haskell-effectful/effectful

Lazy `State` effect

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MagicRB commented

Is there a reason why the State effect is strict? I would need one which is lazy, due to keeping in there a data structure which is read from disk and is quite slow to read. It is build with unsafeInterleaveIO. I am building a filesystem so State being strict quite hampers the laziness which I need.

arybczak commented

Is there a reason why the State effect is strict?

Yes, it's far too easy to introduce a space leak by accidentally using modify instead of modify' 🙂

I would need one which is lazy

The workaround is quite simple, if you create data Lazy a = Lazy a and then use State (Lazy a) instead of State a, you'll get the desired semantics. How does that sound?

MagicRB commented

That sounds completely doable :) thanks!

MagicRB commented

Oh, does it have to be data? I'd guess so, using newtype would erase that indirection at runtime.

arybczak commented

Yes, has to be data to create the indirection.

MagicRB commented

Thanks, I seem to be still having some strictness issues, but I think that's unrelated to effectful, gotta check if Vec from vector is lazy no it is not