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Today's Topics:
1. Why must inflix constructors begin with a colon? (Olumide)
2. Parametrizing [] as an instance of the Functor type class
(Olumide)
3. Re: Parametrizing [] as an instance of the Functor type
class (Alexander Berntsen)
4. Re: Parametrizing [] as an instance of the Functor type
class (Daniel Trstenjak)
5. Re: Why must inflix constructors begin with a colon?
(Francesco Ariis)
6. Re: Why must inflix constructors begin with a colon?
(Norbert Melzer)
----------------------------------------------------------------------
Message: 1
Date: Thu, 31 Dec 2015 12:52:44 +0000
From: Olumide <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Why must inflix constructors begin with a
colon?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
I'm new to Haskell and wondering why this restriction?
For example (from LYH):
infixr 5 :-:
data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord)
Thanks,
- Olumide
------------------------------
Message: 2
Date: Thu, 31 Dec 2015 13:06:18 +0000
From: Olumide <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Parametrizing [] as an instance of the
Functor type class
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
According to LYH, the list is an instance of the Functor type class:
instance Functor [] where
fmap = map
Why does 'f' not appear in this expression?, considering that Functor is
defined as
class Functor f where
fmap :: (a -> b) -> f a -> f b
Overall, I'm a bit confused about the relationship between the type
constructor f and [].
Thanks,
- Olumide
------------------------------
Message: 3
Date: Thu, 31 Dec 2015 14:10:18 +0100
From: Alexander Berntsen <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Parametrizing [] as an instance of
the Functor type class
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252
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On 31/12/15 14:06, Olumide wrote:
> Overall, I'm a bit confused about the relationship between the type
> constructor f and [].
f = []. In other words, [] *is* the type constructor.
In Haskell, [] is both the type constructor for lists *and* the term
level value for an empty list. This is unfortunate. In ghci you can
see this.
? :t []
[] :: [t] -- term level
? :k []
[] :: * -> * -- type level
- --
Alexander
[email protected]
https://secure.plaimi.net/~alexander
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Message: 4
Date: Thu, 31 Dec 2015 14:35:36 +0100
From: Daniel Trstenjak <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Parametrizing [] as an instance of
the Functor type class
Message-ID: <20151231133536.GA4227@octa>
Content-Type: text/plain; charset=us-ascii
On Thu, Dec 31, 2015 at 01:06:18PM +0000, Olumide wrote:
> Why does 'f' not appear in this expression?, considering that Functor is
> defined as
instance Functor [] where
fmap = map
is the eta reduced - search for eta reduction for the details - of a
version like:
instance Functor [] where
fmap g f = map g f
Greetings,
Daniel
------------------------------
Message: 5
Date: Thu, 31 Dec 2015 14:33:06 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Why must inflix constructors begin
with a colon?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Thu, Dec 31, 2015 at 12:52:44PM +0000, Olumide wrote:
> I'm new to Haskell and wondering why this restriction?
>
> For example (from LYH):
>
> infixr 5 :-:
> data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord)
Not a committee member, but there is a similar restriction (dichotomy,
rather) with 'functions' (first char lowercase) and 'constructors'
(first char uppercase), so mirroring it to infix operators/constructors
seems reasonable.
------------------------------
Message: 6
Date: Thu, 31 Dec 2015 13:42:50 +0000
From: Norbert Melzer <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Why must inflix constructors begin
with a colon?
Message-ID:
<CA+bCVsuuYTx6m04LtyYe2azvq=phhta7tparqjpuijjgpej...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I'm not a member of the commitee either, but I do think it is to make the
grammar context free and give the parser an easy way to distinguish.
Because of the design as it is now, already a lexer can decide whether we
have a function or an constructor operator.
Francesco Ariis <[email protected]> schrieb am Do., 31. Dez. 2015 14:35:
> On Thu, Dec 31, 2015 at 12:52:44PM +0000, Olumide wrote:
> > I'm new to Haskell and wondering why this restriction?
> >
> > For example (from LYH):
> >
> > infixr 5 :-:
> > data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord)
>
> Not a committee member, but there is a similar restriction (dichotomy,
> rather) with 'functions' (first char lowercase) and 'constructors'
> (first char uppercase), so mirroring it to infix operators/constructors
> seems reasonable.
> _______________________________________________
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> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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