[Haskell-cafe] Monad explanation

Gregg Reynolds dev at mobileink.com
Mon Feb 9 06:24:15 EST 2009

On Mon, Feb 9, 2009 at 4:38 AM, Tony Morris <tmorris at tmorris.net> wrote:

> Hash: SHA1
> 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 :)

My bad, I restate:  a value cannot be both static and dynamic.  Or an object
and a morphism.  Or an element and a function.  Sure, you can treat a
morphism as an object, but only by moving to a higher (or different) level
of abstraction.  That doesn't erase the difference between object and
morphism.  If you do erase that difference you end up with mush.  getChar
/looks/ like an object, but semantically it must be a morphism.  But it
can't be a function, since it is non-deterministic.   So actually the
logical contradiction comes from the nature of the beast.

Another reason it's confusing to newcomers:  it's typed as "IO Char", which
looks like a type constructor.  One would expect getChar to yield a value of
type IO Char, no?  But it delivers a Char instead.  This is way confusing.
So I take "type IO foo" to mean "type foo, after a side effect".  In a sense
"getChar :: IO Char" isn't even a true type signature.

In any case, many thanks to all who have contributed to the thread.  It's
sharpened my thinking revealed weaknesses in my terminology, and I expect
I'll make my inevitable contribution to the infinite Haskell tutorial on the
topic before too long.

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