David Roundy wrote:
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.
How can a Haskell type define a data dependency? Haskell functions are
non-strict, so they may choose to ignore arguments, and ignored
arguments are not evaluated at all.
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.
> runIdentity $ do {x <- return 2; undefined; return (x * x) }
4
Tillmann
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