CODEWITHSUNDEEP
haskellhaskell-lens
RandomB
RandomB

Reputation: 3737

How to use setter with polymorphic type?

How to use lens (setter) in this case?

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 .........

Btw, this trick works with tuples (they are polymorphic too), but not with such type.

Upvotes: 3

Views: 114

Answers (1)

duplode
duplode

Reputation: 34378

The HasBbb class generated by makeFields does not offer a type changing lens:

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

Contrast it with the Xxx-specific lens generated by makeLenses:

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

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]

Upvotes: 4

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