On Mon, Feb 9, 2009 at 5:32 AM, Sittampalam, Ganesh < [email protected]> 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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