runState (liftM (+100) pop) [1, 2, 3, 4] (Example from LYH)
Dear List, Apologies for the awkward question title -- its the best I can do ATM. I'm still working my way through chapter 13 of LYH (title for a few monads more) and I came across the following two beasts ghci> runState (liftM (+100) pop) [1,2,3,4] (101,[2,3,4]) ghci> runState (fmap (+100) pop) [1,2,3,4] (101,[2,3,4]) See http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions Even though I'm still struggling to wrap my mind around monads, I sort of understand what's going on here. The problem is that I can't explain why the function (+100) is applied to _only_ the value 1 in (1,[2,3,4]). Regards, - Olumide
On Tue, Oct 24, 2017 at 10:17:46AM +0100, Olumide wrote:
ghci> runState (fmap (+100) pop) [1,2,3,4] (101,[2,3,4])
Even though I'm still struggling to wrap my mind around monads, I sort of understand what's going on here. The problem is that I can't explain why the function (+100) is applied to _only_ the value 1 in (1,[2,3,4]).
Hello Olumide, if we look at the instance of `fmap` for State we'll find (more or less): fmap :: (a -> b) -> State s a -> State s b So fmap modifies the *result*, not the *state* itself. If we also recall that `State s a` is nothing but `\s -> (a, s)` then it is easy to see that only the first element of the tuple (the so called result, `a`) will be modified. Does this clear your doubts?
participants (2)
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Francesco Ariis -
Olumide