[Haskell-beginners] print all days of calendar

[kane96 at gmx.de]

-------- Original-Nachricht --------
> Datum: Mon, 21 Dec 2009 22:00:58 +0100
> Von: Daniel Fischer <daniel.is.fischer at web.de>
> An: kane96 at gmx.de
> CC: beginners at haskell.org
> Betreff: Re: [Haskell-beginners] print all days of calendar

> Am Montag 21 Dezember 2009 21:39:38 schrieb kane96 at gmx.de:
> > -------- Original-Nachricht --------
> >
> > > Datum: Mon, 21 Dec 2009 21:12:48 +0100
> > > Von: Daniel Fischer <daniel.is.fischer at web.de>
> > > An: beginners at haskell.org
> > > CC: kane96 at gmx.de
> > > Betreff: Re: [Haskell-beginners] print all days of calendar
> > >
> > > Am Montag 21 Dezember 2009 20:47:38 schrieb kane96 at gmx.de:
> > > > > Does (==) ring a few bells?
> > > >
> > > > not really...
> > >
> > > Prelude> let okay k = k^3 `mod` 13 == 5 in filter okay [1 .. 30]
> > > [7,8,11,20,21,24]
> > >
> > > Now, you don't want the numbers between 1 and 30 inclusive whose cube
> > > modulo 13 is 5, but
> > > the dates in a given month.
> > >
> > > getMonth :: Month -> Calendar -> Calendar
> > > getMonth month [(1,Jan,y),(2,Jan,y) ... (31,Dec,y)]
> >
> > more like this:
> > getMonth month calendar = calendar filter ([1..31], month, year)
> > but it doesn't make sense
> 
> No.
> type Calendar = [Date]
> type Day = Int
> data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct |
> Nov | Dec 
> deriving (Eq,Enum,Show)
> type Year = Int
> type Date = (Day,Month,Year)
> 
> So the calendar in "getMonth month calendar" is a list of date-triples as
> illustrated 
> above.
> 
> Prelude> :t filter
> filter :: (a -> Bool) -> [a] -> [a]
> 
> So filter takes
> 1) a predicate (a function of type (a -> Bool))
> 2) a list
> as arguments.
> 
> getMonth :: Month -> [Date] -> [Date]
> 
> One of the arguments to getMonth is a list, and the result should be a
> list of the same 
> type, thus it's natural to pass that list unchanged to filter.
> 
> getMonth month calendar = filter predicateThatYouNeed calendar
> 
> What remains is to define predicateThatYouNeed. It will somehow involve
> the other argument 
> to getMonth, namely month. And it must map (Day,Month,Year) triples to
> Bool.

so I have to say that it have to match (Day, month, Year), but how...

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