# sum-type-boilerplate: A Haskell sum type library

[![CircleCI](https://circleci.com/gh/jdreaver/sum-type-boilerplate.svg?style=svg)](https://circleci.com/gh/jdreaver/sum-type-boilerplate)

This library allows users to easily construct and manipulate sum types via
Template Haskell It was born out of the author's desire to reduce the
boilerplate associated with lots of large sum types while keeping the type
safety they provide.

## Sum Types

A sum type (also called
a [tagged union](https://en.wikipedia.org/wiki/Tagged_union)) looks like this
in Haskell:

```haskell
data MySum
  = MySumTypeA TypeA
  | MySumTypeB TypeB
  | MySumTypeC TypeC
```

Constructing this when you have a large number of types can be error-prone if
you intend to have a uniform naming scheme, and if you intend to only have each
type present once.

If you have many sum types with overlapping sets of types, then functions to
convert between them can be full of boilerplate.

```haskell
data OtherSum
  = OtherSumTypeA TypeA
  | OtherSumTypeB TypeB

otherSumToMySum :: OtherSum -> MySum
otherSumToMySum (OtherSumTypeA typeA) = MySumTypeA typeA
otherSumToMySum (OtherSumTypeB typeB) = MySumTypeB typeB

mySumToOtherSum :: MySum -> Maybe OtherSum
mySumToOtherSum (MySumTypeA typeA) = Just $ OtherSumTypeA typeA
mySumToOtherSum (MySumTypeB typeB) = Just $ OtherSumTypeB typeB
mySumToOtherSum other = Nothing
```

The boilerplate in examples isn't too bad, but consider writing this when your
sum type has dozens of types in it!

## Where does this library come in?

Using `sum-type-boilerplate`, you can reduce all of the previous code into the
following:

```haskell
constructSumType "MySum" defaultSumTypeOptions [''TypeA, ''TypeB, ''TypeC]
constructSumType "OtherSum" defaultSumTypeOptions [''TypeA, ''TypeB]
sumTypeConverter "otherSumToMySum" ''OtherSum ''MySum
partialSumTypeConverter "mySumToOtherSum" ''MySum ''OtherSum
```

## More features

* This library has an extremely simple implementation. (In fact, it is useful
  as an example if you are learning `template-haskell` for the first time.)
* The only dependency is on `template-haskell`. That means adding it as a
  dependency probably doesn't introduce transitive dependencies.
* The template haskell used here just produces vanilla Haskell data types. No
  crazy type-level magic is going on. That means if you want to ditch this
  library later on, just copy the generated code into your project.

## Motivation

While working on
my [event sourcing library](https://github.com/jdreaver/eventful) I found it is
super common for me to have to construct fairly large sum types of events. In
the system I was working on, each event handler has their own event sum type
that represents what events that handler can listen to. I also had a sum type
that holds all the events in the system for the purpose of serialization and
interacting with the database.

Converting between all of these event types became a huge pain, and I was
writing a ton of boilerplate. I didn't want to lose type safety or
encapsulation by just using the "big sum type" everywhere, but I also didn't
want to keep writing new sum types and conversion functions. Thus, I created
the Template Haskell functions that are now in `sum-type-boilerplate` to ease
the burden of writing this mechanical code.

I thought this might be useful not only for users of `eventful`, but also for
anyone that needs to deal with large sum types.