Monthly Downloads: 17
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Data    
Latest version: v0.1.0.3

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Convenient typeclass for defining arbitrary-index enums, which allow for safe derivation of Enum using DerivingVia.

The Problem

In certain cases (e.g. server emulation), we may find ourselves stuck with reimplementing shoddy serverside logic due to how the original game chose to encode its enumerations for de/serialization in the networking and datafiles. These enums tend to be huge (up to 200+ entries), contain unks, and at times have "holes" in the valid indices of the enumeration.

Haskell's Enum typeclass has two major issues that stand in the way of this:

  • Safety

Although Enum is already "dangerous" as-is due to the use of error upon an invalid index lookup, it is even more dangerous in the case of discontinuous enumeration ranges.

  • Redundant Declarations

To implement Enum, one has to write "duplicate" code for both toEnum and fromEnum. This leads to what I call the 2*n declaration problem. If you have 200 items in your enum ADT, you have to write 400 declarations. The best part is, writing updating such large and redundant Enum implementations is highly error-prone and may lead to an unintentional violation of Enum's isomorphicity law (fromEnum . toEnum = id).

The Solution

woe introduces the typeclass IsoEnum and the wrapper newtype WOE (Write-Once Enum) to solve the above problems. The acronym sounds a bit lame and unoriginal, but I digress.

For safety, woe provides toEnumSafely and fromEnumSafely for instances of IsoEnum. These return a Maybe result in the event of a unimplemented enum value or bad index. Additionally, WOE allows one to derive Enum on their type using deriving (Enum) via WOE MyType, thereby avoiding the "sin" of a blanket Enum instance over all IsoEnum instances.

To solve redundant declarations, IsoEnum provides a single method, mapping :: [(Int, a)], that allows you to declare each index-value exactly once. Should you update it, you will only have to update a single pair instead of two. Should you want an (arguably unsafe) Enum instance for your type instead of relying on just to/fromEnumSafely, again, use deriving (Enum) via WOE MyType.


import Data.WOE

data FriendRegistrationResult
  = FriendRegistered
  | NoEmptyFriendSlots
  | RequestRefused
  | RecipientAlreadyAFriend
  | RecipientNotOnline
  | RecipientHasNoEmptyFriendSlots
  | RecipientBlockedFriendRequests
  | GenericError
  deriving (Eq)
  deriving (Enum, Describable)
    via WOE FriendRegistrationResult

Note: Describable is a separate typeclass from my describe package. But this demonstrates an additional benefit of deriving via WOE: avoiding blanket de/serialization typeclass instances over all implementations of Enum or IsoEnum, which can (and will) lead to ambiguities.