Re: [Haskell-cafe] Reply-To: Header in Mailinglists (was: About "Fun with type functions" example)

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:

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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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