find element of tupels

kane96

17 Dec 2009 17 Dec '09

10:03 a.m.

Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay Date is: 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) Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = not (myDay <= 0) && myDay >= (find ((== myMonth) . fst) monthAndMaxDay) . snd the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day) that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c' against inferred type `Maybe (Month, Day)' -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/atbrowser

Show replies by date

Magnus Therning

17 Dec 17 Dec

10:21 a.m.

On Thu, Dec 17, 2009 at 3:03 PM, kane96 wrote:

...

Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay

Date is: 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)

Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = not (myDay <= 0) && myDay >= (find ((== myMonth) . fst) monthAndMaxDay) . snd

the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day) that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c' against inferred type `Maybe (Month, Day)'

First of all I'd take a look at the function Prelude.lookup, it'll be useful in this case. Using that function I'd do something like this: legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (myDay <= days) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

kane96＠gmx.de

11:18 a.m.

thanks Magnus, great, works so far. I added to check that myDay have to be greater 0: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days)) Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0 monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate? -------- Original-Nachricht --------

...

Datum: Thu, 17 Dec 2009 15:21:13 +0000 Von: Magnus Therning An: kane96 CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] find element of tupels

...

...
Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay

Date is: 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)

Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = not (myDay <= 0) && myDay >= (find ((== myMonth) . fst) monthAndMaxDay) . snd

the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day)

On Thu, Dec 17, 2009 at 3:03 PM, kane96 wrote: that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c'

...
against inferred type `Maybe (Month, Day)'

First of all I'd take a look at the function Prelude.lookup, it'll be useful in this case.

Using that function I'd do something like this:

legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (myDay <= days)

/M

-- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

-- Preisknaller: GMX DSL Flatrate für nur 16,99 Euro/mtl.! http://portal.gmx.net/de/go/dsl02

Henk-Jan van Tuyl

2:44 p.m.

On Thu, 17 Dec 2009 17:18:41 +0100, wrote:

...

thanks Magnus, great, works so far. I added to check that myDay have to be greater 0:

legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days))

Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year

isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0

That is not entirely correct; from package time: isLeapYear :: Integer -> Bool isLeapYear year = (mod year 4 == 0) && ((mod year 400 == 0) || not (mod year 100 == 0))

...

monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate?

Use fromGregorianValid from package time. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html -- --

Daniel Fischer

6:17 p.m.

Am Donnerstag 17 Dezember 2009 20:44:34 schrieb Henk-Jan van Tuyl:

...

On Thu, 17 Dec 2009 17:18:41 +0100, wrote:

...
thanks Magnus, great, works so far. I added to check that myDay have to be greater 0:

legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days))

Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year

isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0

That is not entirely correct; from package time: isLeapYear :: Integer -> Bool isLeapYear year = (mod year 4 == 0) && ((mod year 400 == 0) || not (mod year 100 == 0))

...
monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate?

Use fromGregorianValid from package time.

Or legalDate (d,Feb,y) = 0 < d && d < (if isLeapYear y then 30 else 29) legalDate (d,m,y) | m `elem` [Apr,Jun,Sep,Nov] = 0 < d && d < 31 | otherwise = 0 < d && d < 32

...

Regards, Henk-Jan van Tuyl

-- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

Henk-Jan van Tuyl

18 Dec 18 Dec

6:01 a.m.

On Fri, 18 Dec 2009 00:17:56 +0100, Daniel Fischer wrote:

...

legalDate (d,Feb,y) = 0 < d && d < (if isLeapYear y then 30 else 29)

This should be: legalDate (d, Feb, y) = 0 < d && d < (if isLeapYear y then 29 else 28)

...

legalDate (d,m,y) | m `elem` [Apr,Jun,Sep,Nov] = 0 < d && d < 31 | otherwise = 0 < d && d < 32

Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

Daniel Fischer

7:03 a.m.

Am Freitag 18 Dezember 2009 12:01:53 schrieb Henk-Jan van Tuyl:

...

On Fri, 18 Dec 2009 00:17:56 +0100, Daniel Fischer

wrote:

...
legalDate (d,Feb,y) = 0 < d && d < (if isLeapYear y then 30 else 29)

This should be: legalDate (d, Feb, y) = 0 < d && d < (if isLeapYear y then 29 else 28)

No, I test for strict inequality, hence 30 resp. 29.

...

...
legalDate (d,m,y)

| m `elem` [Apr,Jun,Sep,Nov] = 0 < d && d < 31 | otherwise = 0 < d && d < 32

Regards, Henk-Jan van Tuyl

kane96＠gmx.de

9:57 a.m.

thx Daniel. Now I implemented it like that to use my monthAndMaxDay list and just use the isLeapYear check for February: legalDate :: Date -> Bool legalDate (myDay, Feb, myYear) = 0 < myDay && myDay <= (if isLeapYear myYear then 29 else 28) legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days)) -------- Original-Nachricht --------

...

Datum: Fri, 18 Dec 2009 00:17:56 +0100 Von: Daniel Fischer An: beginners@haskell.org Betreff: Re: [Haskell-beginners] find element of tupels

...

Am Donnerstag 17 Dezember 2009 20:44:34 schrieb Henk-Jan van Tuyl:

...
On Thu, 17 Dec 2009 17:18:41 +0100, wrote:

...
thanks Magnus, great, works so far. I added to check that myDay have to be greater 0:

legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days))

Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year

isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0

That is not entirely correct; from package time: isLeapYear :: Integer -> Bool isLeapYear year = (mod year 4 == 0) && ((mod year 400 == 0) || not (mod year 100 == 0))

...
monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate?

Use fromGregorianValid from package time.

Or

legalDate (d,Feb,y) = 0 < d && d < (if isLeapYear y then 30 else 29) legalDate (d,m,y) | m `elem` [Apr,Jun,Sep,Nov] = 0 < d && d < 31 | otherwise = 0 < d && d < 32

...
Regards, Henk-Jan van Tuyl

-- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

Magnus Therning

2:09 p.m.

On 18/12/09 14:57, kane96@gmx.de wrote:

...

thx Daniel. Now I implemented it like that to use my monthAndMaxDay list and just use the isLeapYear check for February:

legalDate :: Date -> Bool legalDate (myDay, Feb, myYear) = 0 < myDay && myDay <= (if isLeapYear myYear then 29 else 28) legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days))

I know it's a minor detail, but wouldn't it be clearer to change that last line to read something like this: return $ 0 < myDay && myDay <= days ? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

kane96＠gmx.de

9:35 a.m.

-------- Original-Nachricht --------

...

Datum: Thu, 17 Dec 2009 20:44:34 +0100 Von: "Henk-Jan van Tuyl" An: kane96@gmx.de, beginners@haskell.org Betreff: Re: [Haskell-beginners] find element of tupels

...

On Thu, 17 Dec 2009 17:18:41 +0100, wrote:

...
thanks Magnus, great, works so far. I added to check that myDay have to be greater 0:

legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days))

Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year

isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0

That is not entirely correct; from package time: isLeapYear :: Integer -> Bool isLeapYear year = (mod year 4 == 0) && ((mod year 400 == 0) || not (mod

year 100 == 0))

...

...
monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate?

Use fromGregorianValid from package time.

but the problem is I shall implement it on my own in this exercise

...

Regards, Henk-Jan van Tuyl

-- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

--

-- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/chbrowser

Andrew Sackville-West

17 Dec 17 Dec

11:09 a.m.

On Thu, Dec 17, 2009 at 04:03:59PM +0100, kane96 wrote:

...

Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay

Date is: 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)

Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = not (myDay <= 0) && myDay >= (find ((== myMonth) . fst) monthAndMaxDay) . snd

the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day) that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c' against inferred type `Maybe (Month, Day)'

I'm sure the error message goes on to say something like: in the first argument of (.) namely (find ((== myMonth) . fst) monthAndMaxDay) ... you have to interpret this error message a little bit. It is saying the composition operator (.) is expecting a function of type b -> c in it's first argument (or on the left-hand side), but you have provided something of type Maybe (Month, Day). This type is what that (find...) evaluates to. I'm missing the correct terminology here, but you cannot provide something that is fully evaluable on the left-hand side of (.). It needs to be something that evaluates to a function of one argument. That argument will be supplied by the function provided on the right hand side of (.). So, at a minimum, you need to swap your snd and your (find...), but that doesn't solve the problem of getting the tuple out of the Maybe monad that find returns. A

5632

Age (days ago)

5633

Last active (days ago)

List overview

Download

10 comments

6 participants

participants (6)

Andrew Sackville-West
Daniel Fischer
Henk-Jan van Tuyl
kane96
kane96＠gmx.de
Magnus Therning