The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time. On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly wrote:

Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

...

On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote:

...

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly:

[snip]

...

instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n)

[snip]

...

And here is the error:

Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s)

You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

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Hi Bastian, I generally observe the pattern that non-technical mailing lists tend to set this header to help the user, while technical mailing list assume (rightfully, IMHO) that the readers are fine without Reply-To. The reasons against setting that I recall at the moment are: * Mails meant to be send privately but accidentally sent to the list are worse than mails meant to be public but sent privately. * Users who send to the list with a Reply-To header set do not want that header to be lost. * With some clients, the Reply-To-All feature does not always work as expected when there is a Reply-To-Header. Most clients have a Reply-To-List feature (Evolution on Ctrl-L) that works fine, once you get used to it. Greetings, Joachim Am Freitag, den 19.11.2010, 04:55 +0100 schrieb Bastian Erdnüß:

Hi there,

I just put an answer two this in beginners@haskell.org. It was not on purpose to move the topic. It's just that questions I feel I can answer are usually beginner level questions and so I'm not often writing in the cafe itself.

It would make my life a little bit more easy if the mailing lists on haskell.org would add a Reply-To: header automatically to each message containing the address of the mailing list, the message was sent to. Usually that's the place where others would want to sent the answers to, I would suppose.

Is there a reason that that's not the case? Am I missing something? Or am I supposed to install a more cleaver mail client which can do that for me? Is there one? Probably written in Haskell ;-)

Cheers, Bastian

On Nov 19, 2010, at 1:07, Daniel Peebles wrote:

...

The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time.

On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly wrote:

...

Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

...

On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote:

...

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly:

[snip]

...

instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n)

[snip]

...

And here is the error:

Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s)

You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

-- Joachim "nomeata" Breitner mail: mail@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata@debian.org

On Nov 19, 2010, at 13:54, Tillmann Rendel wrote:

Hi Bastian,

Bastian Erdnüß wrote:

...

It would make my life a little bit more easy if the mailing lists on haskell.org would add a Reply-To: header automatically to each message containing the address of the mailing list, the message was sent to. Usually that's the place where others would want to sent the answers to, I would suppose.

The mailing lists on haskell.org set the List-Post: header to the adress of the list, and many mail clients use this information to allow "reply to list" somehow. If your client doesn't, you can manually work around the problem by "reply to all" and possibly deleting the email adress of the original sender.

I see, the Reply-To header is evil.

I use Mozilla Thunderbird 3.1.6, and I have configured it to show a button "reply to list" between "reply to all" and "forward".

I use Apple Mail and at least I have a reply to all button. I just think that pollutes the others mailboxes (esp. yours, right now). I have to check if one can teach my mail client somehow a reply to list button. That with the List-Post header was a valuable information, thanks. Bastian

I personnally use most of the time gmail, so I don't have access to a Reply-To-List feature (or do I?). I usually do Reply-to-all which I think is as I guess most mailers remove duplicate mails. Am I right? Arnaud On Fri, Nov 19, 2010 at 3:16 PM, Maciej Piechotka wrote:

On Fri, 2010-11-19 at 04:55 +0100, Bastian Erdnüß wrote:

...

Hi there,

I just put an answer two this in beginners@haskell.org.  It was not on purpose to move the topic.  It's just that questions I feel I can answer are usually beginner level questions and so I'm not often writing in the cafe itself.

It would make my life a little bit more easy if the mailing lists on haskell.org would add a Reply-To: header automatically to each message containing the address of the mailing list, the message was sent to.  Usually that's the place where others would want to sent the answers to, I would suppose.

Is there a reason that that's not the case?  Am I missing something?  Or am I supposed to install a more cleaver mail client which can do that for me?  Is there one?  Probably written in Haskell ;-)

Cheers, Bastian

On

Inserting the Reply-To header is against the RFC (if you like I can find exact quote). Reply-To is a header marking where *author* of message wants to receive replies.

There are many reasons why you want to reply privately - for example you want to say about crossing the rules of netiquette etc., disclose some information not intended for public view (remote code execution bug) etc.

You press "reply to author" button and... oops. it wasn't suppose to go public.

There is a specialized header meant to specify mailing list which should and is be used.

Regards

PS. Probably it varies from ML to ML along with top and bottom posting along with 72-character limit in line.

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If the mailing list replaced Reply-To header it would required additional effort for responders instead of just pressing reply-to- all.

If the list were to add a "Reply-To:" header, but only in the case where one was not already present, that would seem to me to be ideal. (None of the internet polemics against Reply-To that I have seen, have considered this modest suggestion.) In the past, I have carefully used the Reply-To header to direct responses to a particular mailing list of many (e.g. when cross- posting an announcement). Yet because there is a culture of "Reply- To: is bad", and most MUAs do not have a "ReplyToList" option, most respondents end up pushing "Reply to all", which ignores my setting of "Reply-To:", and spams more people than necessary. Regards, Malcolm

Hi, Nick Bowler wrote:

There is another header, Mail-Followup-To, which tells MUAs to also drop the To and CC lists.

Interesting. So is it a good idea to use Mail-Followup-To to move a discussion from one list to another? To: haskell@haskell.org CC: haskell-cafe@haskell.org Mail-Followup-To: haskell-cafe@haskell.org Subject: Foo on the iPhone (was: [Haskell] [ANN] Foo released!) Or should I rather swap To and CC? Or leave out haskell@... completely? Tillmann

Reply-to munging has come up many times on this list (and others). See this page for information on why many people do not like Reply-to munging: http://marc.merlins.org/netrants/listreplyto.html - jeremy On Thu, Nov 18, 2010 at 9:55 PM, Bastian Erdnüß wrote:

Hi there,

I just put an answer two this in beginners@haskell.org.  It was not on purpose to move the topic.  It's just that questions I feel I can answer are usually beginner level questions and so I'm not often writing in the cafe itself.

It would make my life a little bit more easy if the mailing lists on haskell.org would add a Reply-To: header automatically to each message containing the address of the mailing list, the message was sent to.  Usually that's the place where others would want to sent the answers to, I would suppose.

Is there a reason that that's not the case?  Am I missing something?  Or am I supposed to install a more cleaver mail client which can do that for me?  Is there one?  Probably written in Haskell ;-)

Cheers, Bastian

On Nov 19, 2010, at 1:07, Daniel Peebles wrote:

...

The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time.

On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly wrote:

...

Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

...

On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote:

...

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly:

[snip]

...

instance (Nat n) => Nat (Succ n) where toInt   _ = 1 + toInt (undefined :: n)

[snip]

...

And here is the error:

Naturals.hs:16:18:   Ambiguous type variable `n' in the constraint:     `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39   Probable fix: add a type signature that fixes these type variable(s)

You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

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A continuation. You can't know, what type your "fromInt n" should be, but you're not going to just leave it anyway, you're gonna do some calculations with it, resulting in something of type r. So, your calculation is gonna be of type (forall n. Nat n => n -> r). So, if you imagine for a moment that "fromInt" is somehow implemented, what you're gonna do with it is something like calculate :: Int -> r calculate i = let n = fromInt i in k n where k is of type (forall n. Nat n => n -> r). Now we capture this pattern in a function: genericCalculate :: Int -> (forall n. Nat n => n -> r) -> r genericCalculate i k = let n = fromInt i in k n Going back to reality, fromInt can't be implemented, but genericCalculate probably can, since it doesn't involve calculating a type. 19.11.2010 10:25, Arnaud Bailly пишет:

Just after hitting the button send, it appeared to me that fromInt was not obvious at all, and probably impossible. Not sure I understand your answer though: What would be the second parameter (forall n . (Nat n) => n -> r) -> r) ?

Thanks Arnaud

On Fri, Nov 19, 2010 at 1:07 AM, Daniel Peebles wrote:

...

The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time.

On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly wrote:

...

Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

...

On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote:

...

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly: [snip]

...

instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n) [snip]

...

And here is the error:

Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s) You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

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The "obvious" way to write the result would be (stealing some syntax made up by ski on #haskell): fromInt :: Int -> (exists n. (Nat n) *> n) fromInt = ... meaning, "at compile time we don't know the type of the result of fromInt, because it depends on a runtime value, but we'll tell you it's _some_ type (an existential) with a Nat instance". Now, GHC haskell doesn't support existentials like that (you can make a custom data type that wraps them) but you can invert your thinking a bit and ask yourself _who_ can consume such an existential. The only kinds of functions that can deal with an arbitrary type like that (with a Nat instance) are those that are polymorphic in it. So instead of Int -> (exists n. (Nat n) *> n), we flip the existential and make a continuation and say Int -> (forall n. (Nat n) => n -> r) -> r, meaning we take the Int, and a consumer of an arbitrary type-level natural, and return what the consumer returns on our computed type-level natural. We're forced to return exactly the value that the continuation returns because we know nothing at all about the `r` type variable. Hope this helps! Dan On Fri, Nov 19, 2010 at 9:09 AM, Arnaud Bailly wrote:

Thanks a lot for the explanation. Summarizing: You can't calculate an exact type, but you don't care if the type of the continuation is properly set in order to "hide" the exact type of n? Am I right?

Arnaud

...

A continuation.

You can't know, what type your "fromInt n" should be, but you're not going to just leave it anyway, you're gonna do some calculations with it, resulting in something of type r. So, your calculation is gonna be of type (forall n. Nat n => n -> r). So, if you imagine for a moment that "fromInt" is somehow implemented, what you're gonna do with it is something like

calculate :: Int -> r calculate i = let n = fromInt i in k n

where k is of type (forall n. Nat n => n -> r). Now we capture this

On Fri, Nov 19, 2010 at 9:24 AM, Miguel Mitrofanov wrote: pattern

...

in a function:

genericCalculate :: Int -> (forall n. Nat n => n -> r) -> r genericCalculate i k = let n = fromInt i in k n

Going back to reality, fromInt can't be implemented, but genericCalculate probably can, since it doesn't involve calculating a type.

19.11.2010 10:25, Arnaud Bailly пишет:

...

Just after hitting the button send, it appeared to me that fromInt was not obvious at all, and probably impossible. Not sure I understand your answer though: What would be the second parameter (forall n . (Nat n) => n -> r) -> r) ?

Thanks Arnaud

On Fri, Nov 19, 2010 at 1:07 AM, Daniel Peebles wrote:

...

The best you can do with fromInt is something like Int -> (forall n. (Nat n) => n -> r) -> r, since the type isn't known at compile time.

On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly wrote:

...

Thanks a lot, that works perfectly fine! Did not know this one... BTW, I would be interested in the fromInt too.

Arnaud

On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink wrote:

...

On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly wrote: > > Hello, > I am trying to understand and use the Nat n type defined in the > aforementioned article. Unfortunately, the given code does not

compile

...

...

...

...

> properly:

[snip]

> instance (Nat n) => Nat (Succ n) where > toInt _ = 1 + toInt (undefined :: n)

[snip]

> And here is the error: > > Naturals.hs:16:18: > Ambiguous type variable `n' in the constraint: > `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 > Probable fix: add a type signature that fixes these type > variable(s)

You need to turn on the ScopedTypeVariables extension (using {-# LANGUAGE ScopedTypeVariables #-} at the top of your file, or -XScopedTypeVariables at the command line). Otherwise, the 'n' in the class declaration and in the function definition are different, and you want them to be the same 'n'.

Erik

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