microlens

name	microlens
description	Replace lens with microlens-* in Haskell packages

Replace lens with the appropriate microlens-* packages. Work one package at a time, committing each in dependency order (dependencies before dependents). No co-author metadata in commit messages.

Package mapping

Module	microlens package
Lens.Micro	microlens
Lens.Micro.Extras	microlens
Lens.Micro.FieldN	microlens
Lens.Micro.GHC	microlens-ghc
Lens.Micro.Mtl	microlens-mtl
Lens.Micro.TH	microlens-th

Symbol routing

Lens.Micro

(^.), (^?), (.~), (%~), (^..), lens, to, _Just, _Left, _Right, each, Lens', Traversal', Traversal, Getting, ASetter, at, ix, and (&) when imported alongside other Lens.Micro symbols.

Lens.Micro.FieldN (package: microlens)

_1, _2, _3, etc. — tuple field lenses. These are not re-exported by Lens.Micro and must be imported from Lens.Micro.FieldN directly.

Lens.Micro.GHC

No new symbols — add this import (and microlens-ghc to build-depends) when using at or ix with Map, IntMap, Set, Seq, or ByteString, to bring the required instances into scope. Do not use for Vector.

Lens.Micro.Mtl

use, (.=), (%=), (+=), zoom, and view when used monadically (i.e. in a MonadReader or state-transformer context).

Lens.Micro.Extras

view when used purely (non-monadic), preview.

Lens.Micro.TH

makeLenses.

Data.Function

(&) when it is the only symbol being imported — never put a lone (&) import into Lens.Micro.

Import rules

• Explicit import lists are required. Never write import Lens.Micro without a symbol list.
• Follow existing import conventions.
  • If Control.Lens was imported qualified, do the same for Lens.Micro.*.
  • Put the Lens.Micro.* import in the same stanza that Control.Lens was in.
  • If the imports were sorted alphabetically, ensure they remain so.
• Do not replace (^.) with view or vice-versa. Leave operator usage as-is.
• Minimize diff. Do not reorganize imports beyond what the migration requires.

Cabal / package metadata

• Remove lens from build-depends in every .cabal file.
• Add whichever microlens-* packages are actually used.
• If the package previously depended on lens for transitive re-exports, verify no symbols are now missing after the swap.

Workflow

1. Identify all packages that depend on lens, sorted so dependencies come first.
2. For each package: a. Update .cabal build-depends. b. Update every Haskell source file's imports following the rules above. c. Build and fix any remaining type errors. d. Commit with a concise message (no co-author line).