ARC-0031: Improving mappings

arc-0031/README.md

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+---
+arc: 0031
+title: Improving mappings
+authors: evan@demoxlabs.xyz, mike@demoxlabs.xyz
+discussion: [ARC-0031: Improving mappings](https://github.com/AleoHQ/ARCs/discussions/20)
+topic: Application
+status: Draft
+created: 10-05-2022
+---
+
+## Abstract
+
+Mappings are how Aleo programs store public state. To make them more useful, we should:
+1. Load the value of mappings directly. 
+2. Store the value of mappings directly.
+3. Support structs & strings as mapping data types.
+
+## Specification
+
+1. A function should be able to load the value of a mapping directly. 
+
+```
+mapping balances:
+    key owner as address.public;
+    value amount as u64.public;
+
+function get_balance:
+    input r0 as address.public;
+    load balances[r0] into r1;
+
+    output r1 as u64.public;
+```
+
+2. A finalize statement should support storing the value of a mapping directly
+```
+mapping balances:
+    key owner as address.public;
+    value amount as u64.public;
+
+function store_balance:
+    input r0 as address.public;
+    input r1 as u64.public;
+
+    finalize r0 r1;
+
+finalize store_balance:
+    input r0 as address.public;
+    input r1 as u64.public;
+
+    store balances[r0] r1;
+```
+
+3. Mapping should support structs & strings
+
+```
+struct nft:
+    id as u64.public;
+    collection_id as u64.public;
+    url as string.public;
+    metadata as string.public;
+
+mapping nft_owners:
+    key owner as address.public;
+    value nft_item as nft.public;
+
+function mint_nft:
+    input r0 as address.public;
+    input r1 as u64.public;
+    input r2 as u64.public;
+    input r3 as string.public;
+    input r4 as string.public;
+
+    cast r1 r2 r3 r4 into r5 as nft;
+
+    finalize r0 r5;
+
+finalize mint_nft:
+    input r0 as address.public;
+    input r1 as nft;
+
+    store nft_owners[r0] r1;
+```
+
+4. The REST API should support endpoints to fetch the mapping.
+
+## Dependencies
+
+This will impact snarkVM and everything that has snarkVM as a dependency.
+
+### Backwards Compatibility
+
+As this is a new feature, no programs should be impacted by adding a new opcode.
+
+## Security & Compliance
+
+There should be no regulatory concerns.