Storage example

Author: mohammadfawaz

storage/README.md

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+# Storage in Leo
+
+This example demonstrates **storage variables** in Leo 4 — on-chain state that replaces single-entry mappings with a simpler, typed API.
+
+## The Scenario
+
+A treasury program tracks its balance, configuration, and deposit history using on-chain storage.  A separate auditor program reads the treasury's storage externally to monitor its state — without the treasury needing to expose any getter functions.
+
+## Program Architecture
+
+```
+treasury.aleo                     auditor.aleo
+  │  storage balance: u64            │  reads treasury.aleo::balance
+  │  storage config: FeeConfig       │  reads treasury.aleo::config
+  │  storage deposit_log: [u64]      │  reads treasury.aleo::deposit_log
+  │  storage is_frozen: bool         │  reads treasury.aleo::is_frozen
+  │  (owns and writes all state)     │  (read-only external access)
+  └──────────────────────────────────┘
+```
+
+## Features Showcased
+
+### Storage Singletons
+
+A `storage` declaration creates a single named on-chain slot.  Values are optional — `none` until set, clearable back to `none`.
+
+```leo
+program treasury.aleo {
+    storage balance: u64;
+    storage is_frozen: bool;
+    storage deposit_count: u32;
+
+    fn initialize() -> Final {
+        return final {
+            balance = 0u64;
+            is_frozen = false;
+        };
+    }
+}
+```
+
+All storage operations happen inside a `return final { ... };` block.  The `Final` return type tells the AVM that the function modifies on-chain state.
+
+### Reading Storage
+
+Storage slots are optional.  Use `unwrap()` when the value must exist, or `unwrap_or()` to provide a fallback:
+
+```leo
+fn deposit(amount: u64) -> Final {
+    return final {
+        let current: u64 = balance.unwrap_or(0u64);  // fallback if uninitialized
+        balance = current + amount;
+    };
+}
+```
+
+### Struct Storage
+
+Structs are stored and read as a single unit:
+
+```leo
+struct FeeConfig {
+    rate: u64,
+    minimum: u64,
+}
+
+program treasury.aleo {
+    storage config: FeeConfig;
+
+    fn update_config(rate: u64, minimum: u64) -> Final {
+        return final {
+            config = FeeConfig { rate: rate, minimum: minimum };
+        };
+    }
+}
+```
+
+### Vector Storage
+
+Vectors are dynamically-sized on-chain lists with `push`, `pop`, `get`, `set`, `clear`, `swap_remove`, and `len`:
+
+```leo
+program treasury.aleo {
+    storage deposit_log: [u64];
+
+    fn deposit(amount: u64) -> Final {
+        return final {
+            deposit_log.push(amount);       // append
+        };
+    }
+
+    fn pop_last_deposit() -> Final {
+        return final {
+            let removed = deposit_log.pop();  // remove last
+            assert(removed != none);
+        };
+    }
+
+    fn reset() -> Final {
+        return final {
+            deposit_log.clear();            // empty the list
+        };
+    }
+}
+```
+
+### Clearing Storage
+
+Assign `none` to clear a singleton, or call `.clear()` on a vector:
+
+```leo
+fn reset() -> Final {
+    return final {
+        balance = none;         // slot reverts to uninitialized
+        config = none;          // struct slot cleared
+        deposit_log.clear();    // vector emptied
+    };
+}
+```
+
+### External Storage Access
+
+Any program can **read** another deployed program's storage using `program.aleo::variable` syntax.  Writes are restricted to the owning program.
+
+```leo
+import treasury.aleo;
+
+program auditor.aleo {
+    fn take_snapshot() -> Final {
+        return final {
+            // Read external singletons
+            let bal: u64 = treasury.aleo::balance.unwrap_or(0u64);
+            let frozen: bool = treasury.aleo::is_frozen.unwrap_or(false);
+
+            // Read external struct
+            let cfg = treasury.aleo::config.unwrap_or(
+                treasury.aleo::FeeConfig { rate: 0u64, minimum: 0u64 }
+            );
+
+            // Read external vector
+            let count: u32 = treasury.aleo::deposit_log.len();
+            let first = treasury.aleo::deposit_log.get(0u32);
+        };
+    }
+}
+```
+
+## Project Structure
+
+```
+storage/
+├── run.sh
+├── treasury/              # Owns and writes all storage
+│   ├── program.json
+│   └── src/
+│       └── main.leo
+└── auditor/               # Reads treasury storage externally
+    ├── program.json       # lists treasury.aleo as a local dependency
+    └── src/
+        └── main.leo
+```
+
+## Running the Example
+
+### Part 1 — Local execution (no devnode)
+
+Individual treasury operations can be tested locally with `leo run`:
+
+```bash
+cd treasury
+leo run initialize
+leo run deposit 5000u64
+leo run withdraw 1000u64
+leo run update_config 500u64 25u64
+```
+
+Note: each `leo run` starts with fresh state, so operations don't accumulate across calls.
+
+### Part 2 — External storage access (requires `leo devnode`)
+
+Cross-program storage reads require both programs to be deployed.  Start a local devnode in a **separate terminal** first:
+
+```bash
+leo devnode start --private-key APrivateKey1zkp8CZNn3yeCseEtxuVPbDCwSyhGW6yZKUYKfgXmcpoGPWH
+```
+
+Then run the full demo from the `storage/` directory:
+
+```bash
+./run.sh
+```
+
+The script:
+1. Runs local demos of treasury operations
+2. Deploys `treasury.aleo` and initializes it with deposits
+3. Deploys `auditor.aleo` (local dependency on treasury — Leo deploys both in order)
+4. Calls `take_snapshot` — reads treasury balance, count, and frozen state
+5. Calls `check_fee_config` — reads the treasury's `FeeConfig` struct
+6. Calls `check_deposits` — reads the treasury's deposit log vector
+7. Freezes the treasury, then snapshots again to show the alert triggers
+8. Resets all treasury storage to `none`
+
+## Key Takeaways
+
+- **Storage singletons** replace single-entry mappings with a cleaner, typed API.
+- **Struct storage** lets you group related fields into a single on-chain slot.
+- **Vector storage** gives you a dynamically-sized on-chain list with push/pop/get/set/clear.
+- **External reads** let any program inspect another's storage — no getter functions needed.
+- **`none`** semantics: all storage starts as `none`; use `unwrap_or()` for safe fallbacks.

storage/auditor/.env

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+NETWORK=testnet
+PRIVATE_KEY=APrivateKey1zkp8CZNn3yeCseEtxuVPbDCwSyhGW6yZKUYKfgXmcpoGPWH
+ENDPOINT=http://localhost:3030

storage/auditor/build/abi.json

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+{
+  "program": "auditor.aleo",
+  "structs": [
+    {
+      "path": [
+        "Snapshot"
+      ],
+      "fields": [
+        {
+          "name": "balance",
+          "ty": {
+            "Primitive": {
+              "UInt": "U64"
+            }
+          }
+        },
+        {
+          "name": "deposit_count",
+          "ty": {
+            "Primitive": {
+              "UInt": "U32"
+            }
+          }
+        },
+        {
+          "name": "is_frozen",
+          "ty": {
+            "Primitive": "Boolean"
+          }
+        }
+      ]
+    }
+  ],
+  "records": [],
+  "mappings": [],
+  "storage_variables": [
+    {
+      "name": "last_snapshot",
+      "ty": {
+        "Plaintext": {
+          "Struct": {
+            "path": [
+              "Snapshot"
+            ],
+            "program": "auditor.aleo"
+          }
+        }
+      }
+    },
+    {
+      "name": "is_alert",
+      "ty": {
+        "Plaintext": {
+          "Primitive": "Boolean"
+        }
+      }
+    }
+  ],
+  "functions": [
+    {
+      "name": "take_snapshot",
+      "is_final": true,
+      "inputs": [],
+      "outputs": [
+        {
+          "ty": "Final",
+          "mode": "None"
+        }
+      ]
+    },
+    {
+      "name": "check_fee_config",
+      "is_final": true,
+      "inputs": [],
+      "outputs": [
+        {
+          "ty": "Final",
+          "mode": "None"
+        }
+      ]
+    },
+    {
+      "name": "check_deposits",
+      "is_final": true,
+      "inputs": [],
+      "outputs": [
+        {
+          "ty": "Final",
+          "mode": "None"
+        }
+      ]
+    }
+  ]
+}