[L03 State] State monad

[David Tchepak]

Next up I thought I'd have a go at L03 State.hs:

https://github.com/tonymorris/course/blob/master/src/L03/State.hs

There are tests for this: L03.State.Tests.main

Questions, comments and sample solutions welcomed.

Regards,
David

[Riaan Rottier]

I have a refactoring question again.  How do you refactor FilterM from my solution which is: 

filterM _ Nil = reeturn Nil

filterM p (x:|xs) = bind (\b -> (bind (\xs' -> reeturn (if b then x:|xs' else xs')) (filterM p xs))) (p x)

to a solution using fmaap?  

[David Tchepak]

Earlier we implemented fmaap as:

fmaap f = bind (\a -> reeturn (f a))

This is in the same shape as this snippet from `filterM`:

bind (\xs' -> reeturn (if b ...))

So we should be able to replace that bind/return with `fmaap` by using `(if b...)` as `f`.

I tend to think about this as unwrapping the monad value, applying the function, then re-wrapping using `return`. In those cases we can just map the function inside instead of using bind/return explicitly.

Hope this gets you un-stuck. Let me know if you'd like a break down of the steps to get it to `fmaap`.

Cheers,

David

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