Re: [Haskell-cafe] Remember the future

Dan Piponi wrote:

On 8/17/07, Dan Piponi wrote:

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On 8/17/07, Andrew Coppin wrote:

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That sounds completely absurd to me... can anybody explain?

Except...you can switch on ghc's special time travel features...

On reflection I decided my example isn't very convincing. For one thing, I've argued in another thread that monads aren't really about sequencing actions. But I concede that there is an exception: the IO monad. Because the IO monad has observable side effects you can actually see whether or not an action has taken place at a particular time, so it really does have to sequence actions. So now consider the following code:

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import IO import Control.Monad.Fix

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test = mdo z <- return $ x+y print "Hello" x <- readLn y <- readLn return z

Evaluate test and you'll be prompted to enter a pair of numbers. You'll then be rewarded with their sum. But the "Hello" message is printed before the prompt for input so we know that's being executed first. And we can see clearly that the summation is performed before the "Hello" message. So clearly this program is computing its result before receiving the input.

At this point your natural reaction should be to replace 'print "Hello"' with 'print z'...

Surely all this means is that the magical "mdo" keyword makes the compiler arbitrarily reorder the expression...?