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Today's Topics:
1. Maybe? (Alexander Chen)
2. Re: Maybe? (Imants Cekusins)
3. Re: Maybe? (Alexander Chen)
4. Re: Maybe? (Velichko Lefterov)
5. Re: Maybe? (Imants Cekusins)
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Message: 1
Date: Wed, 13 May 2020 16:01:54 +0200 (CEST)
From: Alexander Chen <alexan...@chenjia.nl>
To: beginners@haskell.org
Subject: [Haskell-beginners] Maybe?
Message-ID: <2064015795.90167.1589378514...@ichabod.co-bxl>
Content-Type: text/plain; charset="utf-8"
Hi,
safeHead.hs
safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead (x:_) = Just x
prelude> safeHead [23,1,4,2,4,2]
Just 23
Question:
what am i doing wrong?
best,
Alexander
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Message: 2
Date: Wed, 13 May 2020 17:11:47 +0300
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Maybe?
Message-ID:
<cap1qinb7uvx6kkyym8ar8cb95ojmtp_d6q6l-6e3d+vszn3...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
>
> prelude> safeHead [23,1,4,2,4,2]
> Just 23
Looks good.
What did you expect to see?
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Message: 3
Date: Wed, 13 May 2020 16:15:22 +0200 (CEST)
From: Alexander Chen <alexan...@chenjia.nl>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Maybe?
Message-ID: <2080448094.91279.1589379322...@ichabod.co-bxl>
Content-Type: text/plain; charset="utf-8"
I expected
prelude> safeHead [23,1,4,2,4,2]
23
not
Just 23
May 13, 2020 4:11:47 PM CEST Imants Cekusins <ima...@gmail.com> wrote:
prelude> safeHead [23,1,4,2,4,2]
Just 23
Looks good.
What did you expect to see?
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Message: 4
Date: Wed, 13 May 2020 17:17:38 +0300
From: Velichko Lefterov <velichko.lefte...@gmail.com>
To: Alexander Chen <alexan...@chenjia.nl>, The Haskell-Beginners
Mailing List - Discussion of primarily beginner-level topics related
to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Maybe?
Message-ID:
<cajuu68u6pedka7rzsb5iopwpedd9rqfw1tncpnakj3wqvad...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
The function returns Maybe a.
On Wed, May 13, 2020 at 5:16 PM Alexander Chen <alexan...@chenjia.nl> wrote:
> I expected
>
> prelude> safeHead [23,1,4,2,4,2]
> 23
>
> not
> Just 23
>
> May 13, 2020 4:11:47 PM CEST Imants Cekusins <ima...@gmail.com> wrote:
>
> prelude> safeHead [23,1,4,2,4,2]
> Just 23
>
>
> Looks good.
>
> What did you expect to see?
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
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Message: 5
Date: Wed, 13 May 2020 17:32:08 +0300
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Maybe?
Message-ID:
<cap1qinaqiuxidphtn8s-rn5q+pg1zun15avxnwanugirhey...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Maybe a is a sum type.
Show instance for sum types displays the constructor.
For example, if we used another sum type:
Either Int Int,
then
Left 23 /= Right 23
So we'd be interested to see the constructor to tell them apart. 23
wouldn't be sufficiently clear.
On Wed, May 13, 2020 at 5:18 PM Velichko Lefterov <
velichko.lefte...@gmail.com> wrote:
> The function returns Maybe a.
>
> On Wed, May 13, 2020 at 5:16 PM Alexander Chen <alexan...@chenjia.nl>
> wrote:
>
>> I expected
>>
>> prelude> safeHead [23,1,4,2,4,2]
>> 23
>>
>> not
>> Just 23
>>
>> May 13, 2020 4:11:47 PM CEST Imants Cekusins <ima...@gmail.com> wrote:
>>
>> prelude> safeHead [23,1,4,2,4,2]
>> Just 23
>>
>>
>> Looks good.
>>
>> What did you expect to see?
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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