[Haskell-cafe] Monad explanation

[Lennart Augustsson]

Sorry, I should have come down on the original poster too. ;)
Functions are values, after all.

On Mon, Feb 9, 2009 at 10:38 AM, Tony Morris <tmorris at tmorris.net> wrote:
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> I also agree it is a value.
> The original post was attempting to make a distinction that does not
> exist. I deliberately avoided that topic.
>
> "A thing cannot be both a value and a function, but e,g, getChar"
>
> My original intent was to hope the poster reconsidered the whole post.
> You've blown my cover :)
>
>
> Lennart Augustsson wrote:
>> But an (IO Char) is a value.  You can do all the things with it
>> that you can do with values, e.g., pass it as an argument, stick it
>> in a list, etc.  It is a special kind of value, since if it ever
>> "gets in contact with" the top level it will be executed. But the
>> fact that IO types also behave as values makes Haskell a very
>> powerful imperative language.
>>
>> On Mon, Feb 9, 2009 at 11:14 AM, Tony Morris <tmorris at tmorris.net>
>> wrote: You're right - my statement is inaccurate.
>>
>> Implementation details aside, I am referring specifically to the
>> statement "getChar ... has the type signature of a value". It
>> clearly does not.
>>
>> Lennart Augustsson wrote:
>>>>> Not it doesn't.  getChar has the type signature IO Char. The
>>>>> IO type is abstract.  GHC happens to implement it by a state
>>>>> monad. But in, e.g., hbc it is implemented in a totally
>>>>> different way, more like a continuation monad.
>>>>>
>>>>> Peeking inside an implementation of IO can be illuminating,
>>>>> but one must remember that IO is abstract.
>>>>>
>>>>> -- Lennart
>>>>>
>>>>> On Mon, Feb 9, 2009 at 10:26 AM, Tony Morris
>>>>> <tmorris at tmorris.net> wrote: Gregg Reynolds wrote:
>>>>>>>> The point being that the metalanguage commonly used to
>>>>>>>> describe IO in Haskell contains a logical
>>>>>>>> contradiction.  A thing cannot be both a value and a
>>>>>>>> function, but e,g, getChar behaves like a function and
>>>>>>>> has the type signature of a value.
>>>>> getChar has the signature RealWorld -> (RealWorld, Char)
>>>>>
>> _______________________________________________ Haskell-Cafe
>> mailing list Haskell-Cafe at haskell.org
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>>>
>
> - --
> Tony Morris
> http://tmorris.net/
>
> S, K and I ought to be enough for anybody.
>
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