[Haskell-beginners] Fish tank monad

[Adrian May]

Thanks! In the mean time I figured out that I need something completely
different though. Actually, they're not fish, they're assorted programmers,
and I need to approach the hiring plan with tasks (as in the Gantt chart
code at http://nuerd.blogspot.com) calling for certain skills and get a
possibly non-contiguous booking. A Timed Integer won't support that though,
unless I step through it day by day which would seem needlessly goofy. It's
at brainstorming stage right now but I think I can probably plough through
it, then I'll probably be back asking for help with boilerplate reduction.

Adrian.
 On 18 May 2013 14:48, "Ertugrul Söylemez" <es at ertes.de> wrote:

> Adrian May <adrian.alexander.may at gmail.com> wrote:
>
> > I want to model a fish tank as a function of time onto how many fish
> > there are in it at that time. If I already have such a function, I
> > want to operate on it with something like "add 2 fish on day 20" or
> > "take 3 away on day 15" to get a new function of the same form, but
> > the latter should not remove more fish than there are in the tank at
> > that time, and it should tell me how many I get. I don't promise to
> > apply these operators in chronological order.
> >
> > This seems like the kind of thing that would be in the prelude
> > somewhere. But where?
>
> Let's see.  You want to model a "number of fish" value:
>
>     Integer
>
> but that value depends on time:
>
>     Time -> Integer
>
> So you have a "what do I get?" and a "how do I get it?".  The former can
> be abstracted away:
>
>     type Timed a = Time -> a
>     type Timed a = (->) Time a
>     type Timed = (->) Time
>
> And yes, Timed is a monad.  You may know it as Reader Time, but Reader
> is just (->) in disguise.  However, you don't need it to be a monad:
>
>     applyAt :: Time -> (a -> a) -> Timed a -> Timed a
>     applyAt tEv f c t
>         | t >= tEv  = f (c t)
>         | otherwise = c t
>
> Then you can add two fish on day 20:
>
>     applyAt 20 (+ 2)
>
> and take 3 away on day 15:
>
>     applyAt 15 (subtract 3)
>
> Have fun. =)
>
>
> Greets,
> Ertugrul
>
> --
> Not to be or to be and (not to be or to be and (not to be or to be and
> (not to be or to be and ... that is the list monad.
>
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