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Today's Topics:
1. Application of lookup function (Jeon-Young Kang)
2. Re: Application of lookup function (Alex Belanger)
3. Re: More type errors I'm having trouble with (Daniel Trstenjak)
4. Re: More type errors I'm having trouble with (Daniel Trstenjak)
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Message: 1
Date: Thu, 3 Dec 2015 23:46:25 -0500
From: Jeon-Young Kang <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Application of lookup function
Message-ID:
<CALWtiK_pZedoNm0==DrgTMwwMCSc=_NeccH4Nx=mh18zq7b...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi all.
I'd like to apply lookup function (Data.Map) for what I am working on.
Here is my code.
data Person = Person {personId :: Int, name = String}
data People = [Person]
data State = InMembership | NoMemebership
person1 = Person {1 = personId, "James" = name}
person2 = Person {2 = personId, "Tom" = name}
members = People [person1, person2]
class Belonging a where
belonging :: a -> [a] -> Bool -> State
here is the problem...
I don't know how to get to know whether a person is belong to members.
I'd like to find it through a person's name.
Can you suggest any examples??
Sincerely,
Jeon
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Message: 2
Date: Thu, 3 Dec 2015 23:50:51 -0500
From: Alex Belanger <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Application of lookup function
Message-ID:
<CADSky2xaMUavFTBnqXTvZ959yeEgNH=dai3zumagpoefhco...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
The (`elem` members) function will tell you if the person is a member of
members. You can then turn that boolean into your custom State type.
On Dec 3, 2015 11:46 PM, "Jeon-Young Kang" <[email protected]> wrote:
> Hi all.
>
> I'd like to apply lookup function (Data.Map) for what I am working on.
>
> Here is my code.
>
> data Person = Person {personId :: Int, name = String}
> data People = [Person]
>
> data State = InMembership | NoMemebership
>
> person1 = Person {1 = personId, "James" = name}
> person2 = Person {2 = personId, "Tom" = name}
>
> members = People [person1, person2]
>
> class Belonging a where
> belonging :: a -> [a] -> Bool -> State
>
> here is the problem...
> I don't know how to get to know whether a person is belong to members.
> I'd like to find it through a person's name.
>
> Can you suggest any examples??
>
> Sincerely,
> Jeon
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Message: 3
Date: Fri, 4 Dec 2015 09:35:18 +0100
From: Daniel Trstenjak <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] More type errors I'm having trouble
with
Message-ID: <20151204083518.GA1841@octa>
Content-Type: text/plain; charset=us-ascii
Hi Dan,
> io_hash_tuples <- map do_prefix_hash filenames
You're operating inside of the IO monad, so anything on the right hand
side of the `<-` has to have the type `IO`.
If you're looking up the type of `map`, you will see
that it doesn't return the right type.
Most likely you just wanted to create a binding like:
let io_hash_tuples = map do_prefix_hash filenames
> hash_tuples <- sequence io_hash_tuples
`io_hash_tuples` is of type `[(IO String, String)]`, but
`sequence` expects a `[IO a]`.
Looking at your code, it's easier not to put the `IO String` computation
of the hash into a tuple, but first compute all hashes:
hashes <- sequence (map Md5s.prefix_md5 filenames)
And if you want the hash and the filename grouped in a tuple:
zip filenames hashes
Greetings,
Daniel
------------------------------
Message: 4
Date: Fri, 4 Dec 2015 10:16:16 +0100
From: Daniel Trstenjak <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] More type errors I'm having trouble
with
Message-ID: <20151204091616.GA4649@octa>
Content-Type: text/plain; charset=us-ascii
> hashes <- sequence (map Md5s.prefix_md5 filenames)
That could be just written as:
hashes <- mapM Md5s.prefix_md5 filenames
Greetings,
Daniel
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