Class-based optics integration for developers who use microlens specifically to avoid transitive dependency bloat: this package depends strictly on base, polytropos-core, and microlens.

Architectural Overview & Rationale

Unlike full lens, which brings in a large dependency graph, microlens provides the core lens combinators in a lightweight package.

In polytropos-microlens: * Name-based field access goes through fieldLens, whose FieldIndex constraint is solved to a literal slot index by polytropos-plugin. * The each instance traverses every field of a homogeneous record (where all fields have the same type, witnessed by AllFieldsAre) in a single pass over the flat SmallArray.

When to Use This Module

• When your project prioritizes fast build times and minimal transitive dependencies, and requires basic lens access to polytropos records.

When Not to Use This Module

• When you need prisms over sum variants or advanced indexed optics; use Polytropos.Optics or Polytropos.Lens instead.

Worked Example

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeApplications #-}
{-# OPTIONS_GHC -fplugin=Polytropos.Plugin #-}

import Lens.Micro ((^.), (%~), (&))
import Polytropos
import Polytropos.Microlens

-- A genuinely heterogeneous record:
type User = Record '["name" ':= String, "score" ':= Int]

user :: User
user = REC { name = "Alice", score = 95 }

-- Accessing a field via heterogeneous fieldLens:
userName :: String
userName = user ^. fieldLens (Field @"name")  -- "Alice"

-- Modifying a field:
user' :: User
user' = user & fieldLens (Field @"score") %~ (+ 5)  -- score = 100