haskell traversals over any type parameter
The code shows my approach to be able to map/fold/traverse over any of the data's type parameter. The approach is somehow related to lenses and multiplate but I think it's still quite different. Is there something like this already implemented in some existing generic library?
We have the following artificial data type:
data Foo a b
= Val
| OpAA a a
| OpAB a b
| OpBs [b]
And it can be fmapped/folded/traversed over the data type a or b or both as follows:
foos :: [Foo String Int]
foos = [OpBs [1, 2, 3, 4], Val, OpAB "abc" 5, OpAA "d" "ef"]
main = do
-- putStrLn the Strings and print the Ints
appMTFol MTFol{mtfolA = putStrLn, mtfolB = print} `foldMap` foos
-- map Strings to their lengths
print$ appMTFun mtfun0{mtfunA = length} <$> foos
-- more finegrained control (per data constructor):
-- map Strings to their head or last elements, depending on the operand position
print$
appMFun mfun0
{ mfunOpAB = \a b -> OpAB (head a) b
, mfunOpAA = \a1 a2 -> OpAA (head a1) (last a2)
}
<$> foos
-- negate random Ints
let randomNegate x = getStdRandom (randomR (False, True)) <&> \case True -> -x; _ -> x
print =<< (appMTTra mttra0{mttraB = randomNegate} `traverse` foos)