have recursive function definitions, but then used a recursive `let`-expression in the body `fix`!
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This is a valid criticism, so I'd like to briefly talk about how `fix` is used in the context of the
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lambda calculus.
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In the untyped typed lambda calculus, we can just define a term that behaves like `fix` does. The
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most common definition is the \\(Y\\) combinator, defined as follows:
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{{< latex >}}
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Y = \lambda f. (\lambda x. f (x x)) (\lambda x. f (x x ))
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{{< /latex >}}
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When applied to a function, this combinator goes through the following evaluation steps:
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{{< latex >}}
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Y f = f (Y f) = f (f (Y f)) =\ ...
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{{< /latex >}}
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This is the exact sort of infinite series of function applications that we saw above with \\(\\text{lengthF}\\).
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### Recursive Data Types
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We have now seen how we can rewrite a recursive function as a fixed point of some non-recursive function.
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Another cool thing we can do, though, is to transform recursive __data types__ in a similar manner!
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Let's start with something pretty simple.
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 20 20 >}}
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Just like we did with functions, we can extract the recursive occurrences of `MyList` into a parameter.
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 21 21 >}}
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Just like `lengthF`, `MyListF` isn't really a list. We can't write a function `sum :: MyListF -> Int`.
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`MyListF` requires _something_ as an argument, and once given that, produces a type of integer lists.
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Once again, let's try feeding it:
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```
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MyListF a
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```
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From the definition, we can clearly see that `a` is where the "rest of the list" is in the original
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`MyList`. So, let's try fill `a` with a list that we can get out of `MyListF`:
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```
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MyListF (MyListF a)
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```
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And again:
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```
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MyListF (MyListF (MyListF a))
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```
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Much like we used a `fix` function to turn our `lengthF` into `length`, we need a data type,
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which we'll call `Fix` (and which has been [implemented before](https://hackage.haskell.org/package/data-fix-0.3.2/docs/Data-Fix.html)). Here's the definition:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 23 23 >}}
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Looking past the constructors and accessors, we might write the above in pseudo-Haskell as follows:
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```Haskell
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newtype Fix f = f (Fix f)
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```
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This is just like the lambda calculus \\(Y\\) combinator above! Unfortunately, we _do_ have to
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deal with the cruft induced by the constructors here. Thus, to write down the list `[1,2,3]`
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using `MyListF`, we'd have to produce the following:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 25 26 >}}
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This is actually done in practice when using some approaches to help address the
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[expression problem](https://en.wikipedia.org/wiki/Expression_problem); however,
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it's quite unpleasant to write code in this way, so we'll set it aside.
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Let's go back to our infinite chain of type applications. We've a similar pattern before,
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with \\(\\text{length}\\) and \\(\\text{lengthF}\\). Just like we did then, it seems like
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we might be able to write something like the following:
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{{< latex >}}
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\begin{aligned}
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& \text{MyList} = \text{MyListF}(\text{MyListF}(\text{MyListF}(...))) \\
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\Leftrightarrow\ & \text{MyList} = \text{MyListF}(\text{MyList})
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\end{aligned}
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{{< /latex >}}
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In something like Haskell, though, the above is not quite true. `MyListF` is a
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non-recursive data type, with a different set of constructors to `MyList`; they aren't
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_really_ equal. Instead of equality, though, we use the next-best thing: isomorphism.
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{{< latex >}}
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\text{MyList} \cong \text{MyListF}(\text{MyList})
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{{< /latex >}}
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Two types are isomorphic when there exist a
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{{< sidenote "right" "fix-isomorphic-note" "pair of functions, \(f\) and \(g\)," >}}
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Let's a look at the types of <code>Fix</code> and <code>unFix</code>, by the way.
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Suppose that we <em>did</em> define <code>MyList</code> to be <code>Fix MyListF</code>.
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Let's specialize the <code>f</code> type parameter of <code>Fix</code> to <code>MyListF</code>
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for a moment, and check:<br><br>
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In one direction, <code>Fix :: MyListF MyList -> MyList</code><br>
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And in the other, <code>unFix :: MyList -> MyListF MyList</code><br><br>
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The two mutual inverses \(f\) and \(g\) fall out of the definition of the <code>Fix</code>
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data type! If we didn't have to deal with the constructor cruft, this would be more
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ergonomic than writing our own <code>myIn</code> and <code>myOut</code> functions.
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{{< /sidenote >}}
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that take you from one type to the other (and vice versa), such that applying \\(f\\) after \\(g\\),
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or \\(g\\) after \\(f\\), gets you right back where you started. That is, \\(f\\) and \\(g\\)
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need to be each other's inverses. For our specific case, let's call the two functions `myOut`
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and `myIn` (I'm matching the naming in [this paper](https://maartenfokkinga.github.io/utwente/mmf91m.pdf)). They are not hard to define:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 28 34 >}}
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By the way, when a data type is a fixed point of some other, non-recursive type constructor,
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this second type constructor is called a __base functor__. We can verify that `MyListF` is a functor
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by providing an instance (which is rather straightforward):
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 36 38 >}}
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### Recursive Functions with Base Functors
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One neat thing you can do with a base functor is define recursive functions on the actual data type!
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Let's go back to the very basics. When we write recursive functions, we try to think of it as solving
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a problem, assuming that we are given solutions to the sub-problems that make it up. In the more
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specific case of recursive functions on data types, we think of it as performing a given operation,
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assuming that we know how to perform this operation on the smaller pieces of the data structure.
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Some quick examples:
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1. When writing a `sum` function on a list, we assume we know how to find the sum of
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the list's tail (`sum xs`), and add to it the current element (`x+`). Of course,
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if we're looking at a part of a data structure that's not recursive, we don't need to
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perform any work on its constituent pieces.
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```Haskell
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sum [] = 0
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sum (x:xs) = x + sum xs
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```
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2. When writing a function to invert a binary tree, we assume that we can invert the left and right
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children of a non-leaf node. We might write:
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```Haskell
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invert Leaf = Leaf
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invert (Node l r) = Node (invert r) (invert l)
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```
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What does this have to do with base functors? Well, recall how we arrived at `MyListF` from
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`MyList`: we replaced every occurrence of `MyList` in the definition with a type parameter `a`.
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Let me reiterate: wherever we had a sub-list in our definition, we replaced it with `a`.
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The `a` in `MyListF` marks the locations where we _would_ have to use recursion if we were to
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define a function on `MyList`.
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What if instead of a stand-in for the list type (as it was until now), we use `a` to represent
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the result of the recursive call on that sub-list? To finish computing the sum of the list, then,
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the following would suffice:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 40 42 >}}
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Actually, this is enough to define the whole `sum` function. First things first, let's use `myOut`
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to unpack one level of the `Mylist` type:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 28 28 >}}
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We know that `MyListF` is a functor; we can thus use `fmap sum` to compute the sum of the
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remaining list:
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```Haskell
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fmap mySum :: MyListF MyList -> MyListF Int
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```
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Finally, we can use our `mySumF` to handle the last addition:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 40 40 >}}
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Let's put all of these together:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 44 45 >}}
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Notice, though, that the exact same approach would work for _any_ function
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with type:
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```Haskell
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MyListF a -> a
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```
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We can thus write a generalized version of `mySum` that, instead of using `mySumF`,
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uses some arbitrary function `f` with the aforementioned type:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 47 48 >}}
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Let's use `myCata` to write a few other functions:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 50 60 >}}
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#### It's just a `foldr`!
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When you write a function with the type `MyListF a -> a`, you are actually
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providing two things: a "base case" element of type `a`, for when you match `MyNilF`,
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and a "combining function" with type `Int -> a -> a`, for when you match `MyConsF`.
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We can thus define:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 64 66 >}}
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We could also go in the opposite direction, by writing:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 68 69 >}}
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Hey, what was it that we said about types with two functions between them, which
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are inverses of each other? That's right, `MyListF a -> a` and `(a, Int -> a -> a)`
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are isomorphic. The function `myCata`, and the "traditional" definition of `foldr`
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are equivalent!
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#### Base Functors for All!
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We've been playing with `MyList` for a while now, but it's kind of getting boring:
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it's just a list of integers! Furthermore, we're not _really_ getting anything
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out of this new "generalization" procedure -- `foldr` is part of the standard library,
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and we've just reinvented the wheel.
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But you see, we haven't quite. This is because, while we've only been working with `MyListF`,
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the base functor for `MyList`, our approach works for _any recursive data type_, provided
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an `out` function. Let's define a type class, `Cata`, which pairs a data type `a` with
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its base functor `f`, and specifies how to "unpack" `a`:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 71 72 >}}
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We can now provide a more generic version of our `myCata`, one that works for all types
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with a base functor:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 74 75 >}}
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Clearly, `MyList` and `MyListF` are one instance of this type class:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 77 78 >}}
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We can also write a base functor for Haskell's built-in list type, `[a]`:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 80 84 >}}
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We can use our `cata` function for regular lists to define a generic `sum`:
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{{< codelines "Haskell" "catamorphisms/Cata.hs" 86 89 >}}
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It works perfectly:
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```
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ghci> Main.sum [1,2,3]
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6
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||||
ghci> Main.sum [1,2,3.0]
|
||||
6.0
|
||||
ghci> Main.sum [1,2,3.0,-1]
|
||||
5.0
|
||||
```
|
||||
|
||||
What about binary trees, which served as our second example of a recursive data structure?
|
||||
We can do that, too:
|
||||
|
||||
{{< codelines "Haskell" "catamorphisms/Cata.hs" 91 96 >}}
|
||||
|
||||
Given this, here's an implementation of that `invert` function we mentioned earlier:
|
||||
|
||||
{{< codelines "Haskell" "catamorphisms/Cata.hs" 98 101 >}}
|
||||
|
||||
#### What About `Foldable`?
|
||||
If you've been around the Haskell ecosystem, you may know the `Foldable` type class.
|
||||
Isn't this exactly what we've been working towards here? No, not at all. Take
|
||||
a look at how the documentation describes [`Data.Foldable`](https://hackage.haskell.org/package/base-4.16.1.0/docs/Data-Foldable.html):
|
||||
|
||||
> The Foldable class represents data structures that can be reduced to a summary value one element at a time.
|
||||
|
||||
One at a time, huh? Take a look at the signature of `foldMap`, which is sufficient for
|
||||
an instance of `Foldable`:
|
||||
|
||||
```Haskell
|
||||
foldMap :: Monoid m => (a -> m) -> t a -> m
|
||||
```
|
||||
|
||||
A `Monoid` is just a type with an associative binary operation that has an identity element.
|
||||
Then, `foldMap` simply visits the data structure in order, and applies this binary operation
|
||||
pairwise to each monoid produced via `f`. Alas,
|
||||
this function is not enough to be able to implement something like inverting a binary tree;
|
||||
there are different configurations of binary tree that, when visited in-order, result in the
|
||||
same sequence of elements. For example:
|
||||
|
||||
```
|
||||
ghci> fold (Node "Hello" Leaf (Node ", " Leaf (Node "World!" Leaf Leaf)))
|
||||
"Hello, World!"
|
||||
ghci> fold (Node "Hello" (Node ", " Leaf Leaf) (Node "World!" Leaf Leaf))
|
||||
"Hello, World!"
|
||||
```
|
||||
|
||||
As far as `fold` (which is just `foldMap id`) is concerned, the two trees are equivalent. They
|
||||
are very much not equivalent for the purposes of inversion! Thus, whereas `Foldable` helps
|
||||
us work with list-like data types, the `Cata` type class lets us express _any_ function on
|
||||
a recursive data type similarly to how we'd do it with `foldr` and lists.
|
||||
|
||||
#### Catamorphisms
|
||||
Why is the type class called `Cata`, and the function `cata`? Well, a function that
|
||||
performs a computation by recursively visiting the data structure is called a catamorphism.
|
||||
Indeed, `foldr f b`, for function `f` an "base value" `b` is an example of a list catamorophism.
|
||||
It's a fancy word, and there are some fancier descriptions of what it is, especially when
|
||||
you step into category theory (check out the [Wikipedia entry](https://en.wikipedia.org/wiki/Catamorphism)
|
||||
if you want to know what I mean). However, for our purposes, a catamorphism is just a generalization
|
||||
of `foldr` from lists to any data type!
|
||||
Loading…
Reference in New Issue
Block a user