Draft: Dh@table of constants

drafts/current/draft-table-of-constants.md

+---
+title: Table of Micheline feature
+status: Draft
+author: realD4hines
+type: L
+created: 2020-01-05
+date: 2021-01-05
+version: 0
+---
+
+## Summary
+
+We propose adding a global table to the protocol for storing Michelson values,
+accessible from any contract.
+
+## Motivation
+
+Contracts have a fixed upper size limit. Unfortunately, the current size limit
+severely restricts the complexity of smart contracts, forcing users to resort to a
+kind of Michelson code golf to compensate.
+
+Our proposed solution to this problem is a shared table of Michelson values that
+contracts can reference by hash, allowing users to factor large chunks of code
+out of their contract. We expect the primary things to be factored out to be
+lambdas and large types.
+
+Factoring out lambdas and types has many benefits:
+
+- Globally available lambdas form libraries of reusable logic, enabling much
+  more complex applications to fit into the same size.
+- As the table becomes more widely adopted it serves as a reference of the 
+the most frequently reused functions. This supplies us with fine grained data to
+further optimize the most heavily used contracts:
+- Large types for complex storages or parameters are often repeated throughout a
+  contract, taking up precious space. Referencing these types via constants
+  alleviates this problem.
+
+## Specification
+
+- A new manager operation `Register_global` is created, which allows the user to
+  store a valid Micheline representation of a Michelson value in a
+  globally-accessible table and returns the hash of this value.
+  - Expressions registered as constants may contain references to other constants. 
+    However, the depth of a fully-expanded expression is constrained so as to not
+    exceed the depth of a contract that can be type checked (currently the typechecker
+    fails after 10,000 iterations.)
+      - To prevent the need to iterate through deep chains of constants, the depth of
+        a constant is stored along with its value.
+- The storage table is implemented similarly to the existing one for
+    `big_map`'s, but is shared by all contracts instead of accessible only to
+    one.
+- A new Michelson primitive `constant` is added for referencing these values.
+- When applying a transaction, right before type elaboration, we substitute each
+  occurrence of a constant primitive with the value it points to in storage.
+  If no value is found, the transaction fails.
+- The typed Michelson IR has no knowledge of constants, and the Michelson type
+  elaborator and interpreter do not change in any way.
+- Users will be presented with several new `tezos-client` commands (we may add
+  others as we see the need):
+  - `tezos-client register global constant <expression> from <src>`
+
+    Registers a new expression in the table.
+  - `tezos-client get global constant <hash>`
+
+    Returns the value of the hash in the current context (if it exists).
+
+  - `tezos-client expand constants in <contract>`
+
+    Substitutes every constant with its value in the current context (or
+    fails if a hash is not found).
+      
+## Backwards Compatibility
+
+To the author's knowledge, the change does not break any existing functionality
+and is only additive.
+
+## Security Considerations
+
+Once contracts are loading libraries, an obvious attack is to try to trick
+developers into loading malicious libraries. Users must take care to use the
+correct hash and review all the code they load into their contracts.
+
+At the same time, libraries could improve security overall, since code be
+audited and certified at the function level rather than the contract level. As a
+user, I will find it easier to review 100 lines of logic that uses trusted
+libraries than 1000 lines of in-lined code that does the same thing.
+
+## Implementation
+
+The global table of micheline is implemented in MR 2962: https://gitlab.com/tezos/tezos/-/merge_requests/2962
+## Appendix
+
+Much credit goes to Arthur Breitman's
+[post on the Agora](https://forum.tezosagora.org/t/global-constants/2025)
+and Tom Jack's insights and critiques.
+
+## Copyright
+
+Copyright and related rights waived via
+[CC0](https://creativecommons.org/publicdomain/zero/1.0/).