[L03 StateT] Questions - SPOILER ALERT

[Riaan Rottier]

I finally got round to StateT and have a few question:

Question 1:

----------------

For Exercise 1 the answer I get is:

  fmaap g (StateT s) = StateT (\s' -> let m = s s' in fmaap (\(a,s'') -> (g a,s'')) m)

and I have no clue how to even relate that to Tony's:

  fmaap f (StateT k) = StateT (fmaap (\(a, t) -> (f a, t)) . k)

Any ideas?

I am in the same situation for Ex2 but I think if I get Ex 1 then I will be able to figure out 2 by myself.

Thanks

Riaan

[Charles O'Farrell]

Hi Riaan,

They're identical, but Tony has just used composition (.) and currying, also known as point-free. Not sure if this helps, but...

fmaap g (StateT s) = StateT (\s' -> let m = s s' in fmaap (\(a,s'') -> (g a,s'')) m)

fmaap g (StateT s) = StateT (\s' -> fmaap (\(a,s'') -> (g a,s'')) (s s'))

fmaap g (StateT s) = StateT (\s' -> (fmaap (\(a,s'') -> (g a,s'')) . s) s') -- (.) :: (b -> c) -> (a -> b) -> a -> c

fmaap g (StateT s) = StateT (fmaap (\(a,s'') -> (g a,s'')) . s) -- curry the last argument

fmaap f (StateT k) = StateT (fmaap (\(a, t) -> (f a, t)) . k) -- different letters, same thing

Does that make sense?

Charles

Riaan

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[Riaan Rottier]

Perfect sense. Thanks

Riaan