How to memoize a function in PowerShell?

Question

I have a function string --> string in PowerShell that is quite slow to execute, and I would like to memoize it, that to preserve all the input/output pairs to speed-up an execution that calls this function over and over. I can think of many complicated ways of achieving this. Would anyone have anything not too convoluted to propose?

Answer 1

Well, here's a shot at it. I definitely cannot claim this is a good or efficient method; in fact, even coming up with something that works was tricky and a PowerShell expert might do better.

This is massive overkill in a scripting language, by the way (a global variable is far simpler), so this is definitely more of an intellectual exercise.

function Memoize($func) {
    $cachedResults = @{}
    {
        if (-not $cachedResults.ContainsKey("$args")) {
            echo "Remembering $args..." #for illustration
            $cachedResults.Add("$args", $func.invoke($args))
        }
        $cachedResults["$args"]
    }.getnewclosure()
}


function add($a, $b) {
    return $a + $b;
}

$add = Memoize ${function:add};

&$add 5 4
&$add 5 4
&$add 1 2

Answer 2

I don't know if writing a .NET PowerShell module is practical in your case. If it is, you could use the similar technique as I did with one of my projects.

Steps:

1. Rewrite your function as a cmdlet
2. Have a static class with a static hash table where you store the results
3. Add a -Cache flag to the cmdlet so you can run it with and without the cache (if needed)

My scenario is a little different as I am not using a hash. My cache: https://github.com/Swoogan/Octopus-Cmdlets/blob/master/Octopus.Extensions/Cache.cs Usage: https://github.com/Swoogan/Octopus-Cmdlets/blob/master/Octopus.Cmdlets/GetEnvironment.cs

Answer 3

I thought of something pretty simple, like the following:

$array = @{}

function memoized
{
    param ([string] $str)

    if ($array[$str] -eq $null)
    {
        $array["$str"]  = "something complicated goes on here"
    }

    return $array[$str]
}

memoized "testing"