Caching result of evaluation in the notebook?

Question

Sometimes I have a cell which takes a while to re-evaluate, but the result is quite small. It would be nice to have an easy way to "cache" some computations by embedding the result in the notebook, and automatically reusing that result on rerun.

IE, instead of

result = expr

I would do

result = memoizedEvaluation[expr]

Any suggestions to how to implement this?

This was motivated by the cell below, result is small, but it takes couple of minutes to evaluate.

graphNamesOrig = 
 Join @@ Table[GraphData[k], {k, 80, 200}];
graphSpectraOrig = 
 N[GraphData[#, "LaplacianSpectrum"]] & /@ graphNamesOrig;

Answer 1

You can check how many PersistentObjects you have already

Length@PersistentObjects["Hashes/Once/*", "Local"] 

Now you can do your slow calculation using Once, here I'm timing using EchoTiming, and the function is RandomReal so very unlikely to repeat the value.

EchoTiming[
    key = Once[
        CompoundExpression[
            Pause[1],
            Hash[RandomReal[9, 10^7]]
        ]
        , "Local"
    ]
]

Now you can see that you have more PersistentObjects

Length@PersistentObjects["Hashes/Once/*", "Local"] 

You can now exit the Kernel

Exit[]

And repeat all the previous actions.

You should observe that subsequent evaluations take much shorter time, and always return the same value, as in the GIF below.