Send Beginners mailing list submissions to
beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
beginners-requ...@haskell.org
You can reach the person managing the list at
beginners-ow...@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: Functions as Applicatives (Olumide)
2. Re: Functions as Applicatives (Tony Morris)
3. Re: Functions as Applicatives (Imants Cekusins)
----------------------------------------------------------------------
Message: 1
Date: Tue, 23 Aug 2016 12:28:09 +0100
From: Olumide <50...@web.de>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Functions as Applicatives
Message-ID: <4aa266c7-5e26-90a8-9422-21c2dee55...@web.de>
Content-Type: text/plain; charset=utf-8; format=flowed
I must be missing something. I thought f accepts just one argument.
- Olumide
On 23/08/2016 00:54, Theodore Lief Gannon wrote:
> Yes, (g x) is the second argument to f. Consider the type signature:
>
> (<*>) :: Applicative f => f (a -> b) -> f a -> f b
>
> In this case, the type of f is ((->) r). Specialized to that type:
>
> (<*>) :: (r -> a -> b) -> (r -> a) -> (r -> b)
> f <*> g = \x -> f x (g x)
>
> Breaking down the pieces...
> f :: r -> a -> b
> g :: r -> a
> x :: r
> (g x) :: a
> (f x (g x)) :: b
>
> The example is made a bit confusing by tossing in an fmap. As far as the
> definition above is concerned, 'f' in the example is ((+) <$> (+3)) and
> that has to be resolved before looking at <*>.
>
>
> On Mon, Aug 22, 2016 at 9:07 AM, Olumide <50...@web.de
> <mailto:50...@web.de>> wrote:
>
> Hi List,
>
> I'm struggling to relate the definition of a function as a function
>
> instance Applicative ((->) r) where
> pure x = (\_ -> x)
> f <*> g = \x -> f x (g x)
>
> with the following expression
>
> ghci> :t (+) <$> (+3) <*> (*100)
> (+) <$> (+3) <*> (*100) :: (Num a) => a -> a
> ghci> (+) <$> (+3) <*> (*100) $ 5
> 508
>
> From chapter 11 of LYH http://goo.gl/7kl2TM .
>
> I understand the explanation in the book: "we're making a function
> that will use + on the results of (+3) and (*100) and return that.
> To demonstrate on a real example, when we did (+) <$> (+3) <*>
> (*100) $ 5, the 5 first got applied to (+3) and (*100), resulting in
> 8 and 500. Then, + gets called with 8 and 500, resulting in 508."
>
> The problem is that I can't relate that explanation with the
> definition of a function as an applicative; especially f <*> g = \x
> -> f x (g x) . Is (g x) the second argument to f?
>
> Regards,
>
> - Olumide
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org <mailto:Beginners@haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
------------------------------
Message: 2
Date: Tue, 23 Aug 2016 21:39:43 +1000
From: Tony Morris <tonymor...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Functions as Applicatives
Message-ID: <63fa5813-f5ee-e9d7-f0f6-28899ec19...@gmail.com>
Content-Type: text/plain; charset="utf-8"
All functions in Haskell always take one argument.
On 23/08/16 21:28, Olumide wrote:
> I must be missing something. I thought f accepts just one argument.
>
> - Olumide
>
> On 23/08/2016 00:54, Theodore Lief Gannon wrote:
>> Yes, (g x) is the second argument to f. Consider the type signature:
>>
>> (<*>) :: Applicative f => f (a -> b) -> f a -> f b
>>
>> In this case, the type of f is ((->) r). Specialized to that type:
>>
>> (<*>) :: (r -> a -> b) -> (r -> a) -> (r -> b)
>> f <*> g = \x -> f x (g x)
>>
>> Breaking down the pieces...
>> f :: r -> a -> b
>> g :: r -> a
>> x :: r
>> (g x) :: a
>> (f x (g x)) :: b
>>
>> The example is made a bit confusing by tossing in an fmap. As far as the
>> definition above is concerned, 'f' in the example is ((+) <$> (+3)) and
>> that has to be resolved before looking at <*>.
>>
>>
>> On Mon, Aug 22, 2016 at 9:07 AM, Olumide <50...@web.de
>> <mailto:50...@web.de>> wrote:
>>
>> Hi List,
>>
>> I'm struggling to relate the definition of a function as a function
>>
>> instance Applicative ((->) r) where
>> pure x = (\_ -> x)
>> f <*> g = \x -> f x (g x)
>>
>> with the following expression
>>
>> ghci> :t (+) <$> (+3) <*> (*100)
>> (+) <$> (+3) <*> (*100) :: (Num a) => a -> a
>> ghci> (+) <$> (+3) <*> (*100) $ 5
>> 508
>>
>> From chapter 11 of LYH http://goo.gl/7kl2TM .
>>
>> I understand the explanation in the book: "we're making a function
>> that will use + on the results of (+3) and (*100) and return that.
>> To demonstrate on a real example, when we did (+) <$> (+3) <*>
>> (*100) $ 5, the 5 first got applied to (+3) and (*100), resulting in
>> 8 and 500. Then, + gets called with 8 and 500, resulting in 508."
>>
>> The problem is that I can't relate that explanation with the
>> definition of a function as an applicative; especially f <*> g = \x
>> -> f x (g x) . Is (g x) the second argument to f?
>>
>> Regards,
>>
>> - Olumide
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org <mailto:Beginners@haskell.org>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>> <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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: OpenPGP digital signature
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20160823/b8fa9ec5/attachment-0001.sig>
------------------------------
Message: 3
Date: Tue, 23 Aug 2016 14:00:01 +0200
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] Functions as Applicatives
Message-ID:
<CAP1qinYd6QCbmO0DONrKV8+TXuTnHQCDJd=ewdd4ynkvh65...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
> I thought f accepts just one argument.
if f is (+) then
f::a -> a -> a
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20160823/7f03ffa3/attachment-0001.html>
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 98, Issue 16
*****************************************
