24 Oct
2017
24 Oct
'17
5:32 a.m.
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?