# shapely-data: a library for structuralizing Haskell data types

This was a quick, on-a-whim sort of project that ended up taking me all week, as I learned my way around template haskell. It felt like trying to make a sandwich with your feet for the first time, and was about as traumatizing.

Anyway the result is my library named shapely-data which you can get with the usual…

```
cabal install shapely-data
```

and is up on github here.

The library can be used to convert an algebraic data type into a form that
replaces its constructors with a combination of `(,)`

, `Either`

and `()`

. See
the haddocks for usage information, and implementation details.

This first `0.0`

version is very raw. Please let me know if you might find
this useful or have any ideas/feature-requests. If there is any interest I will
polish this thing sooner rather than later.

## Motivation

I wanted to demonstrate that `Either`

and `(,)`

are haskell’s primitive sum and
product types, and to show that we can more or less represent any regular type
using these in combination with the unit type (standing in for the empty
constructor a.la `Nothing`

).

So the type

```
data Foo a = FooPair a a
| FooInt Int
| FooEmpty
```

can be converted into

```
newtype ShapelyFoo a = ShapelyFoo {
shapelyFoo :: Either (a,a) (Either Int ())
}
```

The deeper motivation is that there are a lot of
neat abstractions
(mostly having some relation to category theory) that require some notion of
sums and products, and I don’t see any better way of representing this notion
in the type system. It seems most straightforward to design abstractions
around `(,)`

and `Either`

.

`Arrow`

gets criticized for (among other things) its explicit use of these
types, but a class parameterized over the sum or product type (as in A. Megacz'
“generalized arrows”) becomes
complicated and the abstraction breaks down with no suitable way to talk about
products and sums in general AFAICT.

So this gets at the problem from the other direction: we reduce arbitrary
types to haskell’s primitive sum and product types, allowing code to be written
against these types, and then “flattening” the primitive representation back
onto a normal type of the same shape. I’m imagining list processing using
the `ArrowChoice`

interface on `Either (a, List a) ()`

for instance.

Ultimately maybe this can get us some of the goodness of Structural Typing for certain applications. It’s also a somewhat lighter-weight approach to generic programming, and might be useful for:

- generic view functions and lenses
- conversions between similarly-structured data, or “canonical representations” of types
- incremental
`Category`

-level modification of data structures, e.g. with`Arrow`

- serializing data types

…but again, I’m not an expert and this is just scratching a personal itch of mine.

## TODOs

Here’s a quick list of things on my mind of things to do and questions to be answered for potential future releases:

- lots of misc. polishing like handling record types, derived instances, etc.
- handle constructor-less (bottom) types:
`data Foo`

- handle recursive types in some intelligent way, so that we can do conversions
between e.g. a custom
`List a`

and`[a]`

- figure out how to think about ordering of sum types: how can we make
`Either Int ()`

equivalent to`Either () Int`

during conversions? Should we? - what about “deep” conversions that also look at a constructor’s arguments?