Vault

Vault is an in-memory key-value store — think ETS from Erlang. Values are shared across all actors in a process and persist across function calls. Good for caches, counters, session state, and coordination between concurrent tasks.

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Basic CRUD

let v = Vault.new("my-store")

Vault.set(v, "name", "Alice")
Vault.get(v, "name")    -- Some("Alice")
Vault.get(v, "missing") -- None
Vault.drop(v, "name")
Vault.get(v, "name")    -- None

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Atomic update

Vault.update applies a function to the current value atomically — no race between get and set. The callback receives the unwrapped value and is only called when the key exists:

let counter = Vault.new("counters")
Vault.set(counter, "hits", 0)

Vault.update(counter, "hits", fn n -> n + 1)

Multiple actors can call update concurrently without conflict. If the key doesn’t exist, the update is a no-op.

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TTL (time-to-live)

Vault.set_ttl sets a key with an expiry. It takes (store, key, value, ttl_secs) — TTL is in seconds:

Vault.set_ttl(v, "token", "abc123", 60)   -- expires after 60 seconds

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Namespacing

Different store names isolate keys completely:

let user_cache    = Vault.new("users")
let session_store = Vault.new("sessions")
let rate_limits   = Vault.new("rate-limits")

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Listing keys

let store = Vault.new("config")
Vault.set(store, "host", "localhost")
Vault.set(store, "port", "8080")

let keys = Vault.keys(store)    -- ["host", "port"]

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Complete example: request rate limiter

mod RateLimit do
  let store = Vault.new("rate-limit")

  fn check(client_ip : String, limit : Int) : Bool do
    let key = "hits:" ++ client_ip
    let count = match Vault.get(store, key) do
      None    -> 0
      Some(n) -> n
    end
    if count >= limit do
      false
    else
      if count == 0 do
        -- First hit: set with TTL to auto-expire the window after 60s
        Vault.set_ttl(store, key, 1, 60)
      else
        -- Subsequent hits: increment (key already exists)
        Vault.update(store, key, fn n -> n + 1)
      end
      true
    end
  end

  fn main() do
    let ip = "192.0.2.1"
    let limit = 3
    println("hit 1: " ++ bool_to_string(check(ip, limit)))
    println("hit 2: " ++ bool_to_string(check(ip, limit)))
    println("hit 3: " ++ bool_to_string(check(ip, limit)))
    println("hit 4 (over limit): " ++ bool_to_string(check(ip, limit)))
  end
end