[Haskell-cafe] Monad explanation
ganesh.sittampalam at credit-suisse.com
Mon Feb 9 06:32:58 EST 2009
> 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
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