Overview

Miso.Lens.TH generates Lens definitions via Template Haskell, similar to lens's makeLenses and makeClassy. Fields must be prefixed with _ — the generated lens name is the field name with the underscore dropped.

Enable the extension and splice at the declaration level:

{-# LANGUAGE TemplateHaskell #-}
import Miso.Lens.TH (makeLenses, makeClassy)

makeLenses

Generates a Lens Record Field for each _-prefixed record field:

data Model = Model
  { _count :: Int
  , _text  :: MisoString
  } deriving (Eq)

makeLenses ''Model
-- Generates:
--   count :: Lens Model Int
--   text  :: Lens Model MisoString

update :: Action -> Effect p props Model Action
update Increment   = count += 1
update (SetText t) = text  .= t

makeClassy

Generates a HasFoo typeclass with a self-lens foo :: Lens s Foo and one lens per _-prefixed field. This enables lens composition across record types that embed Foo:

data Foo = Foo { _fooX :: Int, _fooY :: Int }
makeClassy ''Foo
-- Generates:
--   class HasFoo s where
--     foo  :: Lens s Foo
--     fooX :: Lens s Int   -- fooX = foo . lens _fooX ...
--     fooY :: Lens s Int
--   instance HasFoo Foo where foo = this

data Bar = Bar { _barFoo :: Foo, _barZ :: Double }
makeLenses ''Bar
instance HasFoo Bar where foo = barFoo
-- Now barX :: Lens Bar Int  (via foo composition)

Comparison with Generic approach

Template Haskell

Miso.Lens.TH — _ prefix required; explicit splice

Overloaded labels

Miso.Lens.Generic — no TH; derives via Generic

See also

• Miso.Lens — Lens, lens, view, set, and the update operators (.=, +=, %=, …)
• Miso.Lens.Generic — label-based alternative requiring no TH splice