[Haskell-cafe] Ambiguous type variable with lenses

[Keith]

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 <nadine.and.henry at pobox.com> 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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20220818/cd555850/attachment.html>