[Haskell-cafe] unsafeInterleaveIO, lazyness and sharing
haskell at colquitt.org
Tue Aug 26 21:14:39 EDT 2008
> Haskell is non-strict but not necessarily lazy. So it's possible
> that an expression is reduced to WHNF although it is not used yet.
> Could this early reduction also happen to outputs of
> unsafeInterleaveIO actions (which might trigger the action too early)?
> While I'd expect those outputs to be evaluated lazily (reduced as
> late as possible), I cannot find anything in the docs that guarantees
"unsafeInterleaveIO allows IO computation to be deferred lazily. When
passed a value of type IO a, the IO will only be performed when the
value of the a is demanded. This is used to implement lazy file reading,
Is this the kind of guarantee you're looking for?
I'd bet against getting any decent durable, portable guarantees for any
unsafe* functions in Haskell; but the above behavioral description may
be strong enough to suit you.
> In addition, I'd like to know whether unsafeInterleaveIO outputs are
> guaranteed to be evaluated at most once so that the interleaved action is
> executed at most once. Again, I suppose that this is the case while I cannot
> find a guarantee for it.
I'd be surprised if an implementation didn't have that behavior.
I'd also be wary of anyone claiming to guarantee it, beyond compiler X
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