[Haskell-beginners] filtering on a datatype

[Bob Ippolito]

I can see why you’re a bit confused here, the question is poorly written.
You can not use the Prelude filter function to do this, but you do need to
write a function that filters and maps over the data to do this
transformation (onlyDateTimes below is an example type signature for such a
function).

mapMaybe is the sort of function you may to use for this purpose.

Here are some type signatures as a hint for one way to implement it:

dbDateTime :: DatabaseItem -> Maybe UTCTime

onlyDateTimes :: [DatabaseItem] -> [UTCTime]

There are of course other ways to implement onlyDateTimes such as directly
using pattern matching and recursion, or using foldr, concatMap, etc. I
would go with whichever method you’ve learned from the textbook so far.

-bob

On Tue, Jun 2, 2020 at 09:06 Alexander Chen <alexander at chenjia.nl> wrote:

> Hi,
>
> given this:
> import Data.Time
>
> data DatabaseItem = DbString String
>                   | DbNumber Integer
>                   | DbDate   UTCTime
>                   deriving  (Eq, Ord, Show)
>
> theDatabase :: [DatabaseItem]
> theDatabase =
>     [ DbDate (UTCTime
>              (fromGregorian 1911 5 1)
>       (secondsToDiffTime 34250))
>     , DbNumber 9001
>     , DbString "Hello, world!"
>     , DbDate (UTCTime
>              (fromGregorian 1921 5 1)
>              (secondsToDiffTime 34123))
>     ]
>
>
> question from textbook is : write a function that filters for DbDate
> values and returns a list of the UTCTime values inside them.
>
>
> my question could you give me an example of a working function, I don't
> get how i use the filter function on a data type in a list. Hence i am
> kinda stuck.
>
> thanks in advance.
>
> best,
>
>
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