On Mon, Aug 13, 2007 at 05:13:01PM -0400, Brandon S. Allbery KF8NH wrote:
On Aug 13, 2007, at 16:29 , Benjamin Franksen wrote:
Let's take the simplest example: Maybe. The effect in question is the premature abortion of a computation (when Nothing is returned). And of course Maybe sequences these effects, that's what you use it for: the _first_ action to be encountered that returns Nothing aborts the computation. Clearly sequencing goes on here.
Clearly it does, but not as a side effect of the *monad*. It's ordinary Haskell data dependencies at work here, not some mystical behavior of a monad.
It's the *effect* of a monad, not the *side* effect. The type of >>= defines this dependency. And when you have a chain of dependencies, that is sometimes referred to as a sequence. True, it's not mystical, but it's still sequenced. Try executing: do { x <- return 2; undefined; return (x*x); } in any monad you like, and you'll find that regardless of the *data* dependencies (the return value of this monadic action is unambiguous), the undefined is evaluated *before* the value 4 is returned. -- David Roundy Department of Physics Oregon State University