generic-enumeration: Generically derived enumerations.

[ library, mit, unclassified ] [ Propose Tags ]

This package provides a way to generically obtain every possible value of a type, provided that the generic representation of the type is compatible.


[Skip to Readme]
Versions [RSS] [faq] 0.1.0.0, 0.1.0.1
Dependencies base (>=4.13 && <4.15) [details]
License MIT
Copyright 2020 Owens Murray, LLC.
Author Rick Owens
Maintainer rick@owensmurray.com
Home page https://github.com/owensmurray/generic-enumeration
Uploaded by rickowens at 2021-07-01T20:26:05Z
Distributions NixOS:0.1.0.1
Downloads 115 total (13 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by Bayesian average]
Your Rating
  • λ
  • λ
  • λ
Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2021-07-01 [all 1 reports]

Modules

[Index] [Quick Jump]

Downloads

Maintainer's Corner

For package maintainers and hackage trustees

Candidates


Readme for generic-enumeration-0.1.0.1

[back to package description]

generic-enumeration

This is a library that provides generics-based enumeration of all possible values for a type.

It differs from deriving Enum in that:

  • It supports non-nullary data constructors.
  • It does not try for integer encoding/decoding. It works by providing a list containing every value, and also providing succMay and predMay analogs to Enum's succ and pred
  • It does not allow user-defined instances. Instances either work generically or they don't.

Because of this it is a little less powerful that Enum in some cases, but it is also a lot more safe. There are no partial functions, and disallowing user-defined instances means that not even a human can mess it up.

Use case

I wrote this to solve a particular kind of problem I often have. When I have a type:

data A
  = A1
  | A2
  | A3
  deriving (Enum, Bounded)

And I want to produce a list of all the values:

allAs :: [A]
allAs = [minBound .. maxBound]

This deriving (Enum, Bounded) and [minBound .. maxBound] dance is a little awkward, and it is not always the case Enum or Bounded is meaningful for the type except as an implementation detail for getting the full list of values. Worse, this doesn't even work for types with non-nullary constructors. E.g. this won't compile:

data A
  = A1 B
  | A2 C
  | A3
  deriving (Enum, Bounded)

data B
  = B1
  | B2
  deriving (Enum, Bounded)

data C
  = C1
  | C2
  deriving (Enum, Bounded)

On the other hand, this will compile just fine:

data A
  = A1 B
  | A2 C
  | A3
  deriving (Generic)

data B
  = B1
  | B2
  deriving (Generic)

data C
  = C1
  | C2
  deriving (Generic)

allAs :: [A]
allAs = enumeration
  -- where `enumeration` is a function from this package.
  -- Has the value `[A1 B1, A1 B2, A2 C1, A2 C2, A3]`