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haskellfunctorstate-monad

Alternative way to express StateT Functor instance definition

I'm implemented the Functor instance for StateT in the following manner

import Data.Tuple (swap)

newtype StateT s m a =
    StateT { runStateT :: s -> m (a, s) }

instance Functor m => Functor (StateT s m) where
    fmap f (StateT g) = StateT $ \s -> fmap (fmapSwapTwice f) (g s)
        where fmapSwapTwice f tup = swap $ f <$> swap tup

But I'm not wholly satisfied with my solution, because it requires me to import swap from Data.Tuple and fmap it over the tuple twice in order to apply f to the first element.

To me this seems like a common enough pattern that would have its own abstraction or perhaps there is an alternative way to express the Functor instance that I can't think of which doesn't require swap at all

Solution

  • Take advantage of the Bifunctor instance of tuples. Use Data.Bifunctor.first instead of fmapSwapTwice.