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Today's Topics:
1. Re: Beginners Digest, Vol 122, Issue 6 (Anthony Lee)
2. Re: Bound library questions (David McBride)
3. Empty list and null (trent shipley)
4. Re: Empty list and null (Ut Primum)
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Message: 1
Date: Fri, 17 Aug 2018 09:21:47 -0400
From: Anthony Lee <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Beginners Digest, Vol 122, Issue 6
Message-ID:
<CA+pBo5FSLqbgC=l9bonccb0y+upsgq6jpvgq60oukunk4xn...@mail.gmail.com>
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The source code is here: https://github.com/ekmett/bound/
On Fri, Aug 17, 2018 at 8:41 AM <[email protected]> wrote:
> Send Beginners mailing list submissions to
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> or, via email, send a message with subject or body 'help' to
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> than "Re: Contents of Beginners digest..."
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> Today's Topics:
>
> 1. Bound library questions (Anthony Lee)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 17 Aug 2018 06:46:52 -0400
> From: Anthony Lee <[email protected]>
> To: [email protected]
> Subject: [Haskell-beginners] Bound library questions
> Message-ID:
> <CA+pBo5HVraf_8jbs15bR6QAy1S7=z_u-3KBwT=
> [email protected]>
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>
> In Scope.hs there are some functions I feel difficult to understand,
> Why fmap/foldmap/traverse is applied three times?
>
> instance Functor f => Functor (Scope b f) where
> fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a)
> {-# INLINE fmap #-}
>
> -- | @'toList'@ is provides a list (with duplicates) of the free variables
> instance Foldable f => Foldable (Scope b f) where
> foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a
> {-# INLINE foldMap #-}
>
> instance Traversable f => Traversable (Scope b f) where
> traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a
> {-# INLINE traverse #-}
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Message: 2
Date: Fri, 17 Aug 2018 10:23:53 -0400
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Bound library questions
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The code as it currently stands only has two nested fmaps / foldMaps /
traverses.
The reason he can do that is because Scope is defined as an "f (Var b a)".
Since Scope is only a functor if f is also functor, that means you can fmap
over f, regardless of what it is. But in addition to that Var is also a
functor. So you can fmap over f, and then fmap over the Var inside the f,
which ends up being two nested fmaps. That same condition exists for
foldable and traversable.
On Fri, Aug 17, 2018 at 6:46 AM, Anthony Lee <[email protected]> wrote:
> In Scope.hs there are some functions I feel difficult to understand,
> Why fmap/foldmap/traverse is applied three times?
>
> instance Functor f => Functor (Scope b f) where
> fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a)
> {-# INLINE fmap #-}
>
> -- | @'toList'@ is provides a list (with duplicates) of the free variables
> instance Foldable f => Foldable (Scope b f) where
> foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a
> {-# INLINE foldMap #-}
>
> instance Traversable f => Traversable (Scope b f) where
> traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a
> {-# INLINE traverse #-}
>
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Message: 3
Date: Sat, 18 Aug 2018 02:13:25 -0700
From: trent shipley <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] Empty list and null
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Why does Haskell so often seem to treat [] as a general null.
For example I know 0 : 1 : [] gives [0, 1].
But shouldn't it produce a type fault in a consistent world?
Int:Int:List isn't properly a list. It mixes types.
I expect something like:
Let GSN mean general_scalar_null.
1 : 2 : GSN -- works
1 : 2 : [] -- type fault, cannot mix int and empty list in the same list.
And why does [] == [] : []
instead of [[], []] == [] : []
What sorts of nullity are there in core Haskell?
Trent.
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Message: 4
Date: Sat, 18 Aug 2018 11:34:23 +0200
From: Ut Primum <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Empty list and null
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<CANjDmKKcY_CEcOXeeAsT8=xw0wle_mf5mj7sn_+-yf6wbcg...@mail.gmail.com>
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If you consider the type of the operator *(:)* you have:
Prelude> :t (:)
(:) :: a -> [a] -> [a]
So it takes an element of type a and a list. So if you write 1:[2,3,4] the
type is correct, because you give an integer 1 and a list of integers
[2,3,4]. You will obtain the list of integers [1,2,3,4]. Similarly, writing
1:[] is correct and gives you [1] as result.
Then, if you write
0 : 1 : []
(as in your example), is the same as
0 : (1 : [])
so it means 0 : [1], which is [0,1]. So, the operator (:) is right
associative.
If it was left associative, your example would give an error. Indeed
(0 : 1) : []
is not correct in Haskell.
Furthermore, your final examples are both false:
Prelude> [] == [] : []
False
[[], []] == [] : []
False
The following is True:
Prelude> [[]] == [] : []
True
Indeed if you write [] : [] youy mean you want to build a list whose first
element (head) is [] and whose "tail" (i.e. the rest of the list) is the
empty list.
So, if 1:[] is [1], then []:[] is [[]].
Ut
2018-08-18 11:13 GMT+02:00 trent shipley <[email protected]>:
> Why does Haskell so often seem to treat [] as a general null.
>
> For example I know 0 : 1 : [] gives [0, 1].
>
> But shouldn't it produce a type fault in a consistent world?
>
> Int:Int:List isn't properly a list. It mixes types.
>
> I expect something like:
>
> Let GSN mean general_scalar_null.
>
> 1 : 2 : GSN -- works
>
> 1 : 2 : [] -- type fault, cannot mix int and empty list in the same list.
>
> And why does [] == [] : []
> instead of [[], []] == [] : []
>
> What sorts of nullity are there in core Haskell?
>
> Trent.
>
>
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