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Today's Topics:
1. Re: Trouble understanding the type of sequence [Just, Just]
(Mihai Maruseac)
2. Re: Trouble understanding the type of sequence [Just, Just]
(Chas Leichner)
3. Re: Trouble understanding the type of sequence [Just, Just]
(Imants Cekusins)
----------------------------------------------------------------------
Message: 1
Date: Sat, 12 Dec 2015 15:36:18 -0500
From: Mihai Maruseac <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Trouble understanding the type of
sequence [Just, Just]
Message-ID:
<caomsum+uufgp4ywutfu6qkkw7v34nxs+htavfo6hf85rhip...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Hi,
Maybe is a type and Just is one of it's constructors (the other being Nothing):
> data Maybe a = Just a | Nothing
On Sat, Dec 12, 2015 at 3:29 PM, Lim H. <[email protected]> wrote:
> Hi everyone,
>
> Sorry if this email disturbs you. I haven't used a developer's mailing list
> before so I'm not sure if I'm violating any etiquette. If I do, please
> excuse me.
>
> I'm trying to understand the type of sequence [Just, Just]. I can understand
> sequence [Just 1, Just 2] :: Num a => Maybe [a]
>
> because when looking at the type of sequence
>
> sequence :: (Monad m, Traversable t) => t (m a) -> m (t a)
>
> it is clear that this function takes a collection of monadic values and
> return a single monadic value of the collection. Thus, when we call sequence
> [Just 1, Just 2] we should get back a Just of [1,2]. Following that train of
> thoughts, shouldn't sequence [Just, Just] return a single Just?
>
> Here is the corresponding SO question
>
> http://stackoverflow.com/questions/34244574/trouble-understanding-the-type-of-sequence-just-just
>
> Lim
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
Mihai Maruseac (MM)
"If you can't solve a problem, then there's an easier problem you can
solve: find it." -- George Polya
------------------------------
Message: 2
Date: Sat, 12 Dec 2015 12:43:25 -0800
From: Chas Leichner <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Trouble understanding the type of
sequence [Just, Just]
Message-ID:
<CALPPNrzp93RgpHq79CG9Mr345jR23koYfy=ejlydftv58fn...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
The type of just is (a -> Maybe a) so your list has type [a -> Maybe a]
which means the Monad instance that sequence is using isn't Maybe, it's (a
->), the type constructor for function types with its first parameter
partially applied. This means that the type of sequence specialized to this
context uses [] for t, the Traversable and (a ->) for m, the Monad. That is
to say sequence :: [a -> Maybe a] -> (a -> [Maybe a]). (a ->) is one way of
representing the Reader monad so you can treat its a parameter as a context
that computations can run inside. That means that sequence [Just, Just]
takes two functions that construct a Maybe value from the value in the
context and turns it into a function which constructs a list of Maybe
values each one fed from the same context. That is to say that (sequence
[Just, Just] $ 4) == [Just 4, Just 4].
On Saturday, December 12, 2015, Lim H. <[email protected]> wrote:
> Hi everyone,
>
> Sorry if this email disturbs you. I haven't used a developer's mailing
> list before so I'm not sure if I'm violating any etiquette. If I do, please
> excuse me.
>
> I'm trying to understand the type of sequence [Just, Just]. I can
> understand
> sequence [Just 1, Just 2] :: Num a => Maybe [a]
>
> because when looking at the type of sequence
>
> sequence :: (Monad m, Traversable t) => t (m a) -> m (t a)
>
> it is clear that this function takes a collection of monadic values and
> return a single monadic value of the collection. Thus, when we call sequence
> [Just 1, Just 2] we should get back a Just of [1,2]. Following that train
> of thoughts, shouldn't sequence [Just, Just] return a single Just?
> Here is the corresponding SO question
>
>
> http://stackoverflow.com/questions/34244574/trouble-understanding-the-type-of-sequence-just-just
>
> Lim
>
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Message: 3
Date: Sat, 12 Dec 2015 22:07:18 +0100
From: Imants Cekusins <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Trouble understanding the type of
sequence [Just, Just]
Message-ID:
<cap1qinzp1ugcvugak0ixphdwcffbn83g4vrdiodj0orln97...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
> when we call sequence [Just 1, Just 2] we should get back
a Just of [1,2]. Following that train of thoughts, shouldn't sequence
[Just, Just] return a single Just?
What would sequence [Just 1, Nothing] return in this case? Just 1 and
Nothing are of the same type - they must be: they are part of the same list.
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