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. Hutton 2016 ex8.3a (trent shipley)
2. Re: Hutton 2016 ex8.3a (Rishi Rajasekaran)
----------------------------------------------------------------------
Message: 1
Date: Mon, 3 Sep 2018 02:05:32 -0700
From: trent shipley <trent.ship...@gmail.com>
To: Haskell Beginners <beginners@haskell.org>
Subject: [Haskell-beginners] Hutton 2016 ex8.3a
Message-ID:
<caeflybk8+h2np6hkjtlcry5juqlh9mdypq_f6v1u5p_+avm...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Given
data Tree a = Leaf a | Node (Tree a) (Tree a)
Write a leaf counter.
Hutton suggests:
leaves :: Tree a -> Int
leaves (Leaf _) = 1
leaves (Node l r) = leaves l + leaves r
I tried:
leavesTrent :: Tree a -> Int
leavesTrent = leaves' 0
where
leaves' n (Leaf a) = n + 1
leaves' n (Node l r) = (leaves' n l), (leaves' n r)
The idea is:
If it is a leaf, add one to the accumulator. (Following Hutton's
explanation of how sum works if defined with foldl.) If it is a tree,
proceed down the left subtree recursively, until you get to a leaf, then
roll up to the right subtree. The problem (among the problems) is that I
don't know how to tell the compiler to do all lefts, before backing up to
go right. I only know how to do that using a real operator like "+" or foo
(l, r).
Is that kind of no-op recursive branching possible?
Trent.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20180903/acad9f16/attachment-0001.html>
------------------------------
Message: 2
Date: Mon, 3 Sep 2018 15:35:44 +0530
From: Rishi Rajasekaran <rajasekaran.ri...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Hutton 2016 ex8.3a
Message-ID:
<CAKBK7aPCaCXR10k=7O=gh0bvz4fywh+dvw+edae8toheox3...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi Trent
For executing your approach, you can do the following:
1. Compute the number of leaves in the left subtree first.
2. Pass that computed value into leaves' call for the right subtree
Regards
Rishi
On Mon, 3 Sep, 2018, 2:36 PM trent shipley, <trent.ship...@gmail.com> wrote:
> Given
> data Tree a = Leaf a | Node (Tree a) (Tree a)
>
> Write a leaf counter.
>
> Hutton suggests:
>
> leaves :: Tree a -> Int
> leaves (Leaf _) = 1
> leaves (Node l r) = leaves l + leaves r
>
> I tried:
>
> leavesTrent :: Tree a -> Int
> leavesTrent = leaves' 0
> where
> leaves' n (Leaf a) = n + 1
> leaves' n (Node l r) = (leaves' n l), (leaves' n r)
>
> The idea is:
>
> If it is a leaf, add one to the accumulator. (Following Hutton's
> explanation of how sum works if defined with foldl.) If it is a tree,
> proceed down the left subtree recursively, until you get to a leaf, then
> roll up to the right subtree. The problem (among the problems) is that I
> don't know how to tell the compiler to do all lefts, before backing up to
> go right. I only know how to do that using a real operator like "+" or foo
> (l, r).
>
> Is that kind of no-op recursive branching possible?
>
> Trent.
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20180903/0e74b1a7/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 123, Issue 1
*****************************************
