recollections

The splicing module needs these extensions and imports:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DerivingVia #-}

import GHC.Generics (Generically1(..))

Given an enumeration of things like

data Things
  = This
  | That
  | Something
  | Else
  | Entirely
  deriving (Eq, Ord, Show, Enum, Bounded, Generic)

We can derive a record that will behave like a homogenous zip-list of static size:

mkCollection ''Things

-- data Collection a = Collection
--   { this, that, something, else', entirely :: a
--   }
--   deriving stock (Eq, Show, Generic1, Functor, Foldable, Traversable)
--   deriving Applicative via (Generically1 Collection)

-- Fill the record with its indices
mkIndices ''Things

Kmettoverse-lite

-- Distributive
mkDistribute ''Things
-- collect is 

-- Representable
mkIndex ''Things
mkTabulate ''Things

VS

• Vanilla monomorphic records: don't have functor/foldable/traversable/applicative goodies.
• Hand-rolling the functor: be my guest, hand-roll the field enum too, while you're at it.
• Map Things a: All fields are mandatory, all keys are known -> lookups are total and O(1). The optional fields are recoverable, with Collection (Maybe a).
• Things -> a: The collection can be stored and loaded.

Gotchas

All the constructors of the tag type must be nullary — anything else is a compile-time error, since a partial Collection would make index partial too.

The generated names (Collection, indices, distribute, index, tabulate) are fixed, so a tag named e.g. Index will clash with them.