[Haskell-cafe] Monad explanation

[Gregg Reynolds]

On Mon, Feb 9, 2009 at 5:32 AM, Sittampalam, Ganesh <
ganesh.sittampalam at credit-suisse.com> wrote:

>
> > 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.
>
> It does yield a value of type IO Char, which it also happens that you
> can ask the Haskell runtime to interpret by combining it with other
> IO values using >>= and invoking it from the top-level.
> *When interpreted in this way* it delivers a Char, but that's precisely
> the point at which we move to the different level of abstraction you
> mention above.


Right; "implementation of IO" means also an implementation for >>=, not just
the IO operators.  I hadn't thought about that but it's hugely important for
the exposition of monads and IO.

"The IO Char indicates that getChar, when invoked, performs some action
which returns a character." (Gentle Intro, typical of many expositions.)

That, plus the form of \x -> putChar x used with >>=, plus the fact that one
can do getChar at the ghci command line, plus all the other stuff - it all
adds up to exasperation.

Thanks,

gregg
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