[Haskell-cafe] Monad explanation
dev at mobileink.com
Mon Feb 9 07:57:37 EST 2009
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.
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