oddsFrom3 function

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Re: [Haskell-beginners] oddsFrom3...

Debdut Karmakar

17 Aug 2015 17 Aug '15

2:25 a.m.

I am a haskell beginner and wondering how the following function works (in procedure) : oddsFrom3 :: [Integer] oddsFrom3 = map (+2) oddsFrom3 Thanks for your help.

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Debdut Karmakar

17 Aug 17 Aug

2:32 a.m.

Sorry, I wrote a wrong function, the correct version is: oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 -- A GNU [1] LINUX [2] Patron Links: ------ [1] http://gnu.org [2] http://www.linuxfoundation.org/

akash g

2:35 a.m.

Not a problem. And I should have thought about what you wanted too. This version will give you an infinite list of odd numbers from 3. On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar wrote:

...

Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron

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akash g

2:39 a.m.

Do note that if you force it so that it the value needs the last element, it will be stuck in an infinite loop of request-> response loop. Here's what will happen/ last [3,5 ..] -- Same thing as your function This will be forced to last [3,5, 7 ..] which will be forced till it terminates (which is never). So, be careful with that. However, things like this are perfectly fine. head [3,5 ..] yields 3 takeWhile (<10) [3,5 ..] yields [3,5,7,9] On Mon, Aug 17, 2015 at 12:05 PM, akash g wrote:

...

Not a problem. And I should have thought about what you wanted too.

This version will give you an infinite list of odd numbers from 3.

On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Debdut Karmakar

3:05 a.m.

On 2015-08-17 02:35, akash g wrote:

...

Not a problem. And I should have thought about what you wanted too.

This version will give you an infinite list of odd numbers from 3.

On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A GNU [1] LINUX [2] Patron _______________________________________________

Beginners mailing list

...
Beginners@haskell.org

http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [3]

...

Beginners mailing

_______________________________________________ list

...

Beginners@haskell.org

http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [3] I know that it will evaluate to a list of odd numbers >= 3, but how? Thanks, anyway. -- A GNU [1] LINUX [2] Patron Links: ------ [1] http://gnu.org [2] http://www.linuxfoundation.org/ [3] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Rein Henrichs

3:10 a.m.

It will absolutely work. Lists can be infinite in Haskell and infinite lists are productive: λ> take 5 oddsFrom3 [3,5,7,9,11] On Mon, Aug 17, 2015 at 12:05 AM Debdut Karmakar wrote:

...

On 2015-08-17 02:35, akash g wrote:

Not a problem. And I should have thought about what you wanted too.

This version will give you an infinite list of odd numbers from 3.

On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing listBeginners@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

I know that it will evaluate to a list of odd numbers >= 3, but how?

Thanks, anyway. --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

akash g

4:42 a.m.

Hi Rein, The initial version which the OP posted doesn't have a terminal value. The OP had posted the version that he'd wanted clarification on. On Mon, Aug 17, 2015 at 12:40 PM, Rein Henrichs wrote:

...

It will absolutely work. Lists can be infinite in Haskell and infinite lists are productive:

λ> take 5 oddsFrom3 [3,5,7,9,11]

On Mon, Aug 17, 2015 at 12:05 AM Debdut Karmakar wrote:

...
On 2015-08-17 02:35, akash g wrote:

Not a problem. And I should have thought about what you wanted too.

This version will give you an infinite list of odd numbers from 3.

On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing listBeginners@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

I know that it will evaluate to a list of odd numbers >= 3, but how?

Thanks, anyway. --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

akash g

4:46 a.m.

You can also peruse SICP for knowing about how lazy evaluation works. The explanations are nice. It is in lisp, though. https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-24.html#%_sec_3.5 On Mon, Aug 17, 2015 at 2:12 PM, akash g wrote:

...

Hi Rein,

The initial version which the OP posted doesn't have a terminal value. The OP had posted the version that he'd wanted clarification on.

On Mon, Aug 17, 2015 at 12:40 PM, Rein Henrichs wrote:

...
It will absolutely work. Lists can be infinite in Haskell and infinite lists are productive:

λ> take 5 oddsFrom3 [3,5,7,9,11]

On Mon, Aug 17, 2015 at 12:05 AM Debdut Karmakar wrote:

...
On 2015-08-17 02:35, akash g wrote:

Not a problem. And I should have thought about what you wanted too.

This version will give you an infinite list of odd numbers from 3.

On Mon, Aug 17, 2015 at 12:02 PM, Debdut Karmakar
...
wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing listBeginners@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

I know that it will evaluate to a list of odd numbers >= 3, but how?

Thanks, anyway. --

A* GNU http://gnu.org Linux http://www.linuxfoundation.org/* Patron _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Kim-Ee Yeoh

3:21 a.m.

On Mon, Aug 17, 2015 at 1:32 PM, Debdut Karmakar wrote:

...

Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3

You can get some idea of lambda evaluation here: http://chrisuehlinger.com/LambdaBubblePop/ (Alas it doesn't support let expressions much less let rec.) Once you have a modicum of intuition, you're now ready to appreciate the illustrated step-by-step evaluation of this infinite list: http://stackoverflow.com/a/19749422 Based on the SO answer, you can now work out your function on your own. -- Kim-Ee

Debdut Karmakar

4:59 a.m.

On 2015-08-17 03:21, Kim-Ee Yeoh wrote:

...

On Mon, Aug 17, 2015 at 1:32 PM, Debdut Karmakar wrote:

...
Sorry, I wrote a wrong function, the correct version is:

oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3

You can get some idea of lambda evaluation here:

http://chrisuehlinger.com/LambdaBubblePop/ [2]

...

(Alas it doesn't

support let expressions much less let rec.)

...

Once you have a modicum

of intuition, you're now ready to appreciate the illustrated step-by-step evaluation of this infinite list:

...

http://stackoverflow.com/a/19749422 [3]

...

Based on the SO answer, you

can now work out your function on your own.

...

-- Kim-Ee

...

Beginners mailing

_______________________________________________ list

...

Beginners@haskell.org

http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [1] Thanks, this is what I wanted. Thanks and the problem has been resolved. -- A GNU [4] LINUX [5] Patron Links: ------ [1] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [2] http://chrisuehlinger.com/LambdaBubblePop/ [3] http://stackoverflow.com/a/19749422 [4] http://gnu.org [5] http://www.linuxfoundation.org/

akash g

2:32 a.m.

Well, it won't work. Oh, it will compile and you can run it too (stuck in a infinte loop if you try to force it in any way). Compilation happens because this satisfies the type solver. However, the compiler cannot ensure non-termination of said program. Just unrolling it should show you what's wrong with it. Unroll once: oddsForm3 = map (+2) (map (+2) oddsForm3) Unroll again: oddsForm3 = map (+2) (map (+2) (map (+2) oddsForm3) And you can keep on going. It will never evaluate to a terminal value. On Mon, Aug 17, 2015 at 11:55 AM, Debdut Karmakar wrote:

...

I am a haskell beginner and wondering how the following function works (in procedure) :

oddsFrom3 :: [Integer] oddsFrom3 = map (+2) oddsFrom3

Thanks for your help.

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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akash g
Debdut Karmakar
Kim-Ee Yeoh
Rein Henrichs