Experiments on the construction of functions
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The goal of this experiment is to explore ways to construct functions which are richer than the typical Haskell functions (of type (->)). In particular, functions which are known to be bijective, or injective, or surjective, or total, or partial, etc.

Although an experiment, this repository is also a working Haskell package with an application to invertible printers, in which a printer and parser is defined simultaneously. I expect the program text is hard to read. If you have any questions, please send me an email. I'm happy to help.

The foundation is this type, from Data.Algebraic.Function

data F g h s t = F {
      to :: g s t
    , from :: h t s

If g and h are arrows, then F g h is a category, so we can use it as we would the typical function type (->). Judiciously choosing types for g and h gives rise to many familiar mathematical notions:

-- Total functions always produce a value...
type TotalFunction = F (Kleisli Identity) (EmptyArrow)

-- ... but partial functions do not.
-- Notice how the choice of EmptyArrow means we don't have to give any
-- information for the 'from' part of the definition.
type PartialFunction = F (Kleisli Maybe) (EmptyArrow)

-- Bijections can always be inverted...
type TotalBijection = F (Kleisli Identity) (Kleisli Identity)

-- ... injections can be inverted too, but not everything in the codomain
-- has a preimage.
type TotalInjection = F (Kleisli Identity) (Kleisli Maybe)

-- Surjections always give at least one preimage.
type TotalSurjection = F (Kleisli Identity) (Kleisli NonEmpty)

Functions of different types can be composed in such a way that the maximal amount of information is preserved. For instance, if you compose a total bijection with a total injection, you will get a total injection. This is all supported by a very convenient ordering on the arrows in question:

                    Kleisli Identity
                ^                      ^
               /                        \
              /                          \
             /                            \
          Kleisli Maybe          Kleisli NonEmpty
            ^         ^          ^         ^
            |          \        /          |
            |          Kleisli []          |
            |                              |
            \              ^               /
             \             |              /
              \                          /

In this diagram, there is a line from x to y if and only if there is an arrow homomorphism from y to x. Notice the one in the middle: Kleisli []. This is just a normal function: every image has 0 or more preimages.

Really, we're concerned with the greatest-lower-bounds in this ordering, which are captured by the type family GLB and the related class WitnessGLB, instances of which are used to give:

    :: forall f1 g1 f2 g2 s t u .
       ( Composable f1 f2
       , Composable g1 g2
    => F f2 g2 u t
    -> F f1 g1 s u
    -> F (GLB f2 f1) (GLB g2 g1) s t

(<.>) = fcompose

By using (<.>) as a replacement for the typical categorical composition (.), we allow GHC to infer the types of our functions.

plus :: Int -> F Total Bijection Int Int
plus i = F (arr ((+) i))
           (arr (\x -> x - i))

isPositive :: F Total Surjection Int Bool
isPositive = F (arr (\x -> x > 0))
               (Kleisli $ \b -> if b
                   then (1 :| [2..])
                   else fmap negate (0 :| [1..])

boolNot :: F Total Bijection Bool Bool
boolNot = F (arr not)
            (arr not)

-- GHC infers the type
--   example :: F (Kleisli Identity)
--                (Kleisli NonEmpty)
--                Int
--                Bool
-- meaning we have a total surjection from Int to Bool, as witnessed by
--   image :: Int -> Bool
--   image = runIdentity . runKleisli (to example)
--   preimages :: Bool -> NonEmpty Int
--   preimages = runKleisli (from example)
example = boolNot <.> isPositive <.> plus 5

For a simple example of use, check out CaesarCipher.hs in which the Caesar Cipher is defined, for any group, in such a way that its encode and decode functions are given simultaneously.

Constructing functions

So we can build rich functions using a category-like interface; big deal. But things get interesting when we mix in algebraic datatypes. By giving types for sums and products, as we do in Data.Algebraic.Product and Data.Algebraic.Sum, we can construct rich functions componentwise. That's to say, if you give a product of Fs, then you have an F on a corresponding product or sum:

-- GLBFold takes the greatest-lower-bound of 0 or more things.
productF :: (F f1 g1 s1 t1 :*: ... :*: F fn gn sn tn)
         -> F (GLBFold [f1,..,fn])
              (GLBFold [g1,..,gn])
              (s1 :*: ... :*: sn)
              (t1 :*: ... :*: tn)

sumF :: (F f1 g1 s1 t1 :*: ... :*: F fn gn sn tn)
     -> F (GLBFold [f1,..,fn])
          (GLBFold [g1,..,gn])
          (s1 :+: ... :+: sn)
          (t1 :+: ... :+: tn)

These are not the types as they appear in GHCi, since these functions are implemented via type classes, but that's what they essentially are.

When interested in Fs where the domain and codomain are not equally-sized sums or products, we can turn to these functions:

-- There is a total bijection between any product, and that same product with
-- an () inserted anywhere into it. For instance, using normal Haskell tuple
-- notation, (String, Int) and (String, Int, ()) are clearly isomorphic.
introduceTerm :: Index index
              -> F Total
                   (IntroduceTerm () product index)

-- Dually to introduceTerm, we can replace () with Void, which is the identity
-- for sums, and we obtain a total bijection from any sum to that same sum
-- with a Void thrown in.
introduceSummand :: Index index
                 -> F Total
                      (IntroduceSummand Void sum index)

-- We can also eliminate a () from any product.
-- The type doesn't show it, but this function will only work when
-- the component of the product at index is ().
eliminateTerm :: Index index
              -> F Total
                   (EliminateTerm product index)

-- This is only possible when the summand at index is of type Void.
eliminateSummand :: Index index
                 -> F Total
                      (EliminateSummand sum index)

-- Of course, any product or sum is isomorphic to a permutation of itself.
-- Since every permutation is the composition of 2-cycles, we just give
-- swapTerms and swapSummands.
swapTerms :: Index index1
          -> Index index2
          -> F Total
               (SwapTerms product index1 index2)

swapSummands :: Index index1
             -> Index index2
             -> F Total
                  (SwapSummands sum index1 index2)

The fact that we must reach () or Void in order to shrink a product or sum, respectively, is exactly what we expect. It forces an F from a bigger sum to a smaller one to be partial (because a function from t /= Void to Void must be partial), and an F from a bigger product to a smaller one to not be injective. Similar story for the other direction: to make an F from a smaller product or sum to a bigger one, you start by introducing a () or a Void. You can always have a total function to a bigger sum--just never choose any of the new summands--and this comes from the fact that there is a total function from Void to anything. However, this function won't be a bijection, since the new summands have no preimages. That's supported by the fact that there is no function t -> Void unless t = Void.

See the Tuple example for elementary use of productF and eliminateTerm to create tuple projections, and Sum for a similar example involving 2-place sums.

Invertible printers (parsers included)

Suppose we want to print a datatype. We'll need a function of type t -> String, or some other string-like thing. Suppose we'd also like to be able to parse the printed thing: we need something which is left-inverse to the printer. Lessons learnt from exisiting parser libraries show us that a parser is not String -> t, but String -> (t, String) or something similar, noting that it's important to produce another string after parsing, to represent the remaining, unparsed input. So if we want an invertible printer, we'd better have something more like (t, String) -> String, because when you flip this function around, you get the type of a parser. But now we have trouble composing printers. We have printer2 <.> printer1 only when printer2 :: String -> (u, String). If we make the type symmetrical, things are much cleaner. So, we choose (s, String) -> (t, String) for an invertible printer. Bring this into the world of F, and we'll be working with

type PrinterPaser stream g h s t = F g h (s, stream) (t, stream)

Check out the PrinterParser example to see this in action. It's actually pretty cool: when you define a printer/parser for a sum type of two or more summands, the reversal becomes at most a Surjection (but maybe something lesser, like a Function, in case the summands are not Bijections) meaning that when you parse it, you may get more than 1 result, which is just what we want, because the parsers for the summands may overlap! That's to say, if you construct an ambiguous parser, it will produce all possible preimages under the printer!

-- printparseBool is a printer/parser where a Bool is parsed from short or
-- long form: "T" or "True" for True, "F" or "False" for False.

-- Notice how both possibilities are given!
runKleisli (from printparseBool) ((), "True")
>>> [(True, ""), (True, "rue")]

-- We can sequence the printer/parser by making a product of it and using
-- parserPrinterOfProduct:
-- Notice how there's only one member of the output. The short form of True
-- was eliminated because "rueF" does not parse.
let p = parserPrinterOfProduct (printparseBool .*. printparseBool)
runKleisli (from p) ((), "TrueF")
>>> [((True) x (False)), "")]

-- To drive the point home:
runKleisli (from p) ((), "TrueFalse")
>>> [((True) x (False)), ""), ((True) x (False), "alse")]

-- When printing, it's nice to have a total function, rather than a
-- "multifunction" in which 1 or more outputs are given. That means, if there
-- are many strings which parse to your value (as is the case for True and False
-- here) then you must choose a canonical representation, the one that should
-- be printed always.
-- For demonstration, we choose "True" as the canonical form of True, and "F"
-- as the canonical form of False.
runKleisli (to p) (True .*. False, "")
>>> Identity ((), "TrueF")

-- Had we not chosen canonical representations, we'd instead have:
runKleisli (to p) (True .*. False, "")
>>> [((), "TF"), ((), "TrueF"), ((), "TFalse"), ((), "TrueFalse")]
-- each of which parses to (True .*. False, "")