[Haskell-cafe] Re: what is inverse of mzero and return?
ashley at semantic.org
Mon Jan 24 05:21:50 EST 2005
In article <41F4C18C.7030508 at imperial.ac.uk>,
Keean Schupke <k.schupke at imperial.ac.uk> wrote:
> Yes it is, side effects are quite clearly not counted. The value
> of (putStrLn "Hello" >> mzero") is mzero.
I don't believe this represents a good understanding of IO actions as
Haskell values. For instance, 'return ()' and 'putStrLn "Hello"' are the
same type, but are clearly different actions and so are usually
considered to be different values. That the latter prints out text might
be better considered not so much a "side effect" as the actual action
You've introduced the concept of "the value of" an IO action, apparently
as something separated from "side effects". I don't believe you can
properly define this. For instance, what is the "value" of getChar such
that it doesn't involve "side effects"?
Ashley Yakeley, Seattle WA
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