makeFields, polymorphic lens

https://stackoverflow.com/questions/66659693/how-to-use-setter-with-polymorphic-type/66660827#66660827

Module A:

data Xxx a = Xxx {
    _xxxAaa :: Int,
    _xxxBbb :: a
}
makeFields ''Xxx

Module B:

t :: IO Bool
t = do
    let n = "aaa" :: String
    let xs = [Xxx 0 0, Xxx 1 1] :: [Xxx Int]
        ys = [set bbb n $ x | x <- xs]
    pure False

Error is:

• Couldn't match type ‘Int’ with ‘[Char]’
    arising from a functional dependency between:
      constraint ‘HasBbb (Xxx Int) String’ arising from a use of ‘bbb’
      instance ‘HasBbb (Xxx a) a’
        at .........

Pay attention, makeFields generates not a polymorphic lenses, ie, it cannot change the type like with usual record constructors:

let xs = [Xxx 0 0, Xxx 1 1]::[Xxx Int]
    ys = [x {_xxxBbb="aaaa"} | x <- xs]

ie, from Xxx Int to Xxx String. But makeLens generates polymorphic lenses. It can be checked with :i in GHCI:

ghci> :i HasBbb
class HasBbb s a | s -> a where
  bbb :: Lens' s a
  {-# MINIMAL bbb #-}
        -- Defined at A.hs:14:1
instance HasBbb (Xxx a) a -- Defined at A.hs:14:1

we see that makeFields generates class HasBbb. And it looks like Lens' s a is a short variant of Lens ... where type-from and type-to are the same (ie, it does not allow to change the type with a setting operation). But a lens via makeLens:

data Xxx a = Xxx {
    _xxxAaa :: Int,
    _xxxBbb :: a
}
makeFields ''Xxx
makeLenses ''Xxx

Let's check:

ghci> :i xxxBbb
xxxBbb :: Lens (Xxx a1) (Xxx a2) a1 a2  -- Defined at A.hs:15:1
ghci> :t set xxxBbb "aaa" (Xxx 0 0)
set xxxBbb "aaa" (Xxx 0 0) :: Xxx [Char]

We see that xxxBbb is polymorphic and it changes the type Xxx a1 -> Xxx a2 when we set the field of type a1 into a2. Something like this :)

And set xxxBbb "aaa" (Xxx 0 0) works now (and with .~ even).