Day 11: Plutonian Pebbles
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The IORef is like a mutable box you can stick things in, so
readIORefreturns whatever was last put in it (in this case usingmodifyIORef'). “last” makes sense here because operations are sequenced thanks to the IO monad, so yes: values get carried back up the tree to the caller. There’s alsoSTReffor the ST monad, or I could have used the State monad which (kind of) encapsulates a single ref.