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Today's Topics:
1. Re: Foldl (Alexander Chen)
2. data type (Alexander Chen)
3. Re: data type (Francesco Ariis)
4. How to change base version easily? (Treee July)
5. Re: How to change base version easily? (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Fri, 29 May 2020 07:22:20 +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] Foldl
Message-ID: <1003915704.58782.1590729740...@ichabod.co-bxl>
Content-Type: text/plain; charset="utf-8"
May 27, 2020 11:33:58 PM CEST Tony Morris <tonymor...@gmail.com> wrote:foldl
does a loop, foldr does constructor replacement
https://www.youtube.com/watch?v=GPwtT31zKRY
On 5/28/20 4:55 AM, Alexander Chen wrote:
> Hi,
>
> I guess it's time to get acquainted with foldr and foldl.
>
> *prelude>xs = [1..5] ++ undefined*
> *prelude> foldr const 0 xs*
> 1
>
> I assume it goes something like this:
>
> ( 1 `const`(2 `const`(3 `const`(4 `const`(5 `const`(undefined `const`
> 0))))))
> (
> 5 `const` undefined)
> (4 `const` 5)
> (3 `const` 4)
> ( 2 `const` 3)
> (1 `const`2)
> = 1
>
> ========================================================================
>
>
> What i don't get is the opposite:
>
> *prelude> foldl const 0 xs*
> error
>
>
> in my mind this should go like this:
> ((((((0`const`1)`const` 2) `const` 3 )`const` 4)`const` 5)
> `const` undefined)
> (0 `const`2)
> (0`const` 3)
> ( 0`const`4)
>
> (0`const` 5)
>
> (0 `const` undefined )
> = 0
>
>
>
> I have been told that the main difference between foldl and foldr is
> that foldl needs to evaluate the whole spline before it continues. And i
> guess that has something to do with it. What I don't understand is WHY
> foldl need to do this and foldr doesn't.
>
>
> thanks in advance!
>
> best,
>
> _______________________________________________
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> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 2
Date: Fri, 29 May 2020 11:55:31 +0200 (CEST)
From: Alexander Chen <alexan...@chenjia.nl>
To: beginners@haskell.org
Subject: [Haskell-beginners] data type
Message-ID: <615928762.72602.1590746131...@ichabod.co-bxl>
Content-Type: text/plain; charset="utf-8"
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))
]
My question:
I want to get the UTCTime.
So, thisFunction :: [DatabaseItem] -> [UTCTime]
I don't need the answer but more the intuition (a link to a youtube etc. is
also possible) how to handle these types of situations. Because i keep trying
to think in terms of key:values (Python,Julia) which doesn't seem the right
paradigm to approach this.
thanks in advance!
best,
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Message: 3
Date: Fri, 29 May 2020 12:03:48 +0200
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] data type
Message-ID: <20200529100348.GA20444@extensa>
Content-Type: text/plain; charset=utf-8
Hello Alexander.
Il 29 maggio 2020 alle 11:55 Alexander Chen ha scritto:
> 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))
> ]
>
> My question:
>
> I want to get the UTCTime.
>
> So, thisFunction :: [DatabaseItem] -> [UTCTime]
This is an excellent first step. A good candidate function is `map`
λ> :t map
map :: (a -> b) -> [a] -> [b]
and if we fill in the arguments we are sure of and leave the rest out,
ghc will — on reload/recompile — tell us what is missing. E.g:
thisFunction :: [DatabaseItem] -> [UTCTime]
thisFunction ds = map _ ds
will lead to
• Found hole: _ :: DatabaseItem -> UTCTime
• In the first argument of ‘map’, namely ‘_’
Can you write a function with `DatabaseItem -> UTCTime` signature?
------------------------------
Message: 4
Date: Fri, 29 May 2020 18:35:18 +0800
From: Treee July <julytr...@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] How to change base version easily?
Message-ID:
<CAGfc3Ot6OfR9YMdGDegvEeRvXiiStLYhYKS=l25medd1do0...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello,
I installed the latest version of ghc, which caused one of my packages does
not satisfy the constraint of the version. I just wondering if there exists
a convenient way to degrade the version of the base. I use Arch Linux,
which is not easy to install an elder version.
Thanks,
July
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Message: 5
Date: Fri, 29 May 2020 12:56:42 +0200
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] How to change base version easily?
Message-ID: <20200529105642.GA15236@extensa>
Content-Type: text/plain; charset=utf-8
Hello July,
Il 29 maggio 2020 alle 18:35 Treee July ha scritto:
> Hello,
> I installed the latest version of ghc, which caused one of my packages does
> not satisfy the constraint of the version. I just wondering if there exists
> a convenient way to degrade the version of the base. I use Arch Linux,
> which is not easy to install an elder version.
if you are using cabal, --allow-newer can ease some of the pain. Failing
that, -w will work — but you need to download and locally install the
appropriate ghc first.
Let us know if that’s enough
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