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Today's Topics:
1. Hutton 2016 ex8.3a (trent shipley)
2. Re: Hutton 2016 ex8.3a (Rishi Rajasekaran)
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Message: 1
Date: Mon, 3 Sep 2018 02:05:32 -0700
From: trent shipley <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] Hutton 2016 ex8.3a
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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.
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Message: 2
Date: Mon, 3 Sep 2018 15:35:44 +0530
From: Rishi Rajasekaran <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Hutton 2016 ex8.3a
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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, <[email protected]> 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
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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