From ndospark320 at gmail.com  Sat Apr 10 22:48:42 2021
From: ndospark320 at gmail.com (Dannyu NDos)
Date: Sun, 11 Apr 2021 07:48:42 +0900
Subject: Generalize indexing function
Message-ID: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>

I noticed that the list indexing function, (!!), is generalizable. I'm
showing some instances:

{-# LANGUAGE MultiParamTypeClasses #-}

import Data.Complex
import Data.Functor.Compose
import Data.Functor.Product
import Data.Functor.Sum
import Data.List.NonEmpty
import Data.Maybe

infix 9 !?
infixl 9 !

class Indexable i a where
    (!?) :: i b -> a -> Maybe b

(!) :: Indexable i a => i b -> a -> b
x ! n = fromJust (x !? n)

instance Indexable [] Int where
    [] !? _ = Nothing
    (x:_) !? 0 = Just x
    (_:xs) !? n
        | n < 0 = Nothing
        | otherwise = xs !? (n-1)

instance Indexable ((->) a) (Identity a) where
    f !? Identity n = Just (f n)

instance Indexable ((,) a) () where
    (_,x) !? _ = Just x

instance Indexable Complex Bool where
    (x :+ _) !? False = Just x
    (_ :+ y) !? True = Just y

instance (Indexable f a, Indexable g b) => Indexable (Compose f g) (a,b)
where
    Compose z !? (m,n) = do
        y <- z !? m
        y !? n

instance (Indexable f a, Indexable g b) => Indexable (Product f g) (Either
a b) where
    Pair x _ !? Left m = x !? m
    Pair _ y !? Right n = y !? n

instance (Indexable f a, Indexable g a) => Indexable (Sum f g) (Identity a)
where
    InL x !? Identity n = x !? n
    InR y !? Identity n = y !? n

instance Indexable NonEmpty Int where
    (x :| xs) !? n = (x : xs) !? n
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From keith.wygant at gmail.com  Sat Apr 10 22:57:21 2021
From: keith.wygant at gmail.com (Keith)
Date: Sat, 10 Apr 2021 22:57:21 +0000
Subject: Generalize indexing function
In-Reply-To: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
Message-ID: <CAC183DF-C1ED-41E7-8956-4966E880C427@gmail.com>

Might be more convenient to use an associated type family. And in practice, using types like '()' and 'Bool' as indexes is not super useful when you've already committed to using 'Int' as the (partial) index type of '[]'.

- Keith

Sent from my phone with K-9 Mail.

On April 10, 2021 10:48:42 PM UTC, Dannyu NDos <ndospark320 at gmail.com> wrote:
>I noticed that the list indexing function, (!!), is generalizable. I'm
>showing some instances:
>
>{-# LANGUAGE MultiParamTypeClasses #-}
>
>import Data.Complex
>import Data.Functor.Compose
>import Data.Functor.Product
>import Data.Functor.Sum
>import Data.List.NonEmpty
>import Data.Maybe
>
>infix 9 !?
>infixl 9 !
>
>class Indexable i a where
>    (!?) :: i b -> a -> Maybe b
>
>(!) :: Indexable i a => i b -> a -> b
>x ! n = fromJust (x !? n)
>
>instance Indexable [] Int where
>    [] !? _ = Nothing
>    (x:_) !? 0 = Just x
>    (_:xs) !? n
>        | n < 0 = Nothing
>        | otherwise = xs !? (n-1)
>
>instance Indexable ((->) a) (Identity a) where
>    f !? Identity n = Just (f n)
>
>instance Indexable ((,) a) () where
>    (_,x) !? _ = Just x
>
>instance Indexable Complex Bool where
>    (x :+ _) !? False = Just x
>    (_ :+ y) !? True = Just y
>
>instance (Indexable f a, Indexable g b) => Indexable (Compose f g) (a,b)
>where
>    Compose z !? (m,n) = do
>        y <- z !? m
>        y !? n
>
>instance (Indexable f a, Indexable g b) => Indexable (Product f g) (Either
>a b) where
>    Pair x _ !? Left m = x !? m
>    Pair _ y !? Right n = y !? n
>
>instance (Indexable f a, Indexable g a) => Indexable (Sum f g) (Identity a)
>where
>    InL x !? Identity n = x !? n
>    InR y !? Identity n = y !? n
>
>instance Indexable NonEmpty Int where
>    (x :| xs) !? n = (x : xs) !? n
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From lemming at henning-thielemann.de  Sun Apr 11 00:14:33 2021
From: lemming at henning-thielemann.de (Henning Thielemann)
Date: Sun, 11 Apr 2021 02:14:33 +0200 (CEST)
Subject: Generalize indexing function
In-Reply-To: <CAC183DF-C1ED-41E7-8956-4966E880C427@gmail.com>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CAC183DF-C1ED-41E7-8956-4966E880C427@gmail.com>
Message-ID: <cd63799-f4ea-ea74-579-84689c9dd83@henning-thielemann.de>


On Sat, 10 Apr 2021, Keith wrote:

> Might be more convenient to use an associated type family.

I have done something similar here:
    http://hackage.haskell.org/package/comfort-array

From carter.schonwald at gmail.com  Sun Apr 11 03:18:12 2021
From: carter.schonwald at gmail.com (Carter Schonwald)
Date: Sat, 10 Apr 2021 23:18:12 -0400
Subject: Generalize indexing function
In-Reply-To: <cd63799-f4ea-ea74-579-84689c9dd83@henning-thielemann.de>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CAC183DF-C1ED-41E7-8956-4966E880C427@gmail.com>
 <cd63799-f4ea-ea74-579-84689c9dd83@henning-thielemann.de>
Message-ID: <CAHYVw0yaZQvJRXEH9Otooqp4AzBOHrW1TscE=Eu-teN=6UaZoQ@mail.gmail.com>

I really like the landscape of indexinbg flavores you've included henning!
i've def thought about similar stuff, and its nice to see it fleshed out :)

On Sat, Apr 10, 2021 at 8:15 PM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Sat, 10 Apr 2021, Keith wrote:
>
> > Might be more convenient to use an associated type family.
>
> I have done something similar here:
>     http://hackage.haskell.org/package/comfort-array
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
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From george at wils.online  Sun Apr 11 06:32:04 2021
From: george at wils.online (George Wilson)
Date: Sun, 11 Apr 2021 16:32:04 +1000
Subject: Generalize indexing function
In-Reply-To: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
Message-ID: <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>

Does this class have any laws? From what I can tell, this is useful only as
an overloading of some identifiers - I don't see what useful functions I
could write in terms of this as an abstraction.

Cheers,
George

On Sun, 11 Apr 2021, 08:49 Dannyu NDos, <ndospark320 at gmail.com> wrote:

> I noticed that the list indexing function, (!!), is generalizable. I'm
> showing some instances:
>
> {-# LANGUAGE MultiParamTypeClasses #-}
>
> import Data.Complex
> import Data.Functor.Compose
> import Data.Functor.Product
> import Data.Functor.Sum
> import Data.List.NonEmpty
> import Data.Maybe
>
> infix 9 !?
> infixl 9 !
>
> class Indexable i a where
>     (!?) :: i b -> a -> Maybe b
>
> (!) :: Indexable i a => i b -> a -> b
> x ! n = fromJust (x !? n)
>
> instance Indexable [] Int where
>     [] !? _ = Nothing
>     (x:_) !? 0 = Just x
>     (_:xs) !? n
>         | n < 0 = Nothing
>         | otherwise = xs !? (n-1)
>
> instance Indexable ((->) a) (Identity a) where
>     f !? Identity n = Just (f n)
>
> instance Indexable ((,) a) () where
>     (_,x) !? _ = Just x
>
> instance Indexable Complex Bool where
>     (x :+ _) !? False = Just x
>     (_ :+ y) !? True = Just y
>
> instance (Indexable f a, Indexable g b) => Indexable (Compose f g) (a,b)
> where
>     Compose z !? (m,n) = do
>         y <- z !? m
>         y !? n
>
> instance (Indexable f a, Indexable g b) => Indexable (Product f g) (Either
> a b) where
>     Pair x _ !? Left m = x !? m
>     Pair _ y !? Right n = y !? n
>
> instance (Indexable f a, Indexable g a) => Indexable (Sum f g) (Identity
> a) where
>     InL x !? Identity n = x !? n
>     InR y !? Identity n = y !? n
>
> instance Indexable NonEmpty Int where
>     (x :| xs) !? n = (x : xs) !? n
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
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From tanuki at gmail.com  Sun Apr 11 07:01:47 2021
From: tanuki at gmail.com (Akhra Gannon)
Date: Sun, 11 Apr 2021 00:01:47 -0700
Subject: Generalize indexing function
In-Reply-To: <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
Message-ID: <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>

It's distributive under IsList.

fromList (xs !! n) == (fromList xs) !! n

On Sat, Apr 10, 2021, 11:33 PM George Wilson <george at wils.online> wrote:

> Does this class have any laws? From what I can tell, this is useful only
> as an overloading of some identifiers - I don't see what useful functions I
> could write in terms of this as an abstraction.
>
> Cheers,
> George
>
> On Sun, 11 Apr 2021, 08:49 Dannyu NDos, <ndospark320 at gmail.com> wrote:
>
>> I noticed that the list indexing function, (!!), is generalizable. I'm
>> showing some instances:
>>
>> {-# LANGUAGE MultiParamTypeClasses #-}
>>
>> import Data.Complex
>> import Data.Functor.Compose
>> import Data.Functor.Product
>> import Data.Functor.Sum
>> import Data.List.NonEmpty
>> import Data.Maybe
>>
>> infix 9 !?
>> infixl 9 !
>>
>> class Indexable i a where
>>     (!?) :: i b -> a -> Maybe b
>>
>> (!) :: Indexable i a => i b -> a -> b
>> x ! n = fromJust (x !? n)
>>
>> instance Indexable [] Int where
>>     [] !? _ = Nothing
>>     (x:_) !? 0 = Just x
>>     (_:xs) !? n
>>         | n < 0 = Nothing
>>         | otherwise = xs !? (n-1)
>>
>> instance Indexable ((->) a) (Identity a) where
>>     f !? Identity n = Just (f n)
>>
>> instance Indexable ((,) a) () where
>>     (_,x) !? _ = Just x
>>
>> instance Indexable Complex Bool where
>>     (x :+ _) !? False = Just x
>>     (_ :+ y) !? True = Just y
>>
>> instance (Indexable f a, Indexable g b) => Indexable (Compose f g) (a,b)
>> where
>>     Compose z !? (m,n) = do
>>         y <- z !? m
>>         y !? n
>>
>> instance (Indexable f a, Indexable g b) => Indexable (Product f g)
>> (Either a b) where
>>     Pair x _ !? Left m = x !? m
>>     Pair _ y !? Right n = y !? n
>>
>> instance (Indexable f a, Indexable g a) => Indexable (Sum f g) (Identity
>> a) where
>>     InL x !? Identity n = x !? n
>>     InR y !? Identity n = y !? n
>>
>> instance Indexable NonEmpty Int where
>>     (x :| xs) !? n = (x : xs) !? n
>>
>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
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From lemming at henning-thielemann.de  Sun Apr 11 10:03:04 2021
From: lemming at henning-thielemann.de (Henning Thielemann)
Date: Sun, 11 Apr 2021 12:03:04 +0200 (CEST)
Subject: Generalize indexing function
In-Reply-To: <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
Message-ID: <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>


On Sun, 11 Apr 2021, Akhra Gannon wrote:

> It's distributive under IsList.
> fromList (xs !! n) == (fromList xs) !! n

fromList (xs !! n) ?

I think, xs!!n is a single element.

From tanuki at gmail.com  Sun Apr 11 10:11:55 2021
From: tanuki at gmail.com (Akhra Gannon)
Date: Sun, 11 Apr 2021 03:11:55 -0700
Subject: Generalize indexing function
In-Reply-To: <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
 <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
Message-ID: <CAJoPsuC43oEj6nhPJCTu86fWeEQcbu8nM=W01ygum8KPv8KGSw@mail.gmail.com>

Yeah, I mis-composed that. Pretty sure there's a formulation around
take/head that works but I'm clearly not in a headspace to find it!

On Sun, Apr 11, 2021, 3:03 AM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Sun, 11 Apr 2021, Akhra Gannon wrote:
>
> > It's distributive under IsList.
> > fromList (xs !! n) == (fromList xs) !! n
>
> fromList (xs !! n) ?
>
> I think, xs!!n is a single element.
>
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From tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk  Sun Apr 11 10:35:53 2021
From: tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk (Tom Ellis)
Date: Sun, 11 Apr 2021 11:35:53 +0100
Subject: Generalize indexing function
In-Reply-To: <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
 <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
Message-ID: <20210411103553.GB649@cloudinit-builder>

On Sun, Apr 11, 2021 at 12:03:04PM +0200, Henning Thielemann wrote:
> On Sun, 11 Apr 2021, Akhra Gannon wrote:
> 
> > It's distributive under IsList.
> > fromList (xs !! n) == (fromList xs) !! n
> 
> fromList (xs !! n) ?
> 
> I think, xs!!n is a single element.

Presumably `xs !! n == fromList xs !! n`

From keith.wygant at gmail.com  Sun Apr 11 15:28:13 2021
From: keith.wygant at gmail.com (Keith)
Date: Sun, 11 Apr 2021 15:28:13 +0000
Subject: Generalize indexing function
In-Reply-To: <20210411103553.GB649@cloudinit-builder>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
 <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
 <20210411103553.GB649@cloudinit-builder>
Message-ID: <F9B41CE1-1E46-44FE-A005-8045A20C9D82@gmail.com>

`xs !! ()  == fromList xs !! ()` seems to be a type error?

-- Keith

Sent from my phone with K-9 Mail.

On April 11, 2021 10:35:53 AM UTC, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>On Sun, Apr 11, 2021 at 12:03:04PM +0200, Henning Thielemann wrote:
>> On Sun, 11 Apr 2021, Akhra Gannon wrote:
>> 
>> > It's distributive under IsList.
>> > fromList (xs !! n) == (fromList xs) !! n
>> 
>> fromList (xs !! n) ?
>> 
>> I think, xs!!n is a single element.
>
>Presumably `xs !! n == fromList xs !! n`
>_______________________________________________
>Libraries mailing list
>Libraries at haskell.org
>http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
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From keith.wygant at gmail.com  Sun Apr 11 15:33:55 2021
From: keith.wygant at gmail.com (Keith)
Date: Sun, 11 Apr 2021 15:33:55 +0000
Subject: Generalize indexing function
In-Reply-To: <20210411103553.GB649@cloudinit-builder>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
 <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
 <20210411103553.GB649@cloudinit-builder>
Message-ID: <8A3348AE-A309-4592-9A1F-DB9265037AD4@gmail.com>

Maybe `xs !! n  ==  fromList xs !! fromEnum n`?

-- Keith
Sent from my phone with K-9 Mail.

On April 11, 2021 10:35:53 AM UTC, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>On Sun, Apr 11, 2021 at 12:03:04PM +0200, Henning Thielemann wrote:
>> On Sun, 11 Apr 2021, Akhra Gannon wrote:
>> 
>> > It's distributive under IsList.
>> > fromList (xs !! n) == (fromList xs) !! n
>> 
>> fromList (xs !! n) ?
>> 
>> I think, xs!!n is a single element.
>
>Presumably `xs !! n == fromList xs !! n`
>_______________________________________________
>Libraries mailing list
>Libraries at haskell.org
>http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
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From keith.wygant at gmail.com  Sun Apr 11 15:41:35 2021
From: keith.wygant at gmail.com (Keith)
Date: Sun, 11 Apr 2021 15:41:35 +0000
Subject: Generalize indexing function
In-Reply-To: <20210411103553.GB649@cloudinit-builder>
References: <CAKM3oZUC4NTydEbe3qw-pH_nc38pXRb3CWZoGGzEiK26PqmErw@mail.gmail.com>
 <CABzzMaxqWyQerwyE7jz372_3PuAnM3KwsGajEdQ-ffXRM0W5AA@mail.gmail.com>
 <CAJoPsuD2dz2PmbBtunRqTMSC7v1n9XeuZ9PZydG6FAnpbxZyJA@mail.gmail.com>
 <91dbe53-ee-2e8a-4528-78ca9342439f@henning-thielemann.de>
 <20210411103553.GB649@cloudinit-builder>
Message-ID: <535D494D-9EA7-44A8-BCE3-AFE213269ED3@gmail.com>

Sorry to spam, just realized that destroys the Map and (->) instances.

-- Keith

Sent from my phone with K-9 Mail.

On April 11, 2021 10:35:53 AM UTC, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>On Sun, Apr 11, 2021 at 12:03:04PM +0200, Henning Thielemann wrote:
>> On Sun, 11 Apr 2021, Akhra Gannon wrote:
>> 
>> > It's distributive under IsList.
>> > fromList (xs !! n) == (fromList xs) !! n
>> 
>> fromList (xs !! n) ?
>> 
>> I think, xs!!n is a single element.
>
>Presumably `xs !! n == fromList xs !! n`
>_______________________________________________
>Libraries mailing list
>Libraries at haskell.org
>http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
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