Re: [Haskell-cafe] Monads

On 12-10-01 05:34 AM, Jon Fairbairn wrote:

"Albert Y. C. Lai" writes:

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On 12-09-30 06:33 PM, Jake McArthur wrote:

...

When discussing monads, at least, a side effect is an effect that is triggered by merely evaluating an expression. A monad is an interface that decouples effects from evaluation.

I don't understand that definition. Or maybe I do subconsciously.

I have

s :: State Int () s = do { x <- get; put (x+1) }

Is there an effect triggered by merely evaluating s?

I have

m :: IO () m = if True then putStrLn "x" else putChar 'y'

Is there an effect triggered by merely evaluating m?

What counts as "evaluate"?

Evaluation! Consider m `seq` 42. m is evaluated, but to no effect.

Sure thing. So s has no side effect, and m has no side effect. Since they have no side effect, there is no effect to be decoupled from evaluation.