Better Records was Re: [Haskell-cafe] Type Directed Name Resolution

[Stephen Tetley]

On 12 November 2010 21:48, Jonathan Geddes <geddes.jonathan at gmail.com> wrote:

>
> I cringe to imagine what the equivalent is in current Haskell syntax.
> Anyone want to try it? Not me!

Perhaps not pretty - but it is regular and avoids Template Haskell an
manages for the few times I have records-in-records:

doubleInner3OfA :: A -> A
doubleInner3OfA =
    (\s a -> s { inner1 = f1 a }) <*> inner1
  where
    f1 = (\s a -> s { inner2 = f2 a })  <*> inner2
    f2 = (\s a -> s { inner3 = 2 * a }) <*> inner3


data A = A { inner1 :: B }
  deriving (Eq,Show)

data B = B { inner2 :: C }
  deriving (Eq,Show)

data C = C { inner3 :: Int }
  deriving (Eq,Show)


t1 = A (B (C 1))


demo1 = doubleInner3OfA $ t1