Overview

Miso.Lens.Generic derives Lens values for record fields at compile time using GHC.Generics and GHC.Records, without Template Haskell. Fields are addressed by name via OverloadedLabels or the explicit field combinator.

Enable the required extensions:

{-# LANGUAGE OverloadedLabels, DeriveGeneric #-}
import GHC.Generics (Generic)
import Miso.Lens.Generic (HasLens, field)
import Miso.Lens         (Lens, view, set, (.=), (++=))

Quick start

data Counter = Counter { _count :: Int, _label :: MisoString }
  deriving (Generic)

-- Label syntax (requires OverloadedLabels):
countLens :: Lens Counter Int
countLens = #_count

-- Explicit syntax (works without OverloadedLabels):
labelLens :: Lens Counter MisoString
labelLens = field @"_label"

update :: Action -> Effect p props Counter Action
update Increment = #_count += 1
update (SetLabel l) = #_label .= l

How it works

The HasLens instance is resolved via HasField for the getter and a generic traversal (GSet) for the setter. A type-level TotalityCheck produces a descriptive compile error if the field name is absent from (or inconsistent across) the constructors.

Comparison with Template Haskell

Overloaded labels

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

Template Haskell

Miso.Lens.TH — makeLenses / makeClassy

See also

• Miso.Lens — Lens, lens, operators
• Miso.Lens.TH — Template Haskell alternative