lens-composition.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

import Control.Arrow (first)
import Data.Generics.Product.Fields (field')
import Lens.Micro (Lens', (^.), (.~), (&))
import GHC.Generics (Generic)

data Sample = Sample
  { sample1 :: Int
  , sample2 :: Int
  } deriving (Generic, Show)

viewThenSet :: Sample -> Lens' Sample Int -> (Int, Sample)
viewThenSet sample lensInt = (sample ^. lensInt, sample & lensInt .~ 10)

main :: IO ()
main = print . viewThenSet (Sample 1 2) $ field' @"sample1"
--           ^ Replacing this (.) with ($) fixes the type error:
{-
> stack exec ghc -- --version
The Glorious Glasgow Haskell Compilation System, version 8.6.4
> stack exec runghc .\lens-composition.hs

lens-composition.hs:27:16: error:
    • Couldn't match type ‘f’ with ‘f0’
      ‘f’ is a rigid type variable bound by
        a type expected by the context:
          Lens' Sample Int
        at lens-composition.hs:27:16-39
      Expected type: ((Int -> f0 Int) -> Sample -> f0 Sample)
                     -> (Int, Sample)
        Actual type: Lens' Sample Int -> (Int, Sample)
    • In the second argument of ‘(.)’, namely
        ‘viewThenSet (Sample 1 2)’
      In the expression: print . viewThenSet (Sample 1 2)
      In the expression:
        print . viewThenSet (Sample 1 2) $ field' @"sample1"
   |
27 | main = print . viewThenSet (Sample 1 2) $ field' @"sample1"
   |                ^^^^^^^^^^^^^^^^^^^^^^^^

lens-composition.hs:27:43: error:
    • Ambiguous type variable ‘f0’ arising from a use of ‘field'’
      prevents the constraint ‘(Functor f0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘f0’ should be.
      These potential instances exist:
        instance Functor (Either a) -- Defined in ‘Data.Either’
        instance Functor IO -- Defined in ‘GHC.Base’
        instance Functor Maybe -- Defined in ‘GHC.Base’
        ...plus two others
        ...plus 57 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the second argument of ‘($)’, namely ‘field' @"sample1"’
      In the expression:
        print . viewThenSet (Sample 1 2) $ field' @"sample1"
      In an equation for ‘main’:
          main = print . viewThenSet (Sample 1 2) $ field' @"sample1"
   |
27 | main = print . viewThenSet (Sample 1 2) $ field' @"sample1"
   |                                           ^^^^^^^^^^^^^^^^^
-}