I haven't tested this out, but have you tried putting a functional dependency on the class(| a -> b)? Could also put each method in its own class. -- Sent from my phone with K-9 Mail. On 18 August 2022 04:37:47 UTC, Henry Laxen wrote:

-- run this with: ghci -package microlens-platform T7.hs {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE FlexibleInstances #-}

module T7 where

import Lens.Micro.Platform

type Lenstype f a b = (b -> f b) -> a -> f a

data A = A { _a1 :: Int , _a2 :: String} deriving (Show) $(makeLenses ''A)

class B a where lensB1 :: (Functor f) => Lenstype f a Int lensB2 :: (Functor f) => Lenstype f a String

instance B A where lensB1 = a1 lensB2 = a2

xa = A 1 "one" ya1 = xa ^. a1 ya2 = xa ^. lensB1

test1 = ya1 == ya2 -- True

-- But I would like to use a type variable for the String field of A

data C b = C { _c1 :: Int , _c2 :: b} deriving (Show) $(makeLenses ''C)

-- So next I define:

class D a b where -- Need MultiParamTypeClasses here lensD1 :: (Functor f) => Lenstype f a Int -- Need AllowAmbiguousTypes here lensD2 :: (Functor f) => Lenstype f a b

instance D (C b) b where -- Need FlexibleInstances here lensD1 = c1 lensD2 = c2

xc1 = C 1 "one" :: C String xc2 = C 1 'o' :: C Char

yc1 = xc1 ^. c1 :: Int yc2 = xc2 ^. c1 :: Int

test2 = yc1 == yc2 -- True

-- All good until here. Now the trouble begins. If you uncomment the next two lines you get:

-- zc1 = xc1 ^. lensD1 :: Int -- zc2 = xc2 ^. lensD1 :: Int

-- T7.hs:62:14-19: error: -- • Ambiguous type variable ‘b1’ arising from a use of ‘lensD1’ -- prevents the constraint ‘(D (C String) b1)’ from being solved. -- Probable fix: use a type annotation to specify what ‘b1’ should be. -- These potential instance exist: -- instance D (C b) b -- -- Defined at /home/henry/haskell/dev/T7.hs:44:10 -- • In the second argument of ‘(^.)’, namely ‘lensD1’ -- In the expression: xc1 ^. lensD1 :: Int -- In an equation for ‘zc1’: zc1 = xc1 ^. lensD1 :: Int -- | -- 62 | zc1 = xc1 ^. lensD1 :: Int -- | ^^^^^^

-- T7.hs:63:14-19: error: -- • Ambiguous type variable ‘b0’ arising from a use of ‘lensD1’ -- prevents the constraint ‘(D (C Char) b0)’ from being solved. -- Probable fix: use a type annotation to specify what ‘b0’ should be. -- These potential instance exist: -- instance D (C b) b -- -- Defined at /home/henry/haskell/dev/T7.hs:44:10 -- • In the second argument of ‘(^.)’, namely ‘lensD1’ -- In the expression: xc2 ^. lensD1 :: Int -- In an equation for ‘zc2’: zc2 = xc2 ^. lensD1 :: Int -- | -- 63 | zc2 = xc2 ^. lensD1 :: Int -- | ^^^^^^

-- But zc1 and zc2 don't care what is in the second slot. I would -- like to store different types in the second slot of C, but the -- first slot of C will always be an Int. Is there any way to -- manipulate the first slot alone? I got some compiler "suggestions" -- concernting RankNTypes and QuantifiedConstraints but nothing I -- tried ended up working.

-- Thanks in advance for your help. -- Henry Laxen _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.